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Monsoonal influence on variation of hydrochemistry and isotopic signatures: Implications for associated arsenic release in groundwater
Abstract
The present study examines the groundwater and surface water geochemistry of two different geomorphic domains within the Chakdaha Block, West Bengal, in an attempt to decipher potential influences of groundwater abstraction on the hydrochemical evolution of the aquifer, the effect of different water inputs (monsoon rain, irrigation and downward percolation from surface water impoundments) to the groundwater system and concomitant As release. A low-land flood plain and a natural levee have been selected for this purpose. Although the stable isotopic signatures of oxygen (δ18O) and hydrogen (δ2H) are largely controlled by local precipitation, the isotopic composition falls sub-parallel to the Global Meteoric Water Line (GMWL). The Cl/Br molar ratio indicates vertical recharge into the wells within the flood plain area, especially during the post-monsoon season, while influences of both evaporation and vertical mixing are visible within the natural levee wells. Increase in mean DOC concentrations (from 1.33 to 6.29 mg/L), from pre- to post-monsoon season, indicates possible inflow of organic carbon to the aquifer during the monsoonal recharge. Concomitant increase in AsT, Fe(II) and HCO3- highlights a possible initial episode of reductive dissolution of As-rich Fe-oxyhydroxides. The subsequent sharp increase in the mean As(III) proportions (by 223%), particularly in the flood plain samples during the post-monsoon season, which is accompanied by a slight increase in mean AsT (7%) may refer to anaerobic microbial degradation of DOC coupled with the reduction of As(V) to As(III) without triggering additional As release from the aquifer sediments.
... High As concentrations (>10 µg/L; permissible guideline value recommended by WHO) are documented in the groundwater of the shallow grey sand aquifers ('GSA') (at depths <70 m bgl) of the Bengal Basin (Dowling et al., 2002;Harvey et al., 2002;Fendorf et al., 2010;Biswas et al., 2012;Chatterjee et al., 2013;Biswas et al., 2014). Frequent patterns of seasonal variations in As concentrations are documented mainly in the shallow GSA groundwaters of the Bengal Basin (Cheng et al., 2005;Savarimuthu et al., 2006;Planer-Friedrich et al., 2012;Biswas et al., 2014;Majumder et al., 2016Majumder et al., , 2017. In contrast, the brown sand aquifers ('BSA') (at depths 35 -70 m bgl) provide nearly As-free groundwater (<10 µg/L As) Ghosal et al., 2015). ...
... Seasonal variation in As concentrations in the shallow groundwater of the Bengal Basin is documented highly heterogeneous due to the involvement of multiple factors, including variable rainfall, surface water recharge, mixing, groundwater withdrawal, aquifer redox state, and local lithology (Cheng et al., 2005;Savarimuthu et al., 2006;Planer-Friedrich et al., 2012;Biswas et al., 2014;Majumder et al., 2016Majumder et al., , 2017. However, the subject remains poorly addressed due to the lack of comprehensive knowledge on the interconnected roles of multiple factors driving As distribution over time in the shallow groundwaters of the Bengal Basin. ...
... A variable temporal pattern of dissolved total As concentration in the shallow groundwater has been explained based on the seasonal recharge cycles. Multiple studies, registered higher dissolved total As concentrations in shallow groundwater during the wet periods compared to the dry periods in an annual cycle (Cheng et al., 2005;Savarimuthu et al., 2006;Planer-Friedrich et al., 2012;Biswas et al., 2014;Majumder et al., 2016Majumder et al., , 2017. Such an observation is commonly attributed to the monsoonal recharge bringing anoxia either through the inflow of DOC or saturation of sediment pore spaces; conducive for the As release. ...
Arsenic (As) mobilization in groundwater is linked to the dissimilatory reductive dissolution of As(V) coated Fe(III)-oxy-hydroxides from the aquifer sediment coupled with the degradation of available dissolved organic carbon (DOC) due to the presence of anaerobic microbes under anoxic conditions. Understanding the seasonal pattern of As mobilization in the shallow groundwater of the Bengal Basin remains a challenging task due to the heterogeneous character of the shallow aquifers and the involvement of multiple factors. To resolve this issue, in the present study, we showcased a comprehensive effort to advance understanding of the seasonal As mobilization process in the shallow groundwater, utilizing multiple geochemical tracers (i.e., the abundance of dissolved total As, Fe, Mn, NO3⁻, SO4²⁻, DOC, Cl⁻, and δ¹⁸O, δ²H, δ¹³C-DOC isotopic tracers) between post-monsoon and pre-monsoon periods over multiple years from Nadia district, West Bengal, India. We quantified and explained the seasonal variation of dissolved total As concentrations in the groundwaters with the nature of the aquifer lithology (i.e., grey sand aquifer or ‘GSA’ and brown sand aquifer or ‘BSA’). The present study reported elevated dissolved total As concentrations in the shallow groundwater samples during dry pre-monsoon (‘PRM’) periods compared to post-monsoon (‘POM’) time. However, the magnitude of such seasonal changes in groundwater As concentrations (denoted as ΔAs = AsPRM - AsPOM) varied between years depending on the extent of rainfall, surface water infiltration, mixing, and groundwater withdrawal. Our current findings are different from the past studies that reported elevated As concentrations in the shallow groundwater during the monsoon and post-monsoon periods compared to the dry pre-monsoon periods. However, the limitations of the past findings are that most of the previous studies were carried out between seasonal intervals over a single annual cycle without reinvestigating the seasonal trends over different years under variable hydrological conditions. We proposed that the excess groundwater withdrawal during the dry pre-monsoon period drive draw-down and, therefore, trigger infiltration of surface-derived deep pond water into the shallow aquifer. Such surface water infiltration introduces fresh labile organic matter into the shallow aquifer, promoting high As mobilization during the dry pre-monsoon period. A viable alternative approach of ‘squeezing’ of aquifer intercalated clay-peat sedimentary lenses and mixing of organic-rich pore water in the adjoining groundwater can also enhance high As mobilization during the dry time, as examined in this study. This process is triggered by the excessive groundwater withdrawal practices and drawdown encountered during dry time, driving the aquifer intercalated clay-pockets compaction.
... Higher As contamination ([10 lg/ L; a provisional guideline value recommended by the World Health Organization) is recorded in the shallow groundwater tapping grey sand aquifer (\60 m bgl) of Holocene age in the Bengal basin (Dowling et al. 2002;Harvey et al. 2002;Fendorf et al. 2010;Neumann et al. 2010;Biswas et al. 2012;Chatterjee et al. 2013;McArthur et al. 2016). The majority of shallow groundwater in the current study region shows a significant seasonal variability in the dissolved total As content (Biswas et al. 2014;Majumder et al. 2016); however, the factors responsible for such variability are highly debatable. Here, we present the pattern of total As release into the shallow groundwater at seasonal time intervals by simultaneous monitoring of the dissolved total As contents, redox tracers, i.e., oxidation-reduction potential (ORP) and NO 3 À and conservative tracers, including d 18 O, d 2 H and Cl À in the seasonal water samples from the surface and shallow aquifers. ...
... This is consistent with the process observed in the neighbouring region of Bangladesh (Harvey et al. 2002;Neumann et al. 2010). A similar interpretation about the input of water from isolated surface water bodies to the shallow groundwater was provided based on the study of stable isotopic tracers (d 18 O and d 2 H) from the current locations as well (Lawson et al. 2008;Biswas et al. 2014;Majumder et al. 2016;Kulkarni et al. 2018). More recently, the role of clay-pore water in As mobilisation from the aquifer sediment to regional groundwater of the Bengal basin was highlighted Mozumder et al. 2020). ...
... The presence of intercalated clay-pockets/layers is reported in the shallow Holocene aquifers throughout the Bengal basin (Umitsu 1993;Goodbred and Kuehl 2000;Biswas 2003;Horneman et al. 2004;McArthur et al. 2001McArthur et al. , 2004Charlet et al. 2007;Ghosal et al. 2015;Mihajlov et al. 2020;Mozumder et al. 2020), however with more frequent distribution in the sedimentary succession found in India (West Bengal) compared to Bangladesh (M etral et al. 2008). Also, data from both West Bengal (Lawson et al. 2008(Lawson et al. , 2013Sengupta et al. 2008;Datta et al. 2011;Biswas et al. 2014;Majumder et al. 2016;McArthur et al. 2004McArthur et al. , 2016 and Bangladesh (Harvey et al. 2002;Stute et al. 2007;Neumann et al. 2010;Mihajlov et al. 2020) regions share similarity in the signature of high 3 H counts, and similar ranges of stable d 18 O and d 2 H isotope values in the shallow Holocene age groundwater, which is evident from the compiled literature survey. Therefore, we have compared the aquifer-intercalated clayey sediment-trapped pore-water isotopic composition observed in the neighbouring Bangladesh region (Araihazar Upazila) Mozumder et al. 2020) with data from the current study area. ...
Geogenic source of arsenic in groundwater is an indisputable fact. Arsenic (As) contamination in the shallow groundwater of West Bengal is an alarming health issue with progressive growth of regional population and water demand. The shallow groundwater As contamination is facilitated by microbial activities, which depend on the inflow of dissolved organic carbon (DOC), providing a temporary reducing condition conducive to releasing As from the aquifer sediments. Here we present a case where multiple years of seasonal observations on dissolved total As contents and stable isotope ratios in shallow groundwater, ‘deep pond water’ (collected from the deepest part of the pond just above the sediment-water interface), river water, and rainwater from the region of Chakdaha, Haringhata block, Nadia district, West Bengal is documented. Present study shows for the first time a large similarity in the isotopic composition of seasonal deep pond water with the shallow aquifer intercalated clay sediment trapped pore water (data adopted from the literature of Bangladesh region). We highlight the possible processes where seasonal eutrophication during dry time followed by monsoon time flooding and sedimentation allows burial of organic-rich clayey sediments with trapped dissolved organic carbon-rich pore water, serve as a source of clay-pocket pore water. The isotopic composition of clay-pore water formed at a historical time scale resemble the deep bottom water composition of the surface water bodies and provides the ideal composition responsible for triggering the seasonal release of As in the shallow groundwater. The processes like excess withdrawal of groundwater during dry periods and consequent squeezing of aquifer sediment intercalated clay-lenses can expel its organic-rich contents to trigger anaerobic microbial activities and As release in the adjoining water. Using D-excess and δ18O as conservative tracer, we have showed ~13-14% mixing (exclusion) of clay-pore water to the adjoining As uncontaminated (≤10 µg/L) shallow regional groundwater is sufficient to cause high As (>10 µg/L) mobilization.
Keywords: Groundwater, Arsenic, Clay-pore water, Eutrophication, Deep pond water, DOC
... As we have seen, As 5+ sorbs more strongly than As 3+ . However, dissolved organic material can provide food for microorganisms that reduce arsenic from the +5 state to the +3 (Majumder et al. 2016). This creates a situation in which some arsenic is displaced from the iron oxyhydroxides then reduced to the +3 state. ...
... Under reducing conditions, the Fe 3+ in the iron oxyhydroxides can be reduced to Fe 2+ , resulting in the iron oxyhydroxide mineral itself dissolving and any arsenic that is bound to it being remobilized. This process is fueled by dissolved organic carbon that is relatively recently derived from surface sources (Lawson et al. 2016;Majumder et al. 2016). The carbon provides energy to bacteria and its metabolism consumes oxygen ...
... Hutchison and Bortel ◀ Fate of Arsenic in the Flood ▶ 2018 ICC which makes the environment far more reducing in nature (Henke 2009a). It also produces bicarbonate that drives As off of the iron oxyhydroxides (Majumder et al. 2016). The result is the reduction of the iron oxyhydroxides and the release of arsenic, initially as As 5+ but under these conditions a substantial amount will convert to As 3+ (Meng et al. 2016;Molinari et al. 2015). ...
... Previous efforts to track temporal changes in dissolved organic carbon (DOC) concentration in groundwater have also shown contrasting effects of monsoonal recharge. Majumder et al. (2016) observed that the DOC concentration increased during the post-monsoon season in Nadia district in West Bengal (India) and was related to increased reduction of As (V) to As (III). In contrast, Planer-Friedrich et al. (2012) reported an inverse seasonal trend between DOC and dissolved As T , as the lowest DOC concentration coincided with the highest As T concentration. ...
... Ghosh et al. (2015) described the organic matter characteristics of Pleistocene sediments from another site in Nadia district (~12 km south-west of Sahispur site) to contain high molecular weight (HMW) n-alkanes, low molecular weight (LMW) n-alkanes, mono-saturated n-alkanoic acids, LMW n-alkanoic acids, sterol and stanol monomers and lipids. The selected sites for this study are within 25 km distance and receive the similar average annual precipitation (1200 to 3945 mm) during June/July-September/October (Majumder et al., 2016). ...
... Abs 254 can be influenced by Fe (III) concentration, as Fe (III) absorbs significantly near 254 nm (Weishaar et al., 2003). Although Fe (III) species have been shown to be low at this site (Biswas et al., 2011(Biswas et al., , 2014Neidhardt et al., 2013;Majumder et al., 2016), during UV measurement, Fe (II) may oxidize quickly and result in increased absorbance (Poulin et al., 2014). Therefore, Abs 254 and SUVA 254 values do not necessarily represent the higher aromaticity, but may also reflect higher Fe concentrations in samples with high As concentrations. ...
Arsenic (As) mobilization in the Bengal Basin aquifers has been studied for several decades due to the complex redox bio-geochemistry, dynamic hydrogeology and complex nature of dissolved organic matter (DOM). Earlier studies have examined the changes in groundwater As in the dry season before monsoon and during the wet season after monsoonal recharge. To investigate the more immediate influence of recharge during the active monsoon period on As mobilization and DOM character, groundwater samples were analyzed in the pre-monsoon and during the active monsoon period. Groundwater samples were collected from shallow (<40 m) and deep (>40 m) tube-wells in West Bengal, India. Dissolved AsT in shallow groundwater ranged from 50 to 315 μg/L exceeding the WHO guideline of 10 μg/L. Shallow groundwater also showed high total dissolved nitrogen, carbon to nitrogen (C:N) <1, and humic-like DOM with a humic:protein ratio >1. By contrast, deep groundwaters contained AsT between 0.5 and 11 μg/L with carbonaceous and protein-like DOM, C:N >1, and humic:protein <1. Stable isotopes of δ18O and δ2H and Cl/Br results indicated three recharge scenarios in the shallow aquifer including direct recharge of dilute rainwater, evaporated surface water, and anthropogenically impacted surface water. Monsoonal recharge did not cause notable changes in AsT in deep or shallow groundwater, including two As hotspots in the Pleistocene aquifer. However, the monsoon did result in a two-fold decrease in SUVA254, increase in nitrite and nitrate in the shallow groundwater. The DOM in the deep groundwater at the two As hotspots (with AsT 132 and 715 μg/L) had optical properties with much greater humic-like DOM than the surrounding groundwater, which had low AsT and highly protein-like DOM. Overall, these results support that protein-like DOM associated with low groundwater As concentrations and suggest that the monsoonal influence on nitrate and nitrite is limited to shallow aquifers.
... The land-use pattern of the study area consists of farming land (up to ~ 75%), where rice cultivation is the foremost agricultural practice. The climate is characteristically warm and humid (temperature ranges between 10 and 40 0 C, average relative humidity > 60%) with yearly precipitation ranging between 1172 and 1635 mm (mean 1436 mm) (Majumder et al., 2016;Nath et al., 2005). ...
... The factor analysis also indicates that calcium (Ca) and magnesium (Mg) have an association with alkalinity and hardness, which further suggests that carbonate dissolution (calcite as well as dolomite) may contribute to overall composition of groundwater, including arsenic (As) release in the groundwater (Table 3). (Mukherjee and Bromssen 2008;Biswas et al., 2011;Neumann and Ashfaque 2010;Bhattacharyya et al., 2002, Harvey andSwartz 2002;Mc Arthur et al., 2004;Harvey and Swartz 2005;Majumder et al., 2016). Therefore, when the groundwater of shallow aquifers of Chakdah block is used for irrigation purpose, it adds more arsenic and pathogenic microorganisms to the irrigation field. ...
In Bengal Delta Plain (West Bengal and Bangladesh), shallow aquifer (<50 m) groundwa-
ter is often used in irrigation for paddy cultivation. The present study highlights the role
of anthropogenic activities on natural process and focuses on arsenic mobilization in the
shallow irrigation well water of rural Bengal. The major focus is to examine the role of
geochemistry, arsenic mobilization and their association with local microbial community
in irrigation well water. The results suggest that the groundwater of monitored wells is usu-
ally devoid of dissolved oxygen and consists of high concentration of dissolved redox ele-
ments like arsenic (AsT) (mean value 58.7 µgL−1) and iron (FeT) (mean value 2.6 mgL−1),
with low amount of oxyanions like sulphate (SO4
2−) and nitrate (NO3
−) (mean value 8.3
mgL−1 and 1.8 mgL−1, respectively). High concentration of alkaline earth metals like cal-
cium (Ca) (mean value 252.0 mgL−1) and magnesium (Mg) (mean value 96.9 mgL−1) with
high alkalinity (mean value 400.3 mgL−1) suggests that carbonate dissolution (calcite and
dolomite) is the key process in these monitored wells. The factor analysis reveals a positive
and strong correlation (r
2=0.716) between arsenic (As) and iron (Fe). Some of the wells
are contaminated with high concentration of chloride (up to 128.5 mgL−1), total coliforms
(no. of wells contaminated with coliforms=7) and faecal coliforms (no. of wells contam-
inated with faecal coliforms=5), which point towards the local anthropogenic infuence
(leakage of sewage). Some irrigation wells with non-permissible arsenic concentration also
harbour arsenic (As3+) and iron (Fe2+) hyper-tolerant bacteria. The microenvironment of
the irrigation wells consisting of suitable local reducing condition together advocates the
dominance of existing microbial community, particularly arsenic and iron hyper-tolerant
bacteria in them. In this study, an arsenic and iron hyper-tolerant bacterium has been iso-
lated from irrigation well water. Scanning electron microscopy and 16S rDNA sequencing
further established the identity of this bacterium as Enterobacter sp., which is a faculta-
tive anaerobe. The addition of fresh organic matter through local anthropogenic activities
could enhance the microbial activity in these monitored wells. The silver nitrate test reveals the biotransformation potential of this bacterium from arsenite (As3+) to arsenate (As5+).
Altogether, these results point towards the dependence of arsenic mobilization process on
multiple microbial and geochemical factors, where bacterium like Enterobacter sp. might
also play an important role. Such biogeochemical processes are not only responsible for
the unsafe nature of irrigation well water (for both domestic and irrigation purposes) in
rural Bengal, but also convert it into a major potential source for soil arsenic accumulation,
thereby contaminating the food chain.
... Previous studies captured the role of the C-Fe-S redox cycle on groundwater As mobilization process in the Bengal Basin based mostly on one-time sample collection and subsequent monitoring over a hydrologic year (Harvey et al., 2002;Mukherjee and Fryar, 2008;Neumann et al., 2010) without the documentation of seasonal variations. The redox state in shallow aquifers is affected by seasonally variable rainfall amount, groundwater level, and recharge-discharge cycle (Biswas et al., 2014;Cheng et al., 2005;Majumder et al., 2016), which in turn affects the As release procedure. Therefore, the effect of redox-controlled C-Fe-S bio-geochemical cycles on groundwater As behavior at seasonal time intervals is worth further investigation using isotopic tracers. ...
... The field area has been known as one of the 'As hot spot' zones (>10 μg/L As concentration in shallow groundwater, WHO) for >30 years (Pathak et al., 2022a;Biswas et al., 2014;Ghosal et al., 2015;Majumder et al., 2016;Charlet et al., 2007), occurring in proximity to the Hooghly River (now known as Hugli River) at Nadia district (Chakdaha, Haringhata block), West Bengal. The study area (~48 km 2 ) falls under the Gangetic alluvial plain with an elevation of 0-20 m above MSL. ...
The bacterial reduction process of As-coated Fe(III)–OOH and SO4²⁻ in the sediment plays an important yet complex role in contributing to groundwater arsenic (As) concentration. The process utilizes dissolved organic carbon in the aquifer, which varies seasonally. The present observation documented seasonal variability (between dry pre-monsoon and post-monsoon periods) in the potential methanogenesis and intensity of bacterial reduction activity in the shallow and a few deep groundwaters from the West Bengal region, India (Nadia district). The dataset from the present study includes physical parameters (i.e., pH, Eh, electrical conductivity), multiple redox-sensitive solutes (i.e., dissolved total As, Fe, SO4²⁻ concentration), dissolved inorganic carbon (DIC) or HCO3⁻ concentration, major element ratio (HCO3⁻/Na, Ca/Na), and stable isotopes in the dissolved inorganic carbon (δ¹³C-DIC) and dissolved sulfate (δ³⁴S–SO4²⁻) phases in the groundwater samples. Multiple years of observations documented a seasonal pattern of microbially-mediated reduction of As-coated Fe(III)–OOH and potential methanogenesis with limited bacterial sulfate reduction (BSR), causing excess As mobilization in the groundwater registering low sulfate concentration. The process was documented predominantly during the dry pre-monsoon period compared to the post-monsoon period. However, a few seasonal shallow groundwater samples showed abundant sulfate concentration and a signature of potentially active BSR, with low dissolved As concentrations. We showed here that dissolved SO4²⁻ concentration and δ³⁴S–SO4²⁻ isotope enrichment factor can be reliable indicators of the intensity of BSR using the Rayleigh fractionation model simulation.
... Numerical modeling indicated that such ponds contribute 33% of total recharge despite only occupying 7% of the modeled area (Desbarats et al., 2014). Majumder et al. (2016), who also worked in Chakdaha block, observed that δ 18 O and δ 2 H of pond waters fall along the global MWL and that depleted δ 18 O values are associated with elevated As and Cl − in groundwater sampled in December, after the monsoon. We suspect that ponds in Murshidabad district sampled by Datta et al. (2011), which have relatively enriched δ 18 O and δ 2 H values indicative of partial evaporation, were not incised into permeable sediments. ...
... We infer that groundwater flow in the WBA generally follows the topographic gradient eastward toward the B-H, with possible local reversals in hydraulic gradient resulting from pumping or flooding. ) in groundwater may originate from agriculture or human waste, as noted in other studies in the western Bengal basin (Acharyya et al., 2000;McArthur et al., 2012b;Desbarats et al., 2014;Sankar et al., 2014;Majumder et al., 2016). Concentrations of SO 4 2− tend to increase with Cl − , particularly at site C (Fig. S4a). ...
Understanding the controls on spatial variability of groundwater arsenic (As) is critical for mitigating As contamination. The objective of this study is to determine controls on previously unexplained differences in groundwater As concentrations, which are high along the east bank and low along the west bank of the River Bhagirathi-Hoogly (B-H), the primary Indian distributary of the River Ganges, on the western margin of the Bengal basin. A total of 54 wells were sampled after the monsoon season at four sites (two each east and west of the B-H) in Murshidabad district, West Bengal, for field parameters, major and minor solutes, and stable isotopes of water. An additional four boreholes were drilled for analyses of sediment texture, mineralogy, total organic and inorganic carbon, and total As and other metal(loid)s. Results show that higher As in east-bank groundwater (median 0.031 mg/L) is associated with generally more anoxic conditions (higher median total Fe and lower median EH and NO3⁻) relative to west-bank groundwater (median As < 0.001 mg/L), consistent with previous studies. In contrast, concentrations of Mn in the study area are highest in west-bank wells near the B-H. Carbonate and silicate weathering appear to be more important in east- and west-bank groundwater, respectively, which may reflect differences in sediment sources. Ranges of total As are similar in east- and west-bank sediments. Relatively depleted values of δ¹⁸O and δ²H in the east-bank aquifer and streams appear to reflect focused recharge through paleochannels, while relatively enriched west-bank values suggest diffuse recharge to upland aquifers. We speculate that water infiltrating through erosional, stratigraphic “windows” carries organic matter capable of mobilizing As in east-bank groundwater. This comprehensive evaluation of groundwater chemistry provides a more detailed understanding of controls on As variability within the basin.
... In addition, the Cl/Br ratio is a conservative chemical tracer, which has been used to distinguish different water sources and as an indicator of water-rock interactions (particularly halite dissolution), leaching or wastewater influence on a groundwater (e.g. Davis et al., 1998;Alcalá and Custodio, 2005;Cartwright et al., 2006;McArthur et al., 2012;Xie et al., 2012;Majumder et al., 2016;McArthur et al., 2016, and references within). ...
... note p > significance level of .05). The range of values observed are broadly consistent with other studies elsewhere in circum-Himalayan Asia, for example, in the Bengal Basin (McArthur et al., 2012;Majumder et al., 2016;McArthur et al., 2016) and China (Xie et al., 2012). Human and animal (e.g. ...
Chronic exposure to arsenic (As) through the consumption of contaminated groundwaters is a major threat to public health in South and Southeast Asia. The source of As-affected groundwaters is important to the fundamental understanding of the controls on As mobilization and subsequent transport throughout shallow aquifers. Using the stable isotopes of hydrogen and oxygen, the source of groundwater and the interactions between various water bodies were investigated in Cambodia's Kandal Province, an area which is heavily affected by As and typical of many circum-Himalayan shallow aquifers. Two-point mixing models based on δD and δ¹⁸O allowed the relative extent of evaporation of groundwater sources to be estimated and allowed various water bodies to be broadly distinguished within the aquifer system. Model limitations are discussed, including the spatial and temporal variation in end member compositions. The conservative tracer Cl/Br is used to further discriminate between groundwater bodies. The stable isotopic signatures of groundwaters containing high As and/or high dissolved organic carbon plot both near the local meteoric water line and near more evaporative lines. The varying degrees of evaporation of high As groundwater sources are indicative of differing recharge contributions (and thus indirectly inferred associated organic matter contributions). The presence of high As groundwaters with recharge derived from both local precipitation and relatively evaporated surface water sources, such as ponds or flooded wetlands, are consistent with (but do not provide direct evidence for) models of a potential dual role of surface-derived and sedimentary organic matter in As mobilization.
... Here, increasing frequency of drought calls for the sustainable management of water resources for drinking and irrigation purpose (Bhunia et al., 2020;Roy et al., 2023). In West Bengal, monsoon-attributed variations in geochemistry were reported (Farooq et al., 2011;Kulkarni et al., 2018;Majumder et al., 2016;Mohanta and Goel, 2014). However, these studies have been limited in sampling frequency, making it difficult to comprehend groundwater DOM variability during periods of distinct hydrological change, such as the onset of the monsoon season. ...
... The pools of As in lake sediments are primarily governed by the physiochemical characteristics of nutrients and factors that influence its accumulation and degradation. Additionally, it is also affected by monsoon; compared with deeper lakes, shallow lakes are more susceptible to monsoon (Kulkarni et al. 2018;Majumder et al. 2016). Therefore, under the disturbance of the monsoon, the amount of As deposited in the shallower lakes will be more easily released into the overlying water than in the deeper lakes. ...
The behavior and risk of arsenic (As) closely relate to its geochemical fractionation and environmental factors in sediments. The soluble (F1), reducible (F2), oxidizable (F3), and residual fraction (F4) of As were extracted in the sediments from Lake Hulun, Wuliangsuhai, and Dalinor of Inner Mongolia Plateau. Coupled with lakes from Eastern and Northeast Plain, Yunnan-Guizhou and Qinghai-Tibetan Plateau, the responses of As fractions to environmental conditions were investigated according to the spatial distribution of As fractionations in five lake ecoregions at a national scale of China. Generally, F1 was more sensitive to environmental changes, and the pH presented significantly negative effects on the amount of soluble As, while water depth played an important role in regulating the distribution of the fraction F2 and F4. The As pools in surface lake sediments presented a latitudinal zonation due to the gradient effects of climate and anthropogenic activities on nutrient decomposition, and their influence on the capacity of sediments holding As. This work indicated that nutrients played a coordinating role in regulating the impacts of climate and environmental factors on As fractionation in aquatic environments.
... The pools of As in lake sediments are primarily governed by the physiochemical characteristics of nutrients and factors that influence its accumulation and degradation. Additionally, it is also affected by monsoon; compared with deeper lakes, shallow lakes are more susceptible to monsoon (Kulkarni et al. 2018;Majumder et al. 2016). Therefore, under the disturbance of the monsoon, the amount of As deposited in the shallower lakes will be more easily released into the overlying water than in the deeper lakes. ...
The behavior and risk of arsenic (As) closely relate to its geochemical fractionation and environmental factors in sediments. The soluble (F1), reducible (F2), oxidizable (F3), and residual fraction (F4) of As were extracted in the sediments from Lake Hulun, Wuliangsuhai, and Dalinor of Inner Mongolia Plateau. Coupled with lakes from Eastern and Northeast Plain, Yunnan-Guizhou and Qinghai-Tibetan Plateau, the responses of As fractions to environmental conditions were investigated according to the spatial distribution of As fractionations in five lake ecoregions at a national scale of China. Generally, F1 was more sensitive to environmental changes, and the pH presented significantly negative effects on the amount of soluble As, while water depth played an important role in regulating the distribution of the fraction F2 and F4. The As pools in surface lake sediments presented a latitudinal zonation due to the gradient effects of climate and anthropogenic activities on nutrient decomposition, and their influence on the capacity of sediments holding As. This work indicated that nutrients played a coordinating role in regulating the impacts of climate and environmental factors on As fractionation in aquatic environments.
Graphical abstract
... However the As concentration is also largely varying. The variation in As concentration is due to microbial mobilization of As from sediment to ground water where local anthropogenic sources are playing key role [24,25]. During fi eld visit, it has been found that this monitored well are constracted near the sanitary installation where the Fe concentration is varying from 1.08 to 2.15 mgL -1 whereas MPN counts vary from 6 to 9 and FC is varying 4 to 7 counts. ...
In Bengal Delta Plain (BDP), shallow aquifer (<50 m) is often contaminated with Arsenic (As). The phenomenon is wide spread in nature thought the BDP notable in Nadia district of west Bengal. The present study highlights a primary screening of As, Fe, MPN and FC in monitored shallow wells. The study designed for two different sites (site-A, High As and site-B Low As area). The water quality monitoring results suggest that high As concentration (Range- 103-171 μgL-1) has been noticed in site A when compared with site B (range-53-99 μgL-1). In sites A, the Fe concentration is high and low in site B. The correlation study (r2) between arsenic and iron are also determined. The value of r2 is 0.94 for site A and 0.73 for site B. The water quality results suggest that the nature of the monitor aquifer is anoxic in nature with low Eh, DO absent and low NO3- and SO4+. Major anion is HCO3- (376 mgL-1) followed by Cl- (28 mgL-1). However chloride concentration is largely varying in the monitored tube well. Microbial study (MPN & FC count) also indicates some relationship among MPN (r2-0.32) and Fe (r2 -0.24). However the relationship is scatter when As concentration is low. The linear trended has also obtained when both As, MPN and FC are high. The physical observation of plate count (Color reaction in Chromo colt Agar) has also been observed. This is a clear indicator of fecal coli form contamination. The study indicates that the microbial mobilization of As is the key factor for enrichment of As in ground water. The possible sources of the microbes are local land-use pattern (notable pit-latrine). Finally, the study highlights the role of coli forms bacteria (Both facultative and non-facultative) are wide spread in shallow rural aquifer of Bengal. Thus microbial process possibly enriches arsenic in shallow ground water.
... Unfortunately this BDP (Bengal Delta Plain) ground water is contamination due to micro-organism [1][2][3]. ...
... Mukherjee et al. (2011) further pointed out that the increased recharge as an outcome of such extensive pumping generates downward influx of organic carbon (Harvey et al. 2002) and oxygenated (Das et al. 1995) water, which perturbs the redox-reliant biogeochemical reactions within the aquifer framework. Moreover, much of this Asenriched water that is being brought up to the surface is supplied to irrigated fields and the As that is not taken up by the crops would reinfiltrate into the aquifer through processes of enhanced recharge (Roychowdhury et al., 2005;Mukherjee et al., 2011).Previous studies in the HB have also shown the influence of high concentrations of certain agriculture and human sourced anions (SO 4 2− , Cl − ) and nutrients (PO 4 3− , NO 3 − ) on As trends (Acharyya et al., 2000;McArthur et al. 2012;Desbarats et al. 2014;Majumder et al. 2016;Mukherjee et al. 2018). For instance, excess NO 3 − is known to suppresses reductive mobilization of As and enhance As sequestration by oxidation of Fe(II) to ferric hydroxides (Burren 1998). ...
High groundwater arsenic (As) across the globe has been one of the most well researched environmental concerns during the last two decades. Consequently, a large scientific knowledge-base has been developed on As distributions from local to global scales. However, differences in bulk sediment As concentrations cannot account for the As concentration variability in groundwater. Instead, in general, only aquifers in sedimentary basins adjacent to mountain chains (orogenic foreland basins) along continental convergent tectonic margins are found to be As-enriched. We illustrate this association by integrating observations from long-term studies of two of the largest orogenic systems (i.e., As sources) and the aquifers in their associated foreland basins (As sinks), which are located in opposite hemispheres and experience distinct differences in climate and land-use patterns. The Andean orogenic system of South America (AB), an active continental margin, is in principle a modern analogue of the Himalayan orogenic system associated with the Indus-Ganges-Brahmaputra river systems in South Asia (HB).
In general, the differences in hydrogeochemistry between AB and HB groundwaters are conspicuous. Major-solute composition of the arid, oxic AB groundwater exhibits a mixed-ion hydrochemical facies dominated by Na-Ca-Cl-SO4-HCO3. Molar calculations and thermodynamic modeling show that although groundwater of AB is influenced by cation exchange, its hydrochemical evolution is predominated by feldspar dissolution and relationships with secondary clays. In contrast, humid, strongly reducing groundwater of HB is dominated by Ca-HCO3 facies, suggestive of calcite dissolution, along with some weathering of silicates (monosiallitization). This work demonstrates that although hydrogeochemical evolutionary trends may vary with local climate and lithology, the fundamental similarities in global tectonic settings can still lead to the elevated concentrations of groundwater As.
... Large scale atmospheric processes, such as El Niño-Southern Oscillation (ENSO) (Moerman et al., 2013;Sánchez-Murillo et al., 2017;Santos et al., 2019b), the Intertropical Convergence Zone (ITCZ), the South America Convergence Zone (SACZ) (Rozanski and Araguás, 1995;Sánchez-Murillo et al., 2016;Gastmans et al., 2017), and Monsoons in part of tropical Asia (Araguás-Araguás et al., 1998;Ishizaki et al., 2012;Majumder et al., 2016) influence the formation of the precipitation and hence the isotopic variability. Likewise, small scale processes may also be associated with the isotopic variability across the tropical regions, such as sub cloud evaporation (Srivastava et al., 2012) and convective/ stratiform types of rain (Risi et al., 2008;Kurita et al., 2009;Kurita, 2013;Aggarwal et al., 2016;Lacour et al., 2018). ...
... From relationships and analysis of variance against modeled fraction recharge, we examine how differences in the hydrology and recharge sources affect arsenic concentrations, and how human perturbations to the aquifer system caused by urban pumping and irrigation are affecting recharge source and groundwater quality. Differentiating these recharge sources could be important in characterizing aquifer redox status, as recharge from both pond and rivers sources can both enhance arsenic release Majumder et al., 2016;Stahl et al., 2016;van Geen et al., 2013;Xie et al., 2008). We expect that groundwater contains a large fraction of riverine recharge that results in different redox processes contributing varying amounts of DOC and a heterogeneous signature in sulfate that ultimately affects water quality. ...
Widespread contamination of groundwater with geogenic arsenic is attributed to microbial dissolution of arsenic‐bearing iron (oxyhydr)oxides minerals coupled to the oxidation of organic carbon. The recharge sources to an aquifer can influence groundwater arsenic concentrations by transport of dissolved arsenic or reactive constituents that affect arsenic mobilization. To understand how different recharge sources affect arsenic contamination—in particular through their influence on organic carbon and sulfate cycling—we delineated and quantified recharge sources in the arsenic affected region around Hanoi, Vietnam. We constrained potential end‐member compositions and employed a novel end‐member mixing model using an ensemble approach to apportion recharge sources. Groundwater arsenic and dissolved organic carbon concentrations are controlled by the dominant source of recharge. High arsenic concentrations are prevalent regardless of high dissolved organic carbon or ammonium levels, indicative of organic matter decomposition, where the dominant recharge source is riverine. In contrast, high dissolved organic carbon and significant organic matter decomposition are required to generate elevated groundwater arsenic where recharge is largely nonriverine. These findings suggest that in areas of riverine recharge, arsenic may be efficiently mobilized from reactive surficial environments and carried from river‐aquifer interfaces into groundwater. In groundwaters derived from nonriverine recharge areas, significantly more organic carbon mineralization is required to obtain equivalent levels of arsenic mobilization within inland sediments. This method can be broadly applied to examine the connection between hydrology, geochemistry and groundwater quality.
... Downward migration of young organic matter was also expected to occur in shallow As-affected groundwater in Bangladesh (Harvey et al., 2002;Mailloux et al., 2013) and Cambodia (Lawson et al., 2016;Richards et al., 2018). In West Bengal where groundwater was extensively pumped, pond-derived organic matter was transported into shallow groundwater vertically (Farooq et al., 2012;Lawson et al., 2013;Majumder et al., 2016). ...
Organic matter (OM) is widely accepted to be the trigger for arsenic (As) mobilization from sediments into groundwater. Identifications of reactivity and sources of organic matter responsible for driving As liberation remain unresolved. To address these issues, we provide the extensive characterization of OM in shallow groundwater, surface water, and sediments near a permanent wetland and a dried wetland in the Hetao basin with high As groundwater and low As groundwater, respectively, using fluorescence spectroscopy and parallel factor analysis (PARAFAC). Dissolved organic matter (DOM) of shallow groundwater and sediments near the permanent wetland had higher biological index (BIX), and more protein-like components and microbially-derived components relative to that near the dried wetland, showing higher bioreactivity. Fingerprint of water stable isotopes and higher water levels of wetland than shallow groundwater pointed to the recharge of wetland water into shallow groundwater. Laboratory incubations of the sediment with more bioreactive OM obtained near the permanent wetland mobilized more As. Both DOM properties and laboratory incubations indicate that OM with high bioreactivity promoted As mobility mainly by fuelling microbial respiration of Fe oxide reduction. Recharge of wetland water into shallow groundwater introduced bioreactive DOM into shallow aquifer. Groundwater DOM near the permanent wetland was typically sourced from both wetland DOM and sedimentary OM with high bioreactivity. However, sedimentary OM was the major source of groundwater DOM near the dried-wetland. It suggested that downward infiltration of surface-derived OM and perturbations of aquifer sediments by changing groundwater flow regimes increase bioreactivity of groundwater DOM and therefore enhance As mobility.
... For example, shallow groundwater at Van Phuc is recharged by water from the Red River (van Geen et al., 2013); however, the TDP concentration in the Red River was below 0.01 mg L −1 , which would dilute the concentrations in groundwater. Furthermore, some studies argued that land use patterns (e.g., sanitation facilities and agriculture) directly influenced groundwater properties in shallow aquifers of the BDP and the RRD (Majumder et al., 2016;McArthur et al., 2012). The leaching and transport of PO 4 3− from the soil to groundwater was previously observed in other areas of intensive agricultural use (McDowell et al., 2015); however, a downward transport of PO 4 3− was also shown to be mostly associated with well-drained soils and to be restricted to shallow depths of b2 m bls only (Domagalski and Johnson, 2011;Mabilde et al., 2017;McDowell et al., 2015;McGinley et al., 2016;Robertson, 2008). ...
The biogeochemical cycling of phosphorus (P) in South and Southeast Asian floodplain and delta aquifers has received insufficient attention in research studies, even though dissolved orthophosphate (PO4³⁻) in this region is closely linked with the widespread contamination of groundwater with toxic arsenic (As). The overarching aim of this study was to characterize the enrichment of P in anoxic groundwater and to provide insight into the biogeochemical mechanisms underlying its mobilization, subsurface transport, and microbial cycling. Detailed groundwater analyses and in situ experiments were conducted that focused on three representative field sites located in the Red River Delta (RRD) of Vietnam and the Bengal Delta Plain (BDP) in West Bengal, India. The results showed that the total concentrations of dissolved P (TDP) ranged from 0.03 to 1.50 mg L⁻¹ in groundwater, with PO4³⁻ being the dominant P species. The highest concentrations occurred in anoxic sandy Holocene aquifers where PO4³⁻ was released into groundwater through the microbial degradation of organic carbon and the concomitant reductive dissolution of Fe(III)-(hydr)oxides. The mobilization of PO4³⁻ may still constitute an active process within shallow Holocene sediments. Furthermore, a sudden supply of organic carbon may rapidly decrease the redox potential, which causes an increase in TDP concentrations in groundwater, as demonstrated by a field experiment. Considering the subsurface transport of PO4³⁻, Pleistocene aquifer sediments represented effective sinks; however, the enduring contact between oxic Pleistocene sediments and anoxic groundwater also changed the sediments PO4³⁻-sorption capacity over time. A stable isotope analysis of PO4³⁻-bound oxygen indicated the influences of intracellular microbial cycling as well as a specific PO4³⁻ source with a distinct isotopically heavy signal. Consequently, porous aquifers in Asian floodplain and delta regions proved to be ideal natural laboratories to study the biogeochemical cycling of P and its behavior in groundwater environments.
... The groundwater arsenic concentrations at Jianghan plain varied significantly with time, decreasing from October to April/May, then sharply rebounded from May to July, and maintained a high level from August to September . Seasonal variability in As concentrations was also reported at other sites (Frost et al., 1993;Guo et al., 2013;Majumder et al., 2016;Masson et al., 2007). While the observed phenomena were generally attributed to the effect of coupled hydrobiogeochemical processes, the mechanisms leading to the temporal variations in groundwater arsenic concentrations are still ambiguous. ...
The arsenic concentration in groundwater varies significantly with time and space in groundwater-surface water exchange zones. Various processes have been identified that control arsenic concentration distribution and mobility in laboratory systems. However, it is still challenging to identify important processes controlling arsenic concentration distribution at the field scale due to the complex coupling of hydrobiogeochemical processes. In this study, a reactive flow and transport model was used to identify dominant processes controlling arsenic distribution and seasonal variations in groundwater-surface water exchange zones using Jianghan Plain, China, as an example. The results revealed the importance of river water and groundwater interactions on seasonal changes in arsenic concentration; however, the affected region is limited to within 50-m distance to the river. The modeling results, unexpectedly, revealed the predominant importance of groundwater extraction to the seasonal variation in arsenic concentration. The groundwater extraction changed the groundwater flow pattern and induced vertical leakage of oxygen-containing surface water into the aquifer, which triggered a series of biogeochemical reactions that changed groundwater redox conditions and promoted arsenic sorption, resulting in a rapid decrease in arsenic concentrations in groundwater. After groundwater extraction ceased, aquifer recovered to anoxic condition, promoting arsenic release from the sorbed phase, leading to a rapid rebounding in groundwater arsenic concentrations. Overall, this study provided a tool to identify coupled hydrobiogeochemical processes on arsenic spatiotemporal distribution and migration in groundwater.
... This is supported by the fact that the highest NO − 3 concentrations (average ∼12 mg L −1 ) of all sampled wells (n = 51) were observed in Khidirpur, and Mn T concentrations were also high (average ∼0.67 mg L −1 ). The effects of vertical mixing on the geochemistry of shallow groundwater (<40 m) from the Nadia district (∼160 km south of the current study site) have been observed during the post-monsoon period (Majumder et al., 2016). However, further investigations with detailed sediment analyses at Khidirpur would be necessary to understand the mechanism of As T immobilization. ...
HIGHLIGHTS Manganese and arsenic concentrations are elevated in Murshidabad groundwater.
Manganese release appears to be independent of dissolved organic matter quality.
Mineral precipitation and dissolution reactions impact fate of manganese.
Arsenic concentrations are related to dissolved organic matter quantity and quality.
The prevalence of manganese (Mn) in Southeast Asian drinking water has recently become a topic of discussion, particularly when concurrent with elevated arsenic (As). Although Mn groundwater geochemistry has been studied, the link between dissolved organic matter (DOM) quality and Mn release is less understood. This work evaluates characteristics of DOM, redox chemistry, and the distribution of Mn within Murshidabad, West Bengal, India. Shallow aquifer samples were analyzed for cations, anions, dissolved organic carbon, and DOM properties using 3-dimensional fluorescence excitation emission matrices followed by parallel factor modeling analyses. Two biogeochemical regimes are apparent, separated geographically by the river Bhagirathi. East of the river, where Eh and nitrate (NO3−) values are low, humic-like DOM coexists with high dissolved Mn, As, and Fe. West of the river, lower dissolved As and Fe concentrations are coupled with more protein-like DOM and higher NO3− and Eh values. Dissolved Mn concentrations are elevated in both regions. Based on the distribution of available electron acceptors, it is hypothesized that groundwater east of the Bhagirathi, which is more reducing and enriched in dissolved Fe and Mn but depleted in NO3−, is chemically dominated by Mn(IV)/Fe(III) reduction processes. West of the river where NO3− is abundant yet dissolved Fe is absent, NO3− and Mn(IV) likely buffer redox conditions such that Eh values are not sufficiently reducing to release Fe into the dissolved phase. The co-occurrence of humic-like DOM with dissolved As, Fe, and Mn in the more reducing aquifers may reflect complex formation between humic DOM and metals, as well as electron shuttling processes involving humic DOM, which may enhance metal(loid) release. Saturation indices of rhodochrosite (MnCO3) suggest that precipitation is thermodynamically favorable in a greater proportion of the more reducing sites, however humic DOM–Mn complexes may be inhibiting MnCO3 precipitation. Where dissolved arsenic concentrations are low, it is postulated that Mn(IV) reduction is oxidizing As(III) to As(V), increasing the potential for re-adsorption of As(V) onto relatively stable, un-reduced or newly precipitated Fe-oxides. Manganese release appears to be independent of DOM quality, as it persists in both humic and protein-like DOM environments.
... This is supported by the fact that the highest NO − 3 concentrations (average ∼12 mg L −1 ) of all sampled wells (n = 51) were observed in Khidirpur, and Mn T concentrations were also high (average ∼0.67 mg L −1 ). The effects of vertical mixing on the geochemistry of shallow groundwater (<40 m) from the Nadia district (∼160 km south of the current study site) have been observed during the post-monsoon period (Majumder et al., 2016). However, further investigations with detailed sediment analyses at Khidirpur would be necessary to understand the mechanism of As T immobilization. ...
The prevalence of manganese (Mn) and arsenic (As) and their impacts on human health have been documented extensively in the groundwater of West Bengal, India. Although the general mechanisms behind the release of these elements are understood, the link between dissolved (>0.7μm) organic matter (DOM) quality and Mn and As release is less explored. This study illustrates associations between characteristics of DOM and the spatial distribution of groundwater Mn and As. We present a dataset collected between 2011 and 2015 from six different sites within the Murshidabad district of West Bengal, India. A total of 51 shallow (<40m) groundwater samples were analyzed for cations, anions, and various DOM properties (e.g., Freshness Index (β:α), Humification Index (HIX), etc.) using 3-dimensional fluorescence excitation emission matrices followed by parallel factor (PARAFAC) modeling analyses. Mn, As, and Fe in sediments were quantified via bulk and sequential extractions, as well as µ-XRF mapping coupled with XANES and EXAFS spectroscopy to evaluate specific elemental associations in solid phases. Results indicate that two distinct biogeochemical regimes are identified. East of the river Bhagirathi, where Eh values are low and DOC is abundant (1.83 mg L-1), dissolved Mn (0.83 mg L-1), As (329.9 μg L-1), and Fe (3.63 mg L-1) are elevated. Furthermore, fluorescent signatures of DOM degradation (β:α: 0.73; HIX: 11.1) are observed, suggesting that microbial oxidation of DOM is linked to Fe and Mn-oxide reduction, releasing Mn, As, and Fe into the groundwater. West of the Bhagirathi, however, where Eh is higher and DOC values are slightly lower (1.34 mg L-1), dissolved Mn remains elevated (1.11 mg L-1) while Fe (0.31 mg L-1) and As (8.98 μg L-1) are considerably lower. Here, signatures of DOM degradation are not as pronounced (β:α: 0.93; HIX: 5.32), and thus it is hypothesized that microbially derived reductive processes are less active. As a result, Mn-oxide reduction, independent of Fe-oxide reduction and perhaps even abiotically, appears to be the dominant redox driver. Finally, associations between Mn, As, and Fe in sediments reinforce the idea that Fe and Mn-oxide reduction is most likely conducive to the release of As from aquifer sediments in Murshidabad.
... Surface-derived DOC may serve as a natural exogenous source of organic C that that could stimulate dissimilatory As(V)/Fe(III) reduction in the subsurface (Harvey et al., 2002Lawson et al., 2013Lawson et al., , 2016Mailloux et al., 2013;Majumder et al., 2016;Neumann et al., 2010), though this is a matter of controversy (McArthur et al., 2011a;Sengupta et al., 2008;Stute et al., 2007). Studies in Bangladesh show that pond water of ∼50-year-old 3 H-3 He age recharges the groundwater at the depth range (30-40 m) of maximum aqueous As concentration and where irrigation wells are screened (Harvey et al., 2002;Klump et al., 2006;Neumann et al., 2010). ...
Arsenic (As), a toxic metalloid common throughout the Earth's crust, accounts for the most widespread poisoning of a human population in history. Within the major deltas of South and Southeast (S/SE) Asia, rivers annually deposit As-bearing iron oxides, oxyhydroxides, and hydroxides (collectively referred to as Fe oxides hereafter) derived from the Himalaya. The high primary productivity and monsoonal flooding in the tropical deltas promote microbially driven As release to groundwater through dissimilatory As(V)/Fe(III) reduction. Groundwater is a primary source of drinking and irrigation water in the region, especially within rural areas. Prolonged consumption of As-contaminated groundwater can lead to a multitude of serious health complications, including cancer and cardiovascular disease. Here we define the parameters controlling the locations of active microbially driven As release to groundwater, including suboxic/anoxic conditions, microbial communities capable of mediating As(V)/Fe(III) reduction, the reactivity of As-bearing Fe oxides, and the sources and reactivity of organic carbon (C). Conditions for microbially driven As release are optimized where the reactivity of both As-bearing Fe oxides and organic C is greatest. Optimal conditions for As release are found in near-surface sediments of the Red River, under permanent wetlands of the Mekong River, and at depth (∼20 m) in the Yangtze River Basin, whereas findings are variable within the Bengal Basin. Land and water management changes resulting in increased flood duration in deltaic environments may result in new locations of active microbial As release to groundwater.
Consumption of arsenic (As)‐contaminated groundwater adversely impacts the health of almost 20 million people in China. Determining the sources of As‐affected groundwaters may help to improve our understanding of the controlling processes on As mobilization in groundwater systems. In this study, stable hydrogen and oxygen isotopes of water (δ ¹⁸ O and δD) were employed to delineate the groundwater recharge sources and the interactions between river/pond and groundwater in Shahu village, a typical high‐As groundwater area in Jianghan Plain, central China. Utilizing a two‐component mixing model based on δ ¹⁸ O and δD, we successfully calculated the river water contribution to groundwater including its uncertainty analysis and roughly distinguished the different water bodies within the aquifer system. Cl/Br was used to further identify the recharge contributions for shallow (10 m below the ground surface, low As concentrations), intermediate (25 m below the ground surface, high‐As concentrations) and deep (50 m below the ground surface, high‐As concentrations) groundwaters. The hydrogen and oxygen stable isotope signatures of high As and high total organic carbon (TOC) groundwaters (intermediate and deep aquifer) generally plotted near the local meteoric water line (LMWL). However, the δ ¹⁸ O and δD signatures of low As and low TOC groundwaters (shallow aquifer) tended to shift away from the LMWL along evaporation lines. These relationships revealed that the low As groundwater principally derived from surface water (river and pond), while the high‐As groundwater mainly recharged from local precipitation through preferential channel as well as the bedrock and/or adjacent aquifer. Our results will enhance the comprehension of the genesis of high‐As groundwater in Jianghan Plain.
Assessing groundwater geochemical formation processes and pollution circumstances is significant for sustainable watershed management. In the present study, 58 shallow groundwater samples were taken from the Dongwen River Basin (DRB) to comprehensively assess the hydrochemical sources, groundwater quality status, and potential risks of NO3– to human health. Based on the Box and Whisker plot, the cation’s concentration followed the order of Ca²⁺ > Mg²⁺ > Na⁺ > K⁺, while anions’ mean levels were HCO3⁻ > SO4²⁻ > NO3⁻ > Cl⁻. The NO3⁻ level in groundwater samples fluctuated between 4.2 and 301.3 mg/L, with 67.2% of samples beyond the World Health Organization (WHO) criteria (50 mg/L) for drinking. The Piper diagram indicated the hydrochemical type of groundwater and surface water were characterized as Ca·Mg-HCO3 type. Combining ionic ratio analysis with principal component analysis (PCA) results, agricultural activities contributed a significant effect on groundwater NO3⁻, with soil nitrogen input and manure/sewage inputs also potential sources. However, geogenic processes (e.g., carbonates and evaporite dissolution/precipitation) controlled other ion compositions in the study area. The groundwater samples with higher NO3⁻ values were mainly found in river valley regions with intense anthropogenic activities. The entropy weight water quality index (EWQI) model identified that the groundwater quality rank ranged from excellent (70.7%) and good (25.9%) to medium (3.4%). However, the hazard quotient (HQ) used in the human health risk assessment (HHRA) model showed that above 91.38% of groundwater samples have a NO3⁻ non-carcinogenic health risk for infants, 84.48% for children, 82.76% for females, and 72.41% for males. The findings of this study could provide a scientific basis for the rational development and usage of groundwater resources as well as for the preservation of the inhabitants' health in DRB.
The seasonal variations of shallow groundwater arsenic have been widely documented. To gain insight into the monthly variations and mechanisms behind high groundwater arsenic and arsenic exposure risk in different climate scenarios, the monthly probability of high groundwater arsenic in Hetao Basin was simulated through random forest model. The model was based on arsenic concentrations obtained from 566 groundwater sample sites, and the variables considered included soil properties, climate, topography, and landform parameters. The results revealed that spatial patterns of high groundwater arsenic showed some fluctuations among months under different future climate scenarios. The probability of high total arsenic and trivalent arsenic was found to be elevated at the start of the rainy season, only to rapidly decrease with increasing precipitation and temperature. The probability then increased again after the rainy season. The areas with an increased probability of high total arsenic and trivalent arsenic and arsenic exposure risk under SSP126 were typically found in the high-arsenic areas of 2019, while those with decreased probabilities were observed in low-arsenic areas. Under SSP585, which involves a significant increase in precipitation and temperature, the probability of high total arsenic and trivalent arsenic and arsenic exposure risk was widely reduced. However, the probability of high total arsenic and trivalent arsenic and arsenic exposure risk was mainly observed in low-arsenic areas from SSP126 to SSP585. In conclusion, the consumption of groundwater for human and livestock drinking remains a threat to human health due to high arsenic exposure under future climate scenarios.
Deterioration of groundwater quality is a long-term incident which leads unending vulnerability of groundwater. The present work was carried out in Murshidabad District, West Bengal, India to assess groundwater vulnerability due to elevated arsenic (As) and other heavy metal contamination in this area. The geographic distribution of arsenic and other heavy metals including physicochemical parameters of groundwater (in both pre-monsoon and post-monsoon season) and different physical factors were performed. GIS-machine learning model such as support vector machine (SVM), random forest (RF) and support vector regression (SVR) were used for this study. Results revealed that, the concentration of groundwater arsenic compasses from 0.093 to 0.448 mg/L in pre-monsoon and 0.078 to 0.539 mg/L in post-monsoon throughout the district; which indicate that all water samples of the Murshidabad District exceed the WHO's permissible limit (0.01 mg/L). The GIS-machine learning model outcomes states the values of area under the curve (AUC) of SVR, RF and SVM are 0.923, 0.901 and 0.897 (training datasets) and 0.910, 0.899 and 0.891 (validation datasets), respectively. Hence, "support vector regression" model is best fitted to predict the arsenic vulnerable zones of Murshidabad District. Then again, groundwater flow paths and arsenic transport was assessed by three dimensions underlying transport model (MODPATH). The particles discharging trends clearly revealed that the Holocene age aquifers are major contributor of As than Pleistocene age aquifers and this may be the main cause of As vulnerability of both northeast and southwest parts of Murshidabad District. Therefore, special attention should be paid on the predicted vulnerable areas for the safeguard of the public health. Moreover, this study can help to make a proper framework towards sustainable groundwater management.
Geogenic arsenic (As) in groundwater has a negative impact on public health. Recent studies have introduced the mechanisms of temporal variation in arsenic concentrations in groundwater. Short-term drastic variations in groundwater arsenic are usually associated with anthropogenic activities, such as contamination and groundwater extraction. Periodic variations in groundwater arsenic are mainly caused by seasonal precipitation/evaporation changes, periodic irrigation/pumping and periodic anthropogenic input. Hydrogeochemical processes of these temporal variations in groundwater arsenic involve the dilution/concentration of As, oxidative dissolution of arsenopyrite, competitive adsorption between As and other ions including HCO3⁻ and PO4³⁻, reductive dissolution of As–Fe/Mn (oxyhydr)oxide, oxidative elimination of aqueous As(III), advection between the aquifers, and the release of As from clay layers. Compared with natural factors, the temporal changes in groundwater As are more susceptible to anthropogenic behaviors; however, topography and climate still play a significant role in diverse patterns of groundwater arsenic concentrations variations. In this study, decreasing groundwater arsenic concentrations in the rainy season and increasing groundwater arsenic concentrations in the dry season were more likely to occur in oxidative aquifers in mountainous areas with fractured bedrock; in coastal areas; or in arid/semiarid inland basins with less anthropogenic interference, such as mines, ponds, farmland, etc. An inverse trend was more likely to occur in reductive aquifers in humid plain/delta areas containing paddy fields and ponds at the surface. More investigations must be conducted to determine the heterogeneity of groundwater arsenic variations at different scales and correlations with the surface environment, including crop cultivation, climate change, land use, and other factors, by using long-term monitoring data.
Geogenic arsenic is a metabolic hazard to global citizens, due to its presence in most of the rocks. Natural processes such as percolation of rainwater through soil layer and water–rock interaction in weathering process principally lead to the dissolution of arsenic-bearing minerals in the aquifer system. In the present study, arsenic (As)-contaminated groundwater was analyzed covering all blocks (26 blocks) of Murshidabad District, West Bengal, India. Principally, the study focused on the assessment of groundwater quality with respect to arsenic along with other metal ions such as iron, manganese, cadmium and selenium. Tube well water samples (N = 348) were collected during pre- and post-monsoon seasons. The spatial distribution of arsenic levels ranges from 0.086 to 0.513 mg/L in pre-monsoon and 0.059–0.431 mg/L in post-monsoon, which indicates that all groundwater samples of the Murshidabad District exceeds the WHO’s permissible limit of arsenic (0.01 mg/L). Water quality index (WQI) data suggested that 5.74% and 10.3% samples are suitable for drinking purpose in the pre-monsoon and post-monsoon season, respectively. Availability of cations are as follows: Ca²⁺ > Mg²⁺ > Na⁺ > K⁺ , and anions as: SO4²⁻ > HCO³⁻ > Cl⁻ > F⁻ > NO³⁻ in both pre-monsoon and post-monsoon seasons. Human health risk due to consumption of groundwater was assessed through USEPA designed methods as follows—hazard quotient (HQ), hazard index (HI), average daily dose of both direct ingestion of groundwater and dermal absorption of groundwater (ADDIngestion, ADDDermal absorption), and exposure frequency (EF). Thus, carcinogenic risk (CR) and non-carcinogenic risk (NCR) were determined. Results revealed that 29% and 37% of inhabitants suffered from carcinogenic and non-carcinogenic risk, respectively. On the basis of occurrence, spatial distribution and health risk assessment results of the targeted area can be marked as a moderate- to high-risk zone. The said zones need special attention for protection of public health.
The special hydrogeological conditions of wetland easily lead to the formation of high-arsenic (As) groundwater. However, it is unclear relative contribution of wetland water (surface water) dissolved organic matter (DOM) and sedimentary organic matter (SOM) to porewater DOM and its relations with dissolved As under the wetland. Porewater, groundwater and sediment samples were taken to monitor spatial-temporal variations in dissolved As affected by fluctuation of surface water levels near a typical wetland with high As groundwater in the arid-semiarid Hetao Basin, China. Hydrochemical data showed that As concentrations of sediment porewater (average: 17.1 μg/L) lay between As concentrations of surface water (average: 6.48 μg/L) and groundwater (average: 184 μg/L). A long-term observation showed concentrations of dissolved As, Fe, and dissolved organic carbon (DOC) of porewater obviously increased when levels of surface water rose during waterlogging period. There were positive correlations between dissolved As and Fe (r = 0.69, p < 0.01) and dissolved As and DOC (r = 0.38, p < 0.01). The sequential extraction showed that the main As form in sediment was amorphous Fe(III) oxide-coprecipitated form (averagely 45.0% of total As). It indicates that the dissolved As in porewater mainly originated from the Fe(III) oxide-bound forms in the sediment. Fluorescence indices and parallel factor analysis of the three-dimensional fluorescence spectra showed that porewater DOM mainly derived mainly from SOM during the drought period and from both surface water DOM and SOM during the waterlogging period. Higher proportions of As(III) in porewaters during waterlogging period were observed relative to those during drought period, indicating that surface water infiltration led to relatively reducing conditions. Surface water infiltration introduced DOM and stimulated the release of SOM into the porewater. DOM, acting as electron donor and electron shuttle, triggered the reductive dissolution of Fe(III) oxides, leading to the As mobilization under the wetland. This study suggests that drinking water supply near wetlands should be concerned, since high concentrations of dissolved As normally occurred in shallow aquifers near the wetland.
The present study depicts the geospatial relation between basinal geomorphology and heterogeneous arsenic (As) distribution in the Bengal Delta Plain (BDP). The distribution pattern largely varies throughout the study area (higher: Karimpur-II AsT average 214.73 μgL⁻¹; lower: Tehatta AsT average 27.84 μgL⁻¹). Both safe (low As) and unsafe (high As) areas are identified within the single shallow aquifer (<50 m), where they are in close vicinity. Statistical analysis shows that Padma river basin is the most contaminated (AsT avg. 214.7 ± 160 μgL⁻¹) and Churni-Ichhamati river basin (AsT avg. 108.54 ± 89.43 μgL⁻¹) is the least contaminated with groundwater As. Moreover, the role of geomorphological features influencing the geospatial distribution of As has been studied and meandering features are found to correlate with high As wells (r² = 0.52), whereas, natural levees are correlated with safer wells (r² = 0.57). In the meandering features, the deposition of sedimentary organic matter (SOM) facilitates the reduction of As bearing Fe(III) oxy-hydroxides and subsequent higher As mobilization. In natural levees, surface derived labile organic matter (DOC and FOM, Fresh Organic Matter) from different land-use patterns (Habitation, degraded waterbodies, cattle dwelling, sanitation, etc.) is transported to shallow aquifers (notably protein rich leakage sewage). The fresh organic carbon transported to the shallow aquifers, thereby triggering As release by microbe-mediated reductive dissolution of hydrated Fe(III)-oxides (HFO). Iron reduction (mostly amorphous) is playing an important role in the release of As depending on basin-wise sedimentation pattern, local recharge, accumulation of silt/clay/micas at the top with corresponding reactive oxidation of organic carbon. These are important components and often helping the cyclic water-rock interaction of As causing such heterogeneous geospatial distribution. The delineation of aquifer with regard to safer and unsafe areas would immensely help to supply safe drinking water to the rural community.
Temporal and seasonal variations of groundwater arsenic concentration are very significant for the exploration of its mobilisation mechanisms and to frame the policies of viable usage of arsenic-contaminated groundwater. Much less is known about the temporal and seasonal pattern of variation of arsenic contamination in shallow groundwater from the present study area (Karimpur blocks, Nadia district, West Bengal, India). So, to extend the understanding, the present study encompasses the steady monitoring of 15 wells of shallow depth from the study area for the period of 2014–2016. It has been observed that geochemical components were varying extensively during the period and the same trends follow for both Fe and As. During the period, the groundwater levels oscillated seasonally which promotes the reductive dissolution of Fe-oxides and/or oxy-hydroxides (As bearing minerals) which is aided by less dispersion of atmospheric oxygen into the shallow aquifers maintaining the reducing environment in the aquifers. The supply of dissolved organic matter into shallow aquifers was also evident by variation of Cl⁻ ion concentration which may help to enhance arsenic mobilisation into shallow groundwater. Moreover, groundwater recharge from both regional rivers and perennial sources influences the evolution of anoxic conditions in the areas with low aquifers permeability consisting mainly of clayey and/or silty intercalations. Likewise, irrigation practices have been augmented to understand its role in arsenic mobilisation.
The mobilization of co-occurring trace elements (CTEs) in aquifers with known metal (loid) contamination may exacerbate potential human health threats where release mechanisms are not geochemically limited. This study investigates the co-mobilization of the potentially toxic CTEs antimony (Sb), molybdenum (Mo), vanadium (V), uranium (U), and selenium (Se) with arsenic (As) and iron (Fe) in contrasting high and low As aquifers surrounding the river Bhagirathi in Murshidabad, West Bengal, India. We sought to determine the dominant release mechanisms and fate of each CTEs as they relate to redox conditions, major element dissolution pathways, and characteristics of dissolved organic matter (DOM). Groundwater samples collected in 2013 and 2015 field campaigns across six villages in the Murshidabad district were analyzed for major and trace elements, as well as the quantity and optical properties of DOM. East of the river Bhagirathi, the Holocene aquifer contains groundwater with compositions reflective of carbonate mineral dissolution and silicate mineral weathering, along with clear signs of Fe reduction and associated mobilization of As, Sb, Mo, V, and U. Relatively oxic groundwater in the Pleistocene aquifer underlying the study sites on the west side of the river showed minimal Fe reduction and signatures of carbonate mineral dissolution and silicate mineral weathering (i.e. Nabagram), or evaporite mineral dissolution (i.e. Kandi). In groundwater samples from the more oxic Pleistocene aquifer, concentrations of As, Sb, Mo, and V were substantially lower than in the Holocene aquifer, however, U concentrations were relatively higher where carbonate mineral dissolution predominated (i.e., Nabagram). Compelling positive correlations between Fe and CTEs in the Holocene but not in the Pleistocene aquifer implicated Fe reductive processes as the probable pathway for CTEs release. The presence of humic-like DOM ligands and inorganic ions (i.e., Ca2+, CO32−) may also enhance CTEs mobility via aqueous complexation reactions, or possibly by inhibiting CTEs adsorption. We further hypothesize that carbonate mineral dissolution mobilizes U in the absence of Fe reduction, and that U solubility in the reducing Holocene aquifer may be facilitated by carbonate mineral dissolution products (e.g., Ca, HCO3−, CO32−) that promote U complexation and the inhibition of (bio)immobilization pathways. Conclusively, potentially toxic CTEs are being mobilized concomitantly with As in the Murshidabad district of West Bengal, probably by microbial Fe reduction. Because these aquifers still contain requisite organic matter and bulk sediment Fe required for Fe reductive processes, it follows that concentrations of CTEs may increase in the future. We therefore argue that – despite moderately low concentrations of CTEs in the current study – mechanistic modeling efforts and more extensive monitoring of trace metal (loid)s should be performed to ensure that drinking water supplies do not pose additional health threats in the coming years.
The phospholipid fatty acids (PLFA) and dissolved organic matter (DOM) 3D fluorescent signatures were appointed to elaborate arsenic (As) mobilization mechanism in groundwater from the Datong Basin. Groundwater samples were collected along the groundwater flow path from recharge zones to discharge zones according to redox sensitive parameters. Variations of Eh, NO3⁻ and SO4²⁻manifested the redox condition shifted from a weakly oxidative environment to highly reductive environment. In recharge zones, some aerobic bacteria may serve as the dominant species, such as Thiothrix (S-oxidizing bacteria: SOB), Gallionella (Fe-oxidizing bacteria: FeOB) and Fluviicola, characterized by 16:1ω7, 16:0 and 18:1ω7 PLFAs. The biogeochemical processes were mainly governed by aerobic bacteria exploiting protein-like DOM as electron donors to maintain metabolism together with depleting of O2 and NO3⁻ (electron acceptors), which restricted the reduction of As. While in discharge zones, the anaerobic microbes played a predominant role, such as Desulfosporosinus (Sulfate reducing bacteria: SRB) and Clostridia (Fe reducing bacteria: FeRB), indicated by cy17:0, cy19:0 and 18:1ω9 PLFAs. SRB and FeRB mainly utilized protein-like DOM as energy sources for respiratory action. Simultaneously, biogenic reductive dissolution of Fe(III) (hydr)oxides and reduction of As(V), SO4²⁻ resulted in geogenic immobilized As reductive desorption into aquifers. The positive linear correlations between humic substances and Fe, As suggested that As-Fe-DOM complexation can enhance the transport of As and Fe in aqueous environments. The negative relationships between SRB/FeRB and As/Fe concentrations demonstrated that the HS⁻ could react with Fe(II) and As(III) to form secondary Fe(II) sulfides or As-bearing sulfides, which subsequently sequestered As from groundwater via sorption or coprecipitation. In conclusion, the utilization of biodegradable protein-like DOM by SRB/FeRB being associated with reductive dissolution of As-bearing Fe (hydr)oxides minerals and As-Fe-DOM complexation promoted As mobility in groundwater aquifers.
Concentrations of Fe, Mn, Pb, Al in both surface water and groundwater, and As, Sr, B, Li and Ba trace elements only in groundwater were observed to have exceeded the WHO and EPA limits in southeastern Imphal valley, Northeast India. Pearson correlation, factor analysis and cross-plots, along with oxygen isotope were used to identify the hydrochemical processes and to delineate the sources of the elevated trace elements. In groundwater samples, 15.6% of the total samples for Sr and 9.4% each for B, Li and Ba were revealed to have elevated values, from piedmont zone, which is related to the evaporite encrustation in Disang shales. 80% and 40% of surface water; and 9.4% and 25% of groundwater showed high concentrations of Al and Pb, respectively. Al and Pb concentrations were originated both from geogenic and anthropogenic sources. Fe and Mn exceed 70% and 20%, respectively, in surface water while 50% each in groundwater. High Fe and Mn might be resulted from reduction of Fe and Mn bearing minerals present in the study area. Excess of As in groundwater was observed from wells located at Kakching subdivision where leaching of As into groundwater from minerals such as kaolinite, montmorillonite, asenopyrite, chalcopyrite, geothite and magnetite and weathering of carbonaceous clay, silt and sand takes place. Other trace elements such as Zn, Bi, Co, Cr and Cd concentrations were all within the WHO limits.
In East and Southeast Asia, the health of over 100 million people is threatened by the consumption of groundwater containing high concentrations of arsenic (>10 μg L-1), which is released from sediments through reductive dissolution of arsenic-bearing iron/manganese oxides. Dissolved organic matter (DOM) is known to play a crucial role in the process of arsenic mobilization in shallow aquifers, and its availability and reactivity are key factors controlling the variation of arsenic concentrations in groundwater. However, it is unclear how human activities influence the transport of DOM and how the transportation affects the DOM molecular properties in high arsenic groundwater. This study provides insights on the sources and molecular compositions of DOM in groundwater from the Jianghan Plain, central China, a newly discovered area with seasonal fluctuations in arsenic concentrations in shallow groundwater. Monitoring of water levels and stable carbon isotope compositions in groundwater from different depths and canal water over a year indicated that terrestrial DOM was the dominant source, accounting for 54.2%-85.5% of groundwater DOM. Electrospray ionization combined with ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry revealed that canal water infiltration transferred aliphatic, tannin-like and leached aromatic DOM from sediments into groundwater. Therefore, groundwater recharge through irrigation using canal water not only inputs terrestrial DOM, but also accelerates the release of sedimentary DOM. Furthermore, carboxylic-rich alicyclic molecule (CRAM)-like DOM that is derived from biomolecules has the highest proportion (60.1%-65.5%) among the identified DOM structures. And, it might be reused in biochemical processes during arsenic mobilization, suggesting a third source of groundwater DOM in addition to canal water and sediments. The findings in this study advance the understanding on transport processes and molecular properties of DOM in high arsenic groundwater under extensive anthropogenic influences.
Regular monitoring of arsenic (As) in groundwater is crucial from public health perspectives as millions of people are suffering due to use of contaminated aquifer water for drinking purposes. The routine analyses, especially in developing nations, are mostly done in localised government/non-government laboratories with limited resources, having the target of analysing large number of samples in each run. Thus apart from analytical sensitivity, cost-effectiveness of the method and eco-friendliness of the experimental operation are key surreptitious factors. This demands optimisation of total As measurement methods and finding a method that gives ‘optimum benefit’ considering all these factors together. The present study therefore evaluates four common As (total) measurement methods [iodometric-colorimetric method, silver diethyl dithiocarbamate method, molybdenum blue method and hydride generation atomic absorption spectrophotometric (HG-AAS) method] practised in the Bengal Delta Plain, in view of their analytical sensitivity, related environmental hazard and experimental costs. It was found that the HG-AAS method is analytically more sensitive, whereas the iodometric-colorimetric method and the molybdenum blue method are better choices in terms of eco-friendliness and cost-effectiveness, respectively. However, when all three factors (analytical reliability, environmental hazard and cost) are considered simultaneously, the molybdenum blue method was found to be placed first in the ‘optimum performance rank’ list. It was also found that both environmental hazard and cost play a more crucial role than analytical reliability, although this is case specific and would differ from place to place around the globe. Finally based on the results, we have hypothesised a water testing strategy for developing countries such as India where the molybdenum blue method can be adapted as a screening method and later the HG-AAS method can be used to precisely identify the groundwater samples with As concentration below the WHO drinking water guideline value of 10 μg/L.
High levels of arsenic in well water are causing widespread poisoning in Bangladesh. In a typical aquifer in southern Bangladesh,
chemical data imply that arsenic mobilization is associated with recent inflow of carbon. High concentrations of radiocarbon-young
methane indicate that young carbon has driven recent biogeochemical processes, and irrigation pumping is sufficient to have
drawn water to the depth where dissolved arsenic is at a maximum. The results of field injection of molasses, nitrate, and
low-arsenic water show that organic carbon or its degradation products may quickly mobilize arsenic, oxidants may lower arsenic
concentrations, and sorption of arsenic is limited by saturation of aquifer materials.
Marketization, globalization and administrative reform in China: a zigzag road to a promising future
The Chinese administrative system has been periodically reformed since the early 1980s. This article mainly focuses on the reforms of the State Council, the central government of China, and then deals with the five main rounds of reform. It gives a full picture of the story of reform with its context and contents, the measures taken, the difficulties encountered and the challenges ahead. It is argued that administrative reform in China has been used to reorient the Communist state to an emerging market-oriented economy and to enhance the capacity of the government to regulate market forces and to respond to the significant impacts made by economic globalization. Given the authoritarian nature of the party-state polity of China, the authors argue that administrative reform in China has also been used as an alternative route to alleviate the crisis of governance resulting from rapid social change; however, its effects are constrained by the implications of politico-administrative apparatus. To build up a modern governing system suitable for a market economy, a proactive approach, including political or constitutional reform, should be put on the agenda.
Points for practitioners
• The reforms of the State Council, the central government of the People’s Republic of China, since the early 1980s have been dealt with;
• A full picture of the story of these administrative reforms is unfolded in the context of China’s market transition and globalization;
• It represents a good example showing how the Communist administrative apparatus reorient themselves in an emerging market-oriented economic setting and why it is successful/unsuccessful in the context of an authoritarian regime;
• This is an administrative reform with Chinese characteristics;
• The reform in China has responded to the world-wide trend of administrative reform.
A portion of the Gambier Plain underlain by an unconfined aquifer with readily definable hydrologic boundaries has been divided into a number of areas within which soil types have similar hydrologic properties. Mean annual recharge has been estimated for each area using both the tritium concentration and the chloride concentration of water within the soil profile. Good agreement was obtained between the two methods with local recharge varying between 50 and 250 mm year-1. Total mean annual recharge for the area has been estimated to be 2.4 ± 0.3 x 108 m3 year-1, and this compares favourably with an estimated discharge of 2.5 ± 0.3 x 108 m3 year-1.
Arsenic concentrations in groundwater extracted from shallow aquifers in some areas of the Ganga Plain in the states of Bihar and Uttar Pradesh, exceed 50μgL−1 and locally reach levels in the 400μgL−1 range. The study covered 535km2 of active flood plain of the River Ganga, in Bihar where a two-tier aquifer system has been delineated in a multi-cyclic sequence of Quaternary sand, clay, sandy clay and silty clay all ⩽∼250m below ground surface. The research used isotopic signatures (δ18O, δ2Η, 3H, 14C) and major chemical constituents HCO3-,SO42-,NO3-,Cl-,Ca2+,Mg2+,Na+,K+,Astotal of groundwater to understand the recharge processes and groundwater circulation in the aquifers. Values of δ18O and δ2Η combined with 3H data indicate that the recharge to the As-enriched top 40m of the deposits is modern (
Arsenic (As) concentrations and stable hydrogen (δ2H) and oxygen (δ18O) isotope ratios of surface and groundwaters from a representative site in West Bengal, India, are reported. Shallow groundwaters (≤35 m) from the study site have among the highest As concentrations measured in the entire Bengal Basin, reaching values in excess of 4600 μg kg−1. Stable isotope ratios of waters from constructed, perennial ponds indicate the ponds are chiefly recharged during the summer monsoon, and subsequently undergo extensive evaporation during the dry (winter) season. In contrast, groundwaters with high As concentrations plot along the local meteoric water line (LMWL) near where the annual, volume-weighted mean precipitation values for δ2H and δ18O would plot. The stable isotope data demonstrate that groundwaters are directly recharged by local precipitation without significant evaporation, and thus are not recharged by, nor mixed with, the pond waters. Furthermore, reactive transport modeling indicates that dissolved organic matter (DOM) derived from pond waters does not fuel microbial respiration and As mobilization at depth in the underlying aquifer because travel times for pond-derived DOM exceed groundwater ages by thousands of years. Instead, organic matter within the aquifer sediments must drive dissimilatory iron reduction and As release to groundwaters.
Elevated arsenic (As) in groundwater is a well-known phenomenon in the Bengal delta plain (BDP). It has been found that mobilization of As can be influenced by the presence of dissolved organic carbon (DOC) and other mineral phases (specially Fe). In this study, groundwater samples were filtered through varying pore size membrane filters to investigate the fractional distribution of As associated with colloids. Arsenic was found to be associated with colloidal particles having organic matter and Fe mineral phases. This finding is very important for explaining the release mechanism of groundwater As, because such data did not exist with respect to BDP, related to the dissolved phase/colloidal retention of As. This study shows that it would be better to consider the dissolved and colloidal phases as potential scavengers of As when assessing its mobilization or mitigation. (C) 2013 The Authors. Published by Elsevier B.V. Selection and/or peer-review under responsibility of Organizing and Scientific Committee of WRI 14-2013
Systematic variations in Cl/Br ratios together with R36Cl values, 14C activities, and δ18O values reflect differences in groundwater recharge and define groundwater flow paths in the Riverine Province of the southeast Murray Basin. In groundwater from the shallowest Shepparton Formation, homogenisation of Cl/Br and δ18O values and a decline in 14C activities with depth imply that vertical flow dominates. The 14C activities define variations in pre-land clearing recharge. In the palaeovalleys of present day rivers (“deep leads”), which contain sandier sediments, higher recharge (0.5 to 1.4 mm/year) produced relatively fresh groundwater (TDS < 3000 mg/L). By contrast, away from the deep leads, lower recharge (0.1 to 0.4 mm/year) resulted in higher degrees of evapotranspiration producing more saline groundwater (TDS up to 60,000 mg/L). Cl/Br ratios correlate with these differences in recharge and groundwater salinity. Groundwater from the deep leads has average molar Cl/Br ratios of 530 to 660, while that from adjacent areas has average molar Cl/Br ratios of 980 to 1090. The variation in Cl/Br ratios is interpreted as due to small differences in the volume of windblown halite that the groundwater has dissolved during recharge, as confirmed by an inverse correlation between Cl/Br ratios and R36Cl values in groundwater from the Goulburn subcatchment. Zones of groundwater with variable Cl/Br ratios in the deeper Calivil–Renmark aquifer also allow the detection of rapid leakage from the surface that has the potential to compromise groundwater resources.
In natural ground water systems, both chlorine and bromine occur primarily as monovalent anions, chloride and bromide. Although dissolution or precipitation of halite, biological activity in the root zone, anion sorption, and exchange can affect chloride/bromide ratios in some settings, movement of the ions in potable ground water is most often conservative. Atmospheric precipitation will generally have mass ratios between 50 and 150; shallow ground water, between 100 and 200; domestic sewage, between 300 and 600; water affected by dissolution of halite, between 1000 and 10,000; and summer runoff from urban streets, between 10 and 100. These, and other distinctive elemental ratios, are useful in the reconstruction of the origin and movement of ground water, as illustrated by case studies investigating sources of salinity in ground water from Alberta, Kansas, and Arizona, and infiltration rates and pathways at Yucca Mountain, Nevada.
Jordan is classified as an arid to semi-arid country with a population according to 1999 estimates of 4·8 millions inhabitants and a growth rate of 3·4%. Efficient use of Jordan's scarce water is becoming increasingly important as the urban population grows. This study was carried out within the framework of the joint European Research project ‘Groundwater recharge in the eastern Mediterranean’ and describes a combined methodology for groundwater recharge estimation in Jordan, the chloride method, as well as isotopic and hydrochemical approaches. Recharge estimations using the chloride method range from 14 mm year−1 (mean annual precipitation of 500 mm) for a shallow and stony soil to values of 3·7 mm year−1 for a thick desert soil (mean annual precipitation of 100 mm) and values of well below 1 mm year−1 for thick alluvial deposits (mean annual rainfall of 250 mm). Isotopically, most of the groundwater in the Hammad basin, east Jordan, falls below the global meteoric water line and far away from the Mediterranean meteoric water line, suggesting that the waters are ancient and were recharged in a climate different than Mediterranean. Tritium levels in the groundwater of the Hammad basin are less than the detection limit (<1·3 TU). However, three samples in east Hammad, where the aquifer is unconfined, present tritium values between 1 and 4 TU. Copyright © 2007 John Wiley & Sons, Ltd.
Groundwater in the Goulburn region of the Murray Basin (Australia) contains solutes derived mainly from evapotranspiration of rainfall-derived marine aerosols, silicate dissolution, and ion exchange. C-14 data indicate that groundwater in the Shepparton Formation recharges vertically across the region, whereas groundwater in the Calivil-Renmark Formation shows a greater component of lateral flow. The overall pattern of geochemical and stable isotope variations implies that long-term vertical groundwater flow into the Calivil-Renmark Formation has occurred over thousands of years. Elevated C, N, and F concentrations, together with variable Cl/Br ratios and delta(18)O values, suggest that short-term (years to decades) flow of surface water into the Calivil-Renmark Formation also occurs locally. The high degree of vertical flow implies that the high-quality groundwater resources of the Calivil-Renmark Formation are vulnerable to surface contamination. Groundwater in both the Shepparton and Calivil-Renmark Formations yields C-14 ages of up to 20 ka that imply that, overall, recharge rates are low and that, consequently, the groundwater resource in both formations could be impacted by over abstraction.
The Bengal basin, Bangladesh, represents one of the most densely populated recent floodplains of the world. The sediment flux through the basin is one of the highest on a global scale. A significant portion of this sediment load find its sink in the basin itself because of its lower elevation and frequent flooding. The textural, mineralogical and chemical nature of the sediments thus have an important bearing on the environmental quality of the basin as well as for the Bay of Bengal.
The sediment load of the Ganges-Brahmaputra-Meghna (GBM) river system consists exclusively of fine sand, silt and clay at their lower reaches within the Bengal basin, Bangladesh, and is deposited under uniformly fluctuating, unidirectional energy conditions. The sediments have a close simitarity in grain size with the sediments of the surrounding floodplain. The mineral assemblage is dominated by quartz and feldspars. Illite and kaolinite are the major clay minerals, and occur in almost equal proportion in bed sediments. The heavy mineral assemblage is dominated by unstable minerals which are mostly derived from high-rank metamorphic rocks. The characteristic smaller grain-size, i.e. having large surface-to-mass ratios, and the mineralogy of sediments suggests that they are susceptible to large chemical adsorptive reactions and thus could serve as a potential trap for contaminants. However, the sediments of the GBM river system in the Bengal basin, Bangladesh, shows lower concentration of Pb, Hg and As, and a marginally higher value for Cd as compared to that of standard shale. Considering population density and extensive agricultural practice in the basin, the sediments can in the long run become contaminated.
Groundwater for domestic and irrigation purposes is produced primarily from shallow parts of the Bengal Basin aquifer system (India and Bangladesh), which contains high concentrations of dissolved arsenic (exceeding worldwide drinking water standards), though deeper groundwater is generally low in arsenic. An essential first step for determining sustainable management of the deep groundwater resource is identification of hydrogeologic controls on flow and quantification of basin-scale groundwater flow patterns. Results from groundwater modeling, in which the Bengal Basin aquifer system is represented as a single aquifer with higher horizontal than vertical hydraulic conductivity, indicate that this anisotropy is the primary hydrogeologic control on the natural flowpath lengths. Despite extremely low hydraulic gradients due to minimal topographic relief, anisotropy implies large-scale (tens to hundreds of kilometers) flow at depth. Other hydrogeologic factors, including lateral and vertical changes in hydraulic conductivity, have minor effects on overall flow patterns. However, because natural hydraulic gradients are low, the impact of pumping on groundwater flow is overwhelming; modeling indicates that pumping has substantially changed the shallow groundwater budget and flowpaths from predevelopment conditions.
Aquifer geochemistry was characterized at a field site in the Munshiganj district of Bangladesh where the groundwater is severely contaminated by As. Vertical profiles of aqueous and solid phase parameters were measured in a sandy deep aquifer (depth >150 m) below a thick confining clay (119 to 150 m), a sandy upper aquifer (3.5 to 119 m) above this confining layer, and a surficial clay layer (<3.5 m). In the deep aquifer and near the top of the upper aquifer, aqueous As levels are low (<10 μg/L), but aqueous As approaches a maximum of 640 μg/L at a depth of 30 to 40 m and falls to 58 μg/L near the base (107 m) of the upper aquifer. In contrast, solid phase As concentrations are uniformly low, rarely exceeding 2 μg/g in the two sandy aquifers and never exceeding 10 μg/g in the clay layers. Solid phase As is also similarly distributed among a variety of reservoirs in the deep and upper aquifer, including adsorbed As, As coprecipitated in solids leachable by mild acids and reductants, and As incorporated in silicates and other more recalcitrant phases. One notable difference among depths is that sorbed As loads, considered with respect to solid phase Fe extractable with 1 N HCl, 0.2 M oxalic acid, and a 0.5 M Ti(III)-citrate-EDTA solution, appear to be at capacity at depths where aqueous As is highest; this suggests that sorption limitations may, in part, explain the aqueous As depth profile at this site. Competition for sorption sites by silicate, phosphate, and carbonate oxyanions appear to sustain elevated aqueous As levels in the upper aquifer. Furthermore, geochemical profiles are consistent with the hypothesis that past or ongoing reductive dissolution of Fe(III) oxyhydroxides acts synergistically with competitive sorption to maintain elevated dissolved As levels in the upper aquifer. Microprobe data indicate substantial spatial comapping between As and Fe in both the upper and deep aquifer sediments, and microscopic observations reveal ubiquitous Fe coatings on most solid phases, including quartz, feldspars, and aluminosilicates. Extraction results and XRD analysis of density/magnetic separates suggest that these coatings may comprise predominantly Fe(II) and mixed valence Fe solids, although the presence of Fe(III) oxyhydroxides can not be ruled out. These data suggest As release may continue to be linked to dissolution processes targeting Fe, or Fe-rich, phases in these aquifers.
Although arsenic contaminated groundwater in Bangladesh is a serious health issue, little is known about the complex transient patterns of groundwater flow that flush solutes from aquifers and carry solutes into the subsurface. Hydrologic modeling results for our field site in the Munshiganj district indicate that groundwater flow is vigorous, flushing the aquifer over time-scales of decades to a century, and also transporting solute loads into the aquifer with recharge from ponds, rivers and rice fields. The combined hydrologic and biogeochemical results from our field site imply that the biogeochemistry of the aquifer system may not be in steady-state, and that the net effect of competing processes could either increase or decrease arsenic concentrations over the next decades. Modeling results suggest that irrigation has greatly changed the location, timing and chemical content of recharge to the aquifer, flushing water through the system more quickly, and also cycling large fluxes of water through rice fields during the dry season that could mobilize arsenic from oxides in near-surface sediments. Furthermore, the hydrologic model reveals that ponds, many of which have been excavated over the last 50 years, now provide much of the groundwater recharge. These ponds receive most of the waste from the villages and thus provide another potential source of organic carbon to the groundwater system.
The contamination of groundwater by arsenic in Bangladesh is the largest poisoning of a population in history, with millions of people exposed. This paper describes the history of the discovery of arsenic in drinking-water in Bangladesh and recommends intervention strategies. Tube-wells were installed to provide "pure water" to prevent morbidity and mortality from gastrointestinal disease. The water from the millions of tube-wells that were installed was not tested for arsenic contamination. Studies in other countries where the population has had long-term exposure to arsenic in groundwater indicate that 1 in 10 people who drink water containing 500 mu g of arsenic per litre may ultimately die from cancers caused by arsenic, including lung, bladder and skin cancers. The rapid allocation of funding and prompt expansion of current interventions to address this contamination should be facilitated. The fundamental intervention is the identification and provision of arsenic-free drinking water. Arsenic is rapidly excreted in urine, and for early or mild cases, no specific treatment is required. Community education and participation are essential to ensure that interventions are successful; these should be coupled with follow-up monitoring to confirm that exposure has ended. Taken together with the discovery of arsenic in groundwater in other countries, the experience in Bangladesh shows that groundwater sources throughout the world that are used for drinking-water should be tested for arsenic.
The time-averaged 36Cl fallout transport rate velocity is 23 ± 17 mm yr-1. The Cl concentration profiles indicate that, prior to about 15 000 yr ago, soil-water fluxes were approximately 20 times the magnitude of modern fluxes, and that the soil water shifted rapidly (on a geological time scale) to the dry regime between 16 000 and 13 000 yr ago. The remarkable uniformity of the water and solute transport processes across the region may be dictated by the water extraction efficiency of the vegetation. -from Author
Natural pollution of groundwater by arsenic adversely affects the health of tens of millions of people worldwide, with the deltaic aquifers of SE Asia being particularly polluted. The pollution is caused primarily by, or as a side reaction of, the microbial reduction of sedimentary Fe(III)-oxyhydroxides, but the organism(s) responsible for As release have not been isolated. Here we report the first isolation of a dissimilatory arsenate reducer from sediments of the Bengal Basin in West Bengal. The bacterium, here designated WB3, respires soluble arsenate and couples its reduction to the oxidation of acetate; WB3 is therefore implicated in the process of arsenic pollution of groundwater, which is largely by arsenite. The bacterium WB3 is also capable of reducing dissolved Fe(III) citrate, solid Fe(III)-oxyhydroxide, and elemental sulphur, using acetate as the electron donor. It is a member of the Desulfuromonas genus and possesses a dissimilatory arsenate reductase that was identified using degenerate polymerase chain reaction primers. The sediment from which WB3 was isolated was brown, Pleistocene sands at a depth of 35.2 metres below ground level (mbgl). This level was some 3 cm below the boundary between the brown sands and overlying reduced, grey, Holocene aquifer sands. The colour boundary is interpreted to be a reduction front that releases As for re-sorption downflow, yielding a high load of labile As sorbed to the sediment at a depth of 35.8 mbgl and concentrations of As in groundwater that reach >1000 µg/L.
Arsenic in groundwater above the WHO maximum permissible limit of 0.05 mg l(-1) has been found in six districts of West Bengal covering an area of 34 000 km(2) with a population of 30 million. At present, 37 administrative blocks by the side of the River Ganga and adjoining areas are affected. Areas affected by arsenic contamination in groundwater are all located in the upper delta plain, and are mostly in the abandoned meander belt. More than 800 000 people from 312 villages/wards are drinking arsenic contaminated water and amongst them at least 175 000 people show arsenical skin lesions. Thousands of tube-well water in these six districts have been analysed for arsenic species. Hair, nails, scales, urine, liver tissue analyses show elevated concentrations of arsenic in people drinking arsenic-contaminated water for a longer period. The source of the arsenic is geological. Bore-hole sediment analyses show high arsenic concentrations in only few soil layers which is found to be associated with iron-pyrites. Various social problems arise due to arsenical skin lesions in these districts. Malnutrition, poor socio-economic conditions, illiteracy, food habits and intake of arsenic-contaminated water for many years have aggravated the arsenic toxicity. In all these districts, major water demands are met from groundwater and the geochemical reaction, caused by high withdrawal of water may be the cause of arsenic leaching from the source. If alternative water resources are not utilised, a good percentage of the 30 million people of these six districts may suffer from arsenic toxicity in the near future.
High arsenic (As) groundwater is a global issue and notably in south-east Asia. Asystematic screening (sampling) operation has been conducted for hydrogeochemical investigation of groundwaters in rural parts of West Bengal, India. The groundwaters are dominantly Ca-HCO 3--type usually occurring in the study area; however, NaCl-enriched samples are also observed. Statistical factor analysis has been employed to delineate the interaction among the analyzed hydrochemical elements and their role to mobilize arsenic from sediments to the groundwater. The study reveals that besides oxihydroxides, Fe-bearing aluminosilicates could also influence As release to groundwater.
Dissolved organic carbon (DOC) and Fe mineral phases are known to influence the mobility of arsenic (As) in groundwater. Arsenic can be associated with colloidal particles containing organic matter and Fe. Currently, no data is available on the dissolved phase/colloidal association of As in groundwater of alluvial aquifers in West Bengal, India. This study investigated the fractional distribution of As (and other metals/metalloids) among the particulate, colloidal and dissolved phases in groundwater to decipher controlling behavior of organic and inorganic colloids on As mobility. The result shows that 83-94% of As remained in the 'truly dissolved' phases (i.e., <0.05μm size). Strong positive correlation between Fe and As (r(2) between 0.65 and 0.94) is mainly observed in the larger (i.e., >0.05μm size) colloidal particles, which indicates the close association of As with larger Fe-rich inorganic colloids. In smaller (i.e., <0.05μm size) colloidal particles strong positive correlation is observed between As and DOC (r(2)=0.85), which highlights the close association of As with smaller organic colloids. As(III) is mainly associated with larger inorganic colloids, whereas, As(V) is associated with smaller organic/organometallic colloids. Scanning Electron Microscopy and Energy Dispersive X-ray spectroscopy confirm the association of As with DOC and Fe mineral phases suggesting the formation of dissolved organo-Fe complexes and colloidal organo-Fe oxide phases. Attenuated total reflectance-Fourier transform infrared spectroscopy further confirms the formation of As-Fe-NOM organometallic colloids, however, a detailed study of these types of colloids in natural waters is necessary to underpin their controlling behavior.
Environment isotopes (δ 18O and δ 2H) and Cl/Br ratios in groundwater have been used to trace groundwater recharge and geochemical processes for arsenic contamination in Datong Basin. The arsenic concentrations of groundwater samples ranged from 0.4 to 434.9μg/L with the average of 51.2μg/L, which exceeded China's drinking water standard (10μg/L). All the groundwater samples are plotted on or close to the meteoric water line of the δ 18O vs. δ 2H plot, indicating their meteoric origin. The relationship between δ 18O values and Cl/Br ratios and Cl concentrations demonstrate that leaching and mixing are the dominant processes affecting the distribution of high arsenic groundwater in this area. The observed non-linearity in the trend between δ 18O and arsenic concentration is due to combined effects of mixing and leaching. The similarity of the trend in Cl/Br ratios and δ 18O values for high arsenic groundwater demonstrate that extensive leaching of irrigation return and salt flushing water flow could be the dominant process driving arsenic mobilization in the groundwater system. Moreover, the long term irrigation practice can cause the drastic change of the biogeochemical and redox condition of in the aquifer system, which in turn promotes the mobilization of arsenic. Therefore, groundwater pumping for irrigation in this area of waterborne endemic arsenic poisoning should be under strict control to protect groundwater quality in this area.
A detailed review was made of chemical indicators used to identify impacts from septic tanks on groundwater quality. Potential impacts from septic tank leachate on groundwater quality were assessed using the mass ratio of chloride–bromide (Cl/Br), concentrations of selected chemical constituents, and ancillary information (land use, census data, well depth, soil characteristics) for wells in principal aquifers of the United States. Chemical data were evaluated from 1848 domestic wells in 19 aquifers, 121 public-supply wells in 6 aquifers, and associated monitoring wells in four aquifers and their overlying hydrogeologic units. Based on previously reported Cl/Br ratios, statistical comparisons between targeted wells (where Cl/Br ratios range from 400 to 1100 and Cl concentrations range from 20 to 100 mg/L) and non-targeted wells indicated that shallow targeted monitoring and domestic wells (<20 m depth below land surface) had a significantly (p < 0.05) higher median percentage of houses with septic tanks (1990 census data) than non-targeted wells. Higher (p = 0.08) median nitrate–N concentration (3.1 mg/L) in oxic (dissolved oxygen concentrations >0.5 mg/L) shallow groundwater from target domestic wells, relative to non-target wells (1.5 mg/L), corresponded to significantly higher potassium, boron, chloride, dissolved organic carbon, and sulfate concentrations, which may also indicate the influence of septic-tank effluent. Impacts on groundwater quality from septic systems were most evident for the Eastern Glacial Deposits aquifer and the Northern High Plains aquifer that were associated with the number of housing units using septic tanks, high permeability of overlying sediments, mostly oxic conditions, and shallow wells. Overall, little or no influence from septic systems were found for water samples from the deeper public-supply wells.
The paper presents a new control method based on Neural Network technique to damp out the power system low frequency oscillations using STATCOM controller. The main objective of this paper is to investigate the power system dynamic stability enhancement by using neural network based FACTS (STATCOM) Controller. This study includes Proportional feedback, lead-lag, Radial basis function Neural Network controller. The effectiveness of the new controller is demonstrated through time domain simulation studies. The results of these studies show that the designed controller has an excellent capability in damping power system oscillations.
HUMIC substances are heterogeneous high-molecular-weight organic
materials which are ubiquitous in terrestrial and aquatic environments.
They are resistant to microbial degradation1 and thus are not
generally considered to be dynamically involved in microbial metabolism,
especially in anoxic habitats. However, we show here that some
microorganisms found in soils and sediments are able to use humic
substances as an electron acceptor for the anaerobic oxidation of
organic compounds and hydrogen. This electron transport yields energy to
support growth. Microbial humic reduction also enhances the capacity for
microorganisms to reduce other, less accessible electron acceptors, such
as insoluble Fe(III) oxides, because humic substances can shuttle
electrons between the humic-reducing microorganisms and the Fe(III)
oxide. The finding that microorganisms can donate electrons to humic
acids has important implications for the mechanisms by which
microorganisms oxidize both natural and contaminant organics in
anaerobic soils and sediments, and suggests a biological source of
electrons for humics-mediated reduction of contaminant metals and
organics.
Possible relationships between groundwater arsenic concentration and alluvial sediment characteristics in a ~19 km2 area in West Bengal have been investigated using a combination of hydrogeochemical, lithogeochemical and geophysical techniques. Arsenic hotspots, typically associated with elevated groundwater Fe and Mn, were found to be correlated to some extent with old river channels (abandoned meanders, oxbow lakes), where sandy aquifers included intercalated fine-grained overbank deposits, rich in As, Fe, Mn and Corg. Otherwise no demonstrably significant overall differences in any of lithology, grain-size distribution, mineral composition or Fe, Mn and organic C content of the sediments were found between two representative sites with contrastingly low (<50 μg l−1) and high (>200 μg l−1) As groundwater contents.
Our results are consistent with microbially mediated redox reactions controlled by the presence of natural organic matter within the aquifer and the occurrence of As-bearing redox traps, primarily formed by Fe and Mn oxides/hydroxides, being the most important factors which control the release of As into shallow groundwaters at the study site.
Microbially mediated reductive processes involving the oxidation of labile organic carbon are widely considered to be critical to the release of arsenic into shallow groundwaters in South and Southeast Asia. In areas where there is significant pumping of groundwater for irrigation the involvement of surface derived organic carbon drawn down from ponds into the underlying aquifers has been proposed but remains highly controversial. Here we present isotopic data from two sites with contrasting groundwater pumping histories that unequivocally demonstrate the ingress of surface pond-derived organic carbon into arsenic containing groundwaters. We show that pond-derived organic carbon is transported to depths of up to 50 m even in an arsenic contaminated aquifer in Cambodia thought to be minimally disturbed by groundwater pumping. In contrast, in the extensively exploited groundwaters of West Bengal, we show that pond-derived organic carbon is transported in shallow groundwater to greater depths, in excess of 100 m in the aquifer. Intensive pumping of groundwaters may potentially drive secular increases in the groundwater arsenic hazard in this region by increasing the contribution of bioavailable pond-derived dissolved organic carbon drawn into these aquifer systems and transporting it to greater depths than would operate under natural flow conditions.
Late Quaternary sediments of the Bengal basin contain a history of river switching and climate change as revealed from sand- and clay-size mineralogy of boreholes and modern riverbed grabs. Epidote to garnet ratios (E/G) in sand fraction sediments are diagnostic of source, with high (>1) E/G indicating Brahmaputra provenance and low (<1) E/G indicating Ganges provenance. In the clay fraction of surficial sediments smectite is diagnostic, with high values (f39%) in the Ganges and low values (f3%) in the Brahmaputra. In contrast, the Brahmaputra contains more kaolinite (29% vs. 18%), more illite (63% vs. 41%), and more chlorite (3% vs. 1%) than the Ganges. Analysis of mineralogic and stratigraphic data indicates that the two rivers have changed position several times during the Holocene. Extended periods of mixed river inputs appear to be isolated to the Early Holocene, suggesting rapidly migrating braided channels during sea level lowstand. Tectonically driven accommodation in the Sylhet Basin may have contributed to the favored easterly course of the Brahmaputra during much of the Holocene. Relative abundances of illite and chlorite (IC) vs. smectite and kaolinite (SK) record varying degrees of physical and chemical weathering, respectively. High IC values in early post-glacial deposits suggest a dominance of physical weathering at that time. However, a general increase in SK concentrations throughout the Holocene appears to reflect enhanced chemical weathering under increasingly warmer and more humid conditions. Notably, a peak in SK concentrations that corresponds to an Early Holocene warm period (f10 – 7 ka) suggests that weathering patterns in the catchment respond quickly to climatic shifts.
Here we report temporal changes of As concentrations in shallow groundwater of the Bengal Delta Plain (BDP). Observed fluctuations are primarily induced by seasonally occurring groundwater movement, but can also be connected to anthropogenic groundwater extraction. Between December 2009 and July 2010, pronounced variations in the groundwater hydrochemistry were recorded in groundwater samples of a shallow monitoring well tapping the aquifer in 22-25m depth, where As(tot) concentrations increased within weeks from 100 to 315μgL(-1). These trends are attributed to a vertically shift of the hydrochemically stratified water column at the beginning of the monsoon season. This naturally occurring effect can be additionally superimposed by groundwater extraction, as demonstrated on a local scale by an in situ experiment simulating extensive groundwater withdrawal during the dry post-monsoon season. Results of this experiment suggest that groundwater extraction promoted an enduring change within the distribution of dissolved As in the local aquifer. Presented outcomes contribute to the discussion of anthropogenic pumping influences that endanger the limited and yet arsenic-free groundwater resources of the BDP.
Arsenic contamination in groundwater used for drinking purposes has been envisaged as a problem of global concern. Exploitation of groundwater contaminated with arsenic within the delta plains in West Bengal has caused adverse health effects among the population within a span of 8-10 years. The sources of arsenic in natural water are a function of the local geology, hydrology and geochemical characteristics of the aquifers. The retention and mobility of different arsenic species are sensitive to varying redox conditions. The delta plains in West Bengal are characterized by a series of meander belts formed by the fluvial processes comprising different cycles of complete or truncated fining upward sequences (sand-silt-clay). The arseniferous groundwater belts are mainly located in the upper delta plain and in abandoned meander channels. Mineralogical investigations have established that arsenic in the silty clay as well as in the sandy layers occurs as coatings on mineral grains. Clayey sediments intercalated with sandy aquifers at depths between 20 and 80 m are reported as a major source of arsenic in groundwater.Integrated knowledge on geological, hydrologicaland geochemical characteristics of the multi-level aquifer system of the upper delta plain is therefore necessary in predicting the origin, occurrence and mobility of arsenic in groundwater in West Bengal. This would also provide a basis for developing suitable low-cost techniques for safe drinking water supply in the region.
Across West Bengal and Bangladesh, concentrations of Cl in much groundwater exceed the natural, upper limit of 10mg/L. The Cl/Br mass ratios in groundwaters range up to 2500 and scatter along mixing lines between waste-water and dilute groundwater, with many falling near the mean end-member value for waste-water of 1561 at 126mg/L Cl. Values of Cl/Br exceed the seawater ratio of 288 in uncommon NO(3)-bearing groundwaters, and in those containing measurable amounts of salt-corrected SO(4) (SO(4) corrected for marine salt). The data show that shallow groundwater tapped by tube-wells in the Bengal Basin has been widely contaminated by waste-water derived from pit latrines, septic tanks, and other methods of sanitary disposal, although reducing conditions in the aquifers have removed most evidence of NO(3) additions from these sources, and much evidence of their additions of SO(4). In groundwaters from wells in palaeo-channel settings, end-member modelling shows that >25% of wells yield water that comprises ≥10% of waste-water. In palaeo-interfluvial settings, only wells at the margins of the palaeo-interfluvial sequence contain detectable waste water. Settings are identifiable by well-colour survey, owner information, water composition, and drilling. Values of Cl/Br and faecal coliform counts are both inversely related to concentrations of pollutant As in groundwater, suggesting that waste-water contributions to groundwater in the near-field of septic-tanks and pit-latrines (within 30m) suppress the mechanism of As-pollution and lessen the prevalence and severity of As pollution. In the far-field of such sources, organic matter in waste-water may increase groundwater pollution by As.
The hydrology of oxygen-18 (18O) isotopes was monitored between 1995 and 1998 in the Allt a' Mharcaidh catchment in the Cairngorm Mountains, Scotland. Precipitation (mean δ18O=−7·69‰) exhibited strong seasonal variation in δ18O values over the study period, ranging from −2·47‰ in the summer to −20·93‰ in the winter months. As expected, such variation was substantially damped in stream waters, which had a mean and range of δ18O of −9·56‰ and −8·45 to −10·44‰, respectively. Despite this, oxygen-18 proved a useful tracer and streamwater δ18O variations could be explained in terms of a two-component mixing model, involving a seasonally variable δ18O signature in storm runoff, mixing with groundwater characterized by relatively stable δ18O levels. Variations in soil water δ18O implied the routing of depleted spring snowmelt and enriched summer rainfall into streamwaters, probably by near-surface hydrological pathways in peaty soils. The relatively stable isotope composition of baseflows is consistent with effective mixing processes in shallow aquifers at the catchment scale. Examination of the seasonal variation in δ18O levels in various catchment waters provided a first approximation of mean residence times in the major hydrological stores. Preliminary estimates are 0·2–0·8 years for near-surface soil water that contributes to storm runoff and 2 and >5 years for shallow and deeper groundwater, respectively. These 18O data sets provide further evidence that the influence of groundwater on the hydrology and hydrochemistry of upland catchments has been underestimated. Copyright © 2000 John Wiley & Sons, Ltd.
In the present study, the trends in the reference evapotranspiration (ETO) estimated through the Penman-Monteith method were investigated over the humid region of northeast (NE) India by using the Mann-Kendall (MK) test after removing the effect of significant lag-1 serial correlation from the time series of ETO by pre-whitening. During the last 22 years, ETO has been found to decrease significantly at annual and seasonal time scales for 6 sites in NE India and NE India as a whole. The seasonal decreases in ETO have, however, been more significant in the pre-monsoon season, indicating the presence of an element of a seasonal cycle. The decreases in ETO are mainly attributed to the net radiation and wind speed, which are also corroborated by the observed trends in these two parameters at almost all the times scales over most of the sites in NE India. The steady decrease in wind speed and decline in net radiation not only balanced the impact of the temperature increases on ETO, but may have actually caused the decreases in ETO over the humid region of northeast India. Copyright © 2011 John Wiley & Sons, Ltd.
This study aims at finding out possible relation between lithology and spatial pattern of dissolved arsenic (As) in groundwater
around Chakdaha municipality, West Bengal, India. Satellite image, coupled with electrical resistivity survey and borehole
drilling helps to delineate surface and sub-surface lithological framework of the As affected alluvial aquifers. The satellite
imagery demonstrate that the high As area are presumably under active flood plain environment (low-lying areas), that constantly
receive organics due to periodic flooding. Thick low resistive (fine-grained) layer was observed at the top around the high
As areas, which, however, not found in low As areas. The result suggests that hydraulic properties of the surface/sub-surface
soil/sediment have an important control on the fate and transport of As in the aquifer. This study demonstrates that electrical
resistivity tools can be effectively used for the reconnaissance survey in characterizing the plausible lithological framework
of an alluvial aquifer containing As.
KeywordsBengal Delta Plain-Groundwater-Arsenic-Geomorphology-Electrical resistivity
The recharge and origin of groundwater and its residence time were studied using environmental isotopic measurements in samples from the Heihe River Basin, China. δ18O and δD values of both river water and groundwater were within the same ranges as those found in the alluvial fan zone, and lay slightly above the local meteoric water line (δD=6.87δ18O+3.54). This finding indicated that mountain rivers substantially and rapidly contribute to the water resources in the southern and northern sub-basins. δ18O and δD values of groundwater in the unconfined aquifers of these sub-basins were close to each other. There was evidence of enrichment of heavy isotopes in groundwater due to evaporation. The most pronounced increase in the δ18O value occurred in agricultural areas, reflecting the admixture of irrigation return flow. Tritium results in groundwater samples from the unconfined aquifers gave evidence for ongoing recharge, with mean residence times of: less than 36 years in the alluvial fan zone; about 12-16 years in agricultural areas; and about 26 years in the Ejina oasis. In contrast, groundwater in the confined aquifers had 14C ages between 0 and 10 ka BP.
The present study aims to understand the hydrochemistry vis-à-vis As-exposure from drinking groundwater in rural Bengal. The
characteristic feature of the groundwaters are low Eh (range, −151 to −37mV; mean, −68mV) and nitrate (range, 0.01–1.7mg/l;
mean, 0.14mg/l) followed by high alkalinity (range, 100–630mg/l; mean, 301mg/l), Fe (range, 0.99–38mg/l; mean, 8.1mg/l),
phosphate (range, 0.01–15mg/l; mean, 0.54mg/l), hardness (range, 46–600mg/l; mean, 245mg/l) and sulphate (range, 0.19–88mg/l;
mean, 7.2mg/l), indicating reducing nature of the aquifer. The land use pattern (sanitation, surface water bodies, sanitation
coupled with surface water bodies and agricultural lands) demonstrates local enrichment factor for As/Fe in groundwater. Among
these, sanitation is the most prevailing where groundwater is generally enriched with As (mean, 269μg/l) and Fe (mean, 9.8mg/l).
Questionnaire survey highlights that ∼70% of the villagers in the study area do not have proper sanitation. This demonstrating
the local unsewered sanitation (organic waste, anthropogenic in origin) could also cause As toxicity in rural Bengal. In the
agricultural lands, higher mean values of alkalinity, phosphate, sulphate, hardness and electrical conductivity was observed,
and could be linked with the excessive use of fertilizers for agricultural production. Bio-markers study indicates that the
accumulation of As in hair and nail is related with the construction of exposure scenario with time dimension. The strength
and weakness of the on-going West Bengal and Bangladesh drinking water supply scenario and achievability towards alternative
options are also evaluated.
Extremely high As concentrations in drinking water of the Ganges Delta (West Bengal and Bangladesh) has emerged as an issue of great concern in the past decade because of its serious impact on the health of millions of people. The distribution pattern of As concentrations in the Ganges Delta region is patchy and there are numerous As “hotspots”. The present study is perhaps the first attempt in West Bengal to characterize such a hotspot by geophysical and geochemical methods, and to model the transport of the enrichment plume using a 1D reactive transport model (PHREEQC). The study site is located along the Hooghly River, 60 km north of Kolkota City, near the city of Chakdaha. Total As concentrations in the groundwater range from 0.5 to more than 6 μmol L−1; the WHO recommended maximum drinking water concentration is 0.13 μmol L−1 (i.e. 10 μg L−1). Results show groundwater is in chemical equilibrium with siderite and calcite, a mineral phase previously shown to be an efficient trap for As(III). Groundwater redox potential is controlled by the Fe(OH)3(am)/Fe2+ couple. The As(III) versus As(V) distribution (42% As(III) and 58% As(V), on average) is not at equilibrium with measured Eh values. No evidence of sulfide solid phases, such as As rich pyrite or arsenopyrite, was found. Although amorphous Fe dissolution is confirmed to play an important role in the release of As, selective dissolution extractions indicate that adsorption of As on carbonates and micas may also be an important component of As cycling in the sediment. Modelling results demonstrate the role of and Fe(II) in mobilizing the As plume, thereby increasing the threat to the 75,000 inhabitants of Chakdaha.
A hydrochemical investigation was conducted in the Minqin Basin to identify the groundwater evolution and recharge in the aquifer. The mBr/Cl ratio is strongly depleted (average 0.000451) compared with sea water (0.0035), indicating an evaporite origin. The ionic ration plot, saturation index (SI), and chloro alkaline indices (CAI) suggest that the dissolution of halite, the glauberite, gypsum, dolomite and calcite determine Na+, Cl−, Ca2+, Mg2+, , and chemistry, but other processes, such as Na+ exchange for Ca2+ and Mg2+, and calcite precipitation also contribute to the water composition. The δ18O and δ2H in precipitation near the study area are linearly correlated, similar to that for the world meteoric water line (WMWL), with an equation of δ2H = 7.49δ18O + 5.11 (r2 = 0.97). According to radiocarbon residence time estimates, the deep groundwater is approximately 40 ka old, and was recharged during a period when the climate was wetter and colder. The radiocarbon content of shallow groundwater shows a clear evolution along the groundwater flow path. From the beginning of the groundwater flow path to ∼31 km the radiocarbon values are >73.6 pmc, whereas beyond this point the values are <42.9 pmc. Based on radiocarbon content, the shallow groundwater is older than 1 ka, and represents palaeowaters mixed with a limited quality of modern recharge.The rain-fed groundwater direct recharge was estimated by chloride mass balance (CMB) method to range from 1.55 to 1.64 mm yr−1, with a mean value of 1.6 mm yr−1. This value represents about 1.5% of local rainfall. The direct recharge volumes is about 0.666 × 108 m3 yr−1. Indirect recharge volumes by the surface water is about 0.945 × 108 m3 yr−1. The total natural recharge in the Minqin Basin is 1.6 × 108 m3 yr−1, whereas the groundwater abstraction has reached 11.6 × 108 m3 yr−1, far exceeding the groundwater natural recharge.
High As groundwater (50–1600 μg l−1) poses the greatest threat to human health in the Holocene alluvial aquifers of the Bengal Delta Plain (BDP) with increasing global concern in recent years. This study deals with groundwater quality and As mobilization vis-à-vis employing ferralite as a remedial option for removal of As from groundwater. The investigation suggests that Fe-rich As traps undergo degeneration to produce Fe oxyhydroxide (HFO) as coating/precipitation on the fine-grained sediment surface and release redox sensitive species (As, Fe and Mn) as well as PO43− into the groundwater under local reducing conditions. Sediment analysis reveals the presence of AsT (average 17.2 mg/kg), FeT (average 0.93 g/kg) and organic matter (average 7.6 g/kg). Sediment AsT and FeT content cannot validate the presence of high groundwater As/Fe. FeII catalysed FeIII reduction, induced by dissimilatory Fe reducing bacteria liberate the more toxic AsIII than AsV. The release of redox sensitive species (As, Fe and Mn) are the functions of bio-available forms of Fe oxides, concentration and distribution of fresh organic matter and availability of electron donors within the sediment. Further attempt is made to establish the role of ferralite, enriched with natural HFO as an As scavenger. Batch studies demonstrate the competency of the material over the natural/commonly used chemical coagulants generally used for water treatment. The high pHpzc value, 8.5 of ferralite along with the adsorption studies over a wide range of pH elucidate the effectiveness of the material in adsorbing both AsIII and AsV from the well-buffered groundwater. The presence of FeII in the system enhances the As removal process. The Langmuir adsorption isotherm further confirms the merit of ferralite as an efficient As scavenger. The material has been shaped for a fixed bed filter medium to remove As from groundwater (both laboratory and field scale). Ferralite is also cost effective (US 16/ton/1000 km whereas US$ 0.6/100 l is required for treatment of contaminated water.
Arsenic in the groundwater of Bangladesh is a serious natural calamity and a public health hazard. Most groundwater from the shallow alluvial aquifers (<150 m), particularly in the Holocene plain lands, are vulnerable to As-enrichment. Delta plains and flood plains of the Ganges–Brahmaputra river system are moderately to severely enriched and more than 60% of the tube wells are affected. Shallow aquifers in the Meghna river basin and coastal plains are extremely enriched with more than 80% of the tube wells affected. Aquifers in the Pleistocene uplands and Tertiary hills are low in As. The vertical lithofacies sequence of the sediments from highly enriched areas of the country show two distinct lithofacies associations—a dominantly sandy channel-fill association and a fine-grained over bank association. The sediments can be grouped into 4 distinct lithofacies, viz. clay, silty clay, silty sand and sand. Thin section petrography of the As-enriched aquifer sands shows that the sands are of quartzolithic type and derived from the collision suture and fold thrust belt of the recycled orogen provenance. Groundwater is characterized by circum-neutral pH with a moderate to strong reducing nature. The waters are generally of Ca–Mg–HCO3 or Ca–Na–HCO3 type, with HCO3− as the principal anion. Low SO42− and NO3−, and high dissolved organic C (DOC) and NH4+ concentrations are typical chemical characteristics of groundwater. The presence of dissolved sulfides in these groundwaters indicates reduction of SO4. Total As concentration in the analyzed wells vary between 2.5 and 846 μg l−1 with a dominance of As(III) species (67–99%). Arsenic(III) concentrations were fairly consistent with the DOC and NH4+ contents. The HNO3 extractable concentrations of As in the sediments (0.5–17.7 mg kg−1), indicate a significant positive correlation with FeNO3, MnNO3, AlNO3 and PNO3. The concentrations of SNO3 (816–1306 mg kg−1) peaked in the clay sediments with high organic matter (up to 4.5 wt.%). Amounts of oxalate extractable As (Asox) and Fe (Feox) ranged between 0.1–8.6 mg kg−1and 0.4–5.9 g kg−1, respectively. Arsenicox was positively correlated with Feox, Mnox and Alox in these sediments. Insignificant amounts of opaque minerals (including pyrite/arsenopyrite) and the presence of high As contents in finer sediments suggests that some As is incorporated in the authigenically precipitated sulfides in the reducing sediments. Moreover, the chemical extractions suggest the presence of siderite and vivianite as solid phases, which may control the aqueous chemistry of Fe and PO43−. Reductive dissolution of Fe oxyhydroxide present as coatings on sand grains as well as altered mica (biotite) is envisaged as the main mechanism for the release of As into groundwater in the sandy aquifer sediments.
Samples of deep groundwater, river water and rainwater were collected for δ18O and δ2H analyses from an area of ∼22,000 km2 in the arsenic-affected districts of West Bengal, India, in order to examine groundwater recharge. A plot of δ18O versus δ2H of groundwater falls subparallel to the constructed local meteoric water line (δ2H = 7.2 δ18O + 7.7), suggesting a predominance of meteoric recharge with some evaporation. The stable isotopic signature of groundwater from the deeper part of the semiconfined main aquifer is similar to that of shallower groundwater, which suggests that deeper groundwater has been recharged in the present-day climatic regime, mostly from monsoonal rainfall. Groundwater in deeper isolated aquifers falls within the isotopic range observed for the main aquifer. A trend of isotopic depletion of groundwater that extends northward and westward from the Bay of Bengal (the monsoonal moisture source area) indicates a rain-out process following Rayleigh-type distillation. Some recharge may have taken place from the rivers Ganges and Bhagirathi-Hoogly, with minimal inflow from other rivers (Jalangi and Ichamati).
Contamination of the environment with arsenic (As) from both anthropogenic and natural sources has occurred in many parts of the world and is recognized as a global problem. Principal anthropogenic sources of As include base metal smelters, gold mines, power plants that burn As-rich coals or treated lumber, disposal sites for wastes from As-processing plants, as well as industrial and municipal dump sites. In many areas, the levels of As in the environment have become one of concern and epidemiological studies have documented various adverse health effects on local populations. Arsenic poisoning episodes from exposure to industrial sources have been reported all over the world; for instance, in Japan, where cases have been associated with pollution around As mines and pollution of groundwater around As-using industries and industrial waste burial sites. Other examples of contaminated environments with increased risk for As poisoning include agricultural lands treated with arsenical pesticides, urban areas, war zones defoliated or sprayed with As compounds, and the superfund sites in the United States and other countries. Although a lot of people get exposed, most often, however, it is not possible to associate the exposure to elevated As levels with adverse human health effects. Nevertheless, long-term cumulative exposure to As in these contaminated environments should be a matter of public health concern and scientific interest.
Sediment cores from the Ganges–Brahmaputra delta in Bangladesh were examined for sedimentological character, clay mineralogy, elemental trends (C, N, S), and 14C geochronology to develop a model for the sedimentary sequence resulting from lower delta plain progradation in the late Holocene. A widespread facies succession from Muddy Sand to Interbedded Mud records progradation of shoal–island complexes and the transition from subtidal to intertidal conditions. Mangrove-vegetated islands and peninsulas represent the final phase of progradation; a Mottled Mud that is deposited by penetration of turbid coastal water into the mangroves during high water events. Organic matter preservation is generally low (<1% TOC) in most of these well-drained deposits that are characterized by a permeable, silt-dominated granulometry. Clay mineralogy in the cores records the relative influence of smectite and kaolinite-rich Ganges sediments and illite and chlorite-rich Brahmaputra material. The lower delta plain west of the modern river mouths was deposited as a Ganges-dominated delta in three phases since 5000 cal years BP, with Brahmaputra influence confined to the Meghna estuary area and to the supratidal section of western delta deposits. Evolution of the lower delta plain in the late Holocene was influenced by regional subsidence patterns in the tectonically active Bengal Basin, which controlled distributary channel avulsion and migration, and the creation of accommodation space.