[show abstract][hide abstract] ABSTRACT: Bacterial communities can exert significant influence on the biogeochemical cycling of arsenic (As). This has globally important implications since As in drinking water affects the health of over 100 million people worldwide, including in the Ganges-Brahmaputra Delta region of Bangladesh where geogenic arsenic in groundwater can reach concentrations of more than 10 times the World Health Organization's limit. Thus, the goal of this research was to investigate patterns in bacterial community composition across gradients in sediment texture and chemistry in an aquifer with elevated groundwater As concentrations in Araihazar, Bangladesh. We characterized the bacterial community by pyrosequencing 16S rRNA genes from aquifer sediment samples collected at three locations along a groundwater flow path at a range of depths between 1.5 and 15 m. We identified significant differences in bacterial community composition between locations in the aquifer. In addition, we found that bacterial community structure was significantly related to sediment grain size, and sediment carbon (C), manganese (Mn), and iron (Fe) concentrations. Deltaproteobacteria and Chloroflexi were found in higher proportions in silty sediments with higher concentrations of C, Fe, and Mn. By contrast, Alphaproteobacteria and Betaproteobacteria were in higher proportions in sandy sediments with lower concentrations of C and metals. Based on the phylogenetic affiliations of these taxa, these results may indicate a shift to more Fe-, Mn-, and humic substance-reducers in the high C and metal sediments. It is well-documented that C, Mn, and Fe may influence the mobility of groundwater arsenic, and it is intriguing that these constituents may also structure the bacterial community.
[show abstract][hide abstract] ABSTRACT: Drinking shallow groundwater with naturally elevated concentrations of arsenic is causing widespread disease in many parts of South and Southeast Asia. In the Bengal Basin, growing reliance on deep (>150 m) groundwater has lowered exposure. In the most affected districts of Bangladesh, shallow groundwater concentrations average 100 to 370 μg L(-1), while deep groundwater is typically < 10 μg L(-1). Groundwater flow simulations have suggested that, even when deep pumping is restricted to domestic use, deep groundwater in some areas of the Bengal Basin is at risk of contamination. However, these simulations have neglected the impedance of As migration by adsorption to aquifer sediments. Here we quantify for the first time As sorption on deeper sediments in situ by replicating the intrusion of shallow groundwater through injection of 1,000 L of deep groundwater modified with 200 μg L(-1) of As into a deeper aquifer. Arsenic concentrations in the injected water were reduced by 70% due to adsorption within a single day. Basin-scale modelling indicates that while As adsorption extends the sustainable use of deep groundwater, some areas remain vulnerable; these areas can be prioritized for management and monitoring.
[show abstract][hide abstract] ABSTRACT: Dissimilatory metal-reducing bacteria can mobilize As, but few studies have studied such processes in deeper orange-colored Pleistocene sands containing 1-2 mg kg(-1) As that are associated with low-As groundwater in Bangladesh. To address this gap, anaerobic incubations were conducted in replicate over 90 days using natural orange sands initially containing 0.14 mg kg(-1) of 1 M phosphate-extractable As (24 h), >99% as As(V), and 0.8 g kg(-1) of 1.2 M HCl-leachable Fe (1 h at 80 °C), 95% as Fe(III). The sediment was resuspended in artificial groundwater, with or without lactate as a labile carbon source, and inoculated with metal-reducing Shewanella sp. ANA-3. Within 23 days, dissolved As concentrations increased to 17 μg L(-1) with lactate, 97% as As(III), and 2 μg L(-1) without lactate. Phosphate-extractable As concentrations increased 4-fold to 0.6 mg kg(-1) in the same incubations, even without the addition of lactate. Dissolved As levels in controls without Shewanella, both with and without lactate, instead remained <1 μg L(-1). These observations indicate that metal-reducers such as Shewanella can trigger As release to groundwater by converting sedimentary As to a more mobilizable form without the addition of high levels of labile carbon. Such interactions need to be better understood to determine the vulnerability of low-As aquifers from which drinking water is increasingly drawn in Bangladesh.
[show abstract][hide abstract] ABSTRACT: Manually operated tube wells that tap into shallow aquifers remain a critical source of untreated drinking water in south Asia and an estimated 37 million people are still exposed to elevated levels of As in Bangladesh(1). This field effort sought to address two questions. What mechanisms control the partitioning of As between groundwater and sediment? How does groundwater transport affect the spatial variability of dissolved As? Understanding the source of groundwater variability is essential for understanding how [As] will change with time, especially as Bangladesh and its water demands develop. Arsenic mobility and transport within the shallow aquifer was investigated at a 0.5 km2 site where [As] increases from 50 mug/L in the village within the next few decades. The rapid economic development of Bangladesh could induce similar changes in groundwater flow, and thus As concentrations, elsewhere. This suggests that periodic monitoring of shallow wells low in As within regions of where the As content of groundwater is variable is particularly important. The size of the pool of As adsorbed on the sediment also indicates that current attempts to flush Bangladeshi aquifers ``clean'' through increased pumping will likely be ineffective. 1. BBS/UNICEF. Bangladesh: Multiple Indicator Cluster Survey, 2009. (Dhaka, Bangladesh, 2009).
[show abstract][hide abstract] ABSTRACT: Millions of people in Bangladesh and West Bengal, India are drinking groundwater from Bengal Basin aquifers with unsafe levels of arsenic. A promising mitigation option is development of deep groundwater in areas where water quality at depth is high. Deep groundwater is already in use in many areas, but proper management is necessary to maintain a safe water supply for this and future generations. Pumping at depth may induce downward migration of arsenic from shallow, contaminated aquifer zones in areas where physical (groundwater flowpaths) and chemical (arsenic sorption or reaction) protection does not occur or is insufficient. Understanding these protection mechanisms and conditions in which they are undermined is critical to determining the sustainability of mitigation options. We use a basin-scale groundwater flow and solute transport model considering advective flowpaths and dispersive solute transport with sorption to investigate the sustainability of deep groundwater pumping options within the high-arsenic area of the Bengal Basin. On a large scale, distributed pumping for both domestic and irrigation use is considered, including future scenarios in which domestic use increases due to population growth or piped supply. Regional modeling indicates that limited pumping at depth (current domestic supply and future supply under sorbing conditions) is critical to sustainability of the deep groundwater resource. However, basin-scale modeling does not consider effects of point pumping from community or municipal wells, as may be installed for this mitigation option, or local geologic conditions. Small-scale models are developed to consider these effects, and recommendations for the future are made.
[show abstract][hide abstract] ABSTRACT: Simply turning on the tap brings safe, clean, fresh-tasting water to most Americans. Students never need to consider basic concepts about water supply, including their daily water consumption and the quality of the water required for drinking. In stark contrast, the issues of water quality and quantity play a central role in people's daily lives in the developing world. It is difficult to convey this reality to our students through lectures alone and hands-on activities are required. In order to develop an active learning based approach, we transported a traditional cast iron hand-pump and aluminum urns from Bangladesh to the United States. The hand-pump is mounted on a cooler, which acts as a water reservoir, and is now functional and easily transportable. Using this powerful demonstration tool, we have developed an active learning module we call ``How far will you walk for water?''. The goal of the module is to teach students about water quantity, water quality, and resource allocation with a focus on Arsenic and Bangladesh, but the system could be applied to other areas of concern. First the students are given a quick lecture on Arsenic, its health impacts, and the extent of contamination in Bangladesh. They are then assigned a specific well, complete with a map of their village and picture of their well and a water sample (pre-spiked with arsenic to be above or below the 10 ug/L WHO limit). Next they pump the wellhead, fill an urn, walk down the hall and back, and measure the distance walked. This is compared to the distance from their village home to their private well, to safe wells belonging to neighbors and to a community well. The students then use the Hach Arsenic test kit to test the arsenic levels in their water samples and learn if their well is safe to drink. Finally, given all this information students must determine if they should continue drinking from their well or switch to a new well, even if that means making multiple, long trips each day. This module has introduced the students to important water resource concepts, such as water quality testing, usage and water delivery options. It also provides students the opportunity to consider how much time and effort to should be allocated to obtaining water given known health risks. On follow up tests we use transfer questions that ask students to select the locations for installing community wells based on a data showing contaminated wells in a village. We have utilized this module with High School, Undergraduate, and Graduate students and it excites and engages students while teaching many basic water resource issues.
[show abstract][hide abstract] ABSTRACT: The role of microbial arsenate and iron reduction in As mobilization can be difficult to distinguish. A set of incubation experiments was conducted anaerobically over 3 months using deep Pleistocene (orange) aquifer sands from Bangladesh containing low HCl-leachable Fe(II)/Fe of 0.05 and phosphate-extractable As of 0.14 mg/kg, all as arsenate. The experiments assess the role of microbial reduction in a setting of great practical importance because the deep Pleistocene aquifer is a viable source of drinking water for Bangladesh where shallow Holocene aquifers often exceed 50 ìg/L of As. Sands, resuspended in artificial groundwater(Agw) were inoculated with wild-type Shewanella sp. ANA-3, capable of Fe(III), Mn(IV), As(V) reduction. The equivalent of 0.035 mg/kg sedimentary As was mobilized on day 23 by this strain. The controls with or without lactate and without strain of Shewanella released less than the equivalent of 0.003 mg/kg As from the solid phase. We observed that As release was limited by microbes with or without lactate whereas Fe and Mn release was limited by lactate. The reduction of Fe do not enhance the As release. Furthermore, Shewanella sp. ANA-3 strain significantly converted the solid phase As to a mobilizable form by 92 days. Microbial processes responsible for this conversion may therefore be the critical process to evaluate to assess the vulnerability of Pleistocene aquifers of Bangladesh.
[show abstract][hide abstract] ABSTRACT: After a decade of research, there is still no broad-scale understanding of why Asian aquifers support such heterogeneous distributions of groundwater arsenic. In countries like Bangladesh, Nepal, and Vietnam, it is often the case that wells spaced a few meters apart and drilled to the same depth have vastly different concentrations of dissolved arsenic (i.e., 100mug/L). While there is a general consensus that older Pleistocene sediments are typically depleted in arsenic relative to younger aquifer sediments, little is known about either the geological and geochemical evolution of these aquifers with time or the exact nature of their 3-dimensional stratigraphy. To better, and more broadly, understand why local groundwater arsenic patterns exhibit such heterogeneity, sedimentological investigations were undertaken in three arsenic-contaminated Asian villages, including: (1) a hyper-avulsive floodplain in Nepal's Terai, (2) an abandoned portion of the Brahmaputra River in Bangladesh, and (3) a meander bend along Vietnam's stable, fault-controlled Red River complex. Stratigraphic cross-sections, facies determinations, and luminescence dating of the aquifer sands along transects (~1 km long and ~15 m deep) from each of these villages indicate that the aquifer deposits are not uniform, that they vary in the subsurface, and that their depositional ages correspond to concentrations of arsenic dissolved in the groundwater. Comparisons of arsenic concentrations with aquifer age show that there is an inverse relationship, indicating that the age of the sediment does play a role in arsenic's availability and distribution. While there is still much to be learned about the exact mechanism(s) and rate(s) by which arsenic is being liberated, our investigations overall support a geologic model where much of the arsenic variance is explainable by stratigraphic variations over small distances (~10 meters) that results from the dynamic depositional conditions created by these active fluvial regions.
[show abstract][hide abstract] ABSTRACT: One of the reasons the processes resulting in As release to groundwater in southern Asia remain poorly understood is the high degree of spatial variability of physical and chemical properties in shallow aquifers. In an attempt to overcome this difficulty, a simple device that collects groundwater and sediment as a slurry from precisely the same interval was developed in Bangladesh. Recently published results from Bangladesh and India relying on the needle-sampler are augmented here with new data from 37 intervals of grey aquifer material of likely Holocene age in Vietnam and Nepal. A total of 145 samples of filtered groundwater ranging in depth from 3 to 36 m that were analyzed for As (1-1000 mug/L), Fe (0.01-40 mg/L), Mn (0.2-4 mg/L) and S (0.04-14 mg/L) are compared. The P-extractable (0.01-36 mg/kg) and HCl-extractable As (0.04-36 mg/kg) content of the particulate phase was determined in the same suite of samples, in addition to Fe(II)/Fe ratios (0.2-1.0) in the acid-leachable fraction of the particulate phase. Needle-sampler data from Bangladesh indicated a relationship between dissolved As in groundwater and P-extractable As in the particulate phase that was interpreted as an indication of adsorptive equilibrium, under sufficiently reducing conditions, across 3 orders of magnitude in concentrations according to a distribution coefficient of 4 mL/g. The more recent observations from India, Vietnam and Nepal show groundwater As concentrations that are often an order of magnitude lower at a given level of P-extractable As compared to Bangladesh, even if only the subset of particularly reducing intervals characterized by leachable Fe(II)/Fe >0.5 and dissolved Fe >0.2 mg/L are considered. Without attempting to explain why As appears to be particularly mobile in reducing aquifers of Bangladesh compared to the other regions, the consequences of increasing the distribution coefficient for As between the particulate and dissolved phase to 40 mL/g for the flushing of shallow aquifers of their initial As content are explored.
[show abstract][hide abstract] ABSTRACT: Microbial Fe reduction is widely believed to be the primary mechanism of As release from aquifer sands in Bangladesh, but alternative explanations have been proposed. Long-term incubation studies using natural aquifer material are one way to address such divergent views. This study addresses two issues related to this approach: (1) the need for suitable abiotic controls and (2) the spatial variability of the composition of aquifer sands. Four sterilization techniques were examined using orange-colored Pleistocene sediment from Bangladesh and artificial groundwater over 8 months. Acetate (10 mM) was added to sacrificial vials before sterilization using either (1) 25 kGy of gamma irradiation, (2) three 1-h autoclave cycles, (3) a single addition of an antibiotic mixture at 1x or (4) 10x the typical dose, and (5) a 10 mM addition of azide. The effectiveness of sterilization was evaluated using two indicators of microbial Fe reduction, changes in diffuse spectral reflectance and leachable Fe(II)/Fe ratios, as well as changes in P-extractable As concentrations in the solid phase. A low dose of antibiotics was ineffective after 70 days, whereas autoclaving significantly altered groundwater composition. Gamma irradiation, a high dose of antibiotics, and azide were effective for the duration of the experiment.
[show abstract][hide abstract] ABSTRACT: Elevated As concentrations in shallow groundwater pose a major health threat in Bangladesh and similarly affected countries, yet there is little consensus on the mechanism of As release to groundwater or how it might be influenced by human activities. In this study, the rate of As release was measured directly with incubations lasting 11 months, using sediment and groundwater collected simultaneously in Bangladesh and maintained under anaerobic conditions throughout the study. Groundwater and gray sediment were collected as diluted slurries between 5 and 38 m in depth, a range over which ambient groundwater As concentrations increased from 20 to 100 microg L(-1). Arsenic was released to groundwater in slurries from 5 and 12 m in depth at a relatively constant rate of 21 +/- 4 (2 sigma) and 23 +/- 6 microg As kg(-1) yr(-1), respectively. Amendment with a modest level of acetate increased the rate of As release only at 12 m (82 +/- 18 mirog kg(-1) yr(-1)). Although the groundwater As concentration was initially highest at 38 m depth, no release of As was observed. These results indicate that the spatial distribution of dissolved As in Bangladesh and local rates of release to groundwater are not necessarily linked. Iron release during the incubations did not occur concurrently with As release, providing further confirmation thatthe two processes are not directly coupled. Small periodic additions of oxygen suppressed the release of As from sediments at all three depths, which supports the notion that anoxia is a prerequisite for accumulation of As in Bangladesh groundwater.
Environmental Science and Technology 06/2007; 41(10):3639-45. · 5.26 Impact Factor
[show abstract][hide abstract] ABSTRACT: Shallow aquifer groundwater arsenic heterogeneity is well documented in many of the fluvial regions of Asia. To this day, the cause for the heterogeneity remains poorly understood in part because of the heterogeneity of sediment properties inherent to a young floodplain depositional environment. In April 2006, a needle-sampler device was used to obtain depth transects of both sediment and porewater samples as the first step towards understanding the heterogeneous subsurface environment. Depth transects were taken between sites with opposing trends in tube-well arsenic, e.g. low As cluster to high As cluster, in a stable, fault controlled river bend in Van Phuc, Vietnam, to determine how sediment properties such as grainsize and reflectance relate to dissolved arsenic. Luminescence dating of aquifer sands and He3/H3 dating of the groundwater was conducted to investigate how aquifer redox conditions and groundwater arsenic evolve in Van Phuc's river-bend over time. Results indicate that localized zones of sediments enriched in leachable arsenic exist at depth within the aquifer and provide a mostly local source of arsenic. It also appears that groundwater flow encountering sediments of different ages can account for some of the observed spatial patterns of groundwater heterogeneity. Compiled with observations that reducing groundwaters with both low dissolved and extractable arsenic exist in the upper deltaic region, it appears that heterogeneities in shallow floodplain arsenic can be driven by magnitude differences in sediment-labile arsenic. Finally, given that these localized packages of arsenic enrichment correspond to a particular in-filling type of facies, it appears that fluvial cycling along the delta is the underlying cause of arsenic heterogeneity in the shallow floodplain.