Spatial and Temporal Variations of Groundwater Arsenic in South and Southeast Asia

Department of Earth System Sciences, Stanford University, Stanford, CA 94305, USA.
Science (Impact Factor: 33.61). 05/2010; 328(5982):1123-7. DOI: 10.1126/science.1172974
Source: PubMed


Over the past few decades, groundwater wells installed in rural areas throughout the major river basins draining the Himalayas
have become the main source of drinking water for tens of millions of people. Groundwater in this region is much less likely
to contain microbial pathogens than surface water but often contains hazardous amounts of arsenic—a known carcinogen. Arsenic
enters groundwater naturally from rocks and sediment by coupled biogeochemical and hydrologic processes, some of which are
presently affected by human activity. Mitigation of the resulting health crisis in South and Southeast Asia requires an understanding
of the transport of arsenic and key reactants such as organic carbon that could trigger release in zones with presently low
groundwater arsenic levels.

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Available from: Holly A. Michael, Dec 01, 2014
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    • "The dataset is broadly consistent with the widely invoked hypotheses that reductive dissolution of (near surface) Fe oxides and/or reductive desorption of As(III) coupled with downward transport are largely responsible for As mobilization in Gangetic floodplain aquifers (e.g. Fendorf et al., 2010). The findings also strongly affirm the critical role that various Fe minerals can play as host-phases for As as it undergoes redox cycling throughout the floodplain landscape. "
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    ABSTRACT: Alluvial aquifers contaminated by geogenic arsenic often lack direct solid-phase speciation data, hindering efforts to understand landscape-scale spatial heterogeneity and controls on As mobility. Here, we examine solid-phase As and Fe speciation of alluvial aquifer, river and hyporheic-seep sediments along a topographic gradient at the Himalayan foothills/upper-Ganges floodplain margin. We employ a combination of X-ray absorption spectrosco-py, selective extracts, electron microscopy and X-ray fluorescence. Source-river sediments were found to contain a mixture of solid-phase As(V) and As(III) species, while vertically heterogeneous borehole sediments also contained a solid-phase As-sulfide species. In general, the abundance of reduced As species [As(III) + As-sulfide] increased with depth below ground surface, as well as down the topographic gradient from the foothills to the floodplain. Although Fe(III) oxides diminished with depth, goethite-rich nodules were present above the seasonal water table minima and contained 10–100 fold more As than the corresponding bulk sediments [~50–800 mg kg −1 ; mainly As(V)]. In contrast, organic-rich clay layers below the seasonal water table contained abundant As-sulfide species (~ 30–80% of solid-phase As), as well as authigenic pyrite enriched in As [up to ~8500 mg kg −1 ]. Results indicate that aquifer sediments contain discrete facies that are strongly enriched in As due to post-depositional diagenetic coupling between As retention and redox-dependent Fe mineralization. In riverine hyporheic-seeps, ferrihydrite traps As(V)/As(III) species (~30–200 mg kg −1) in a readily exchange-able form, creating a pool of solid-phase As vulnerable to downstream reworking and subsequent reductive mobilization. A paucity of Fe(III) oxides in aquifer sediments at tube well-screen depth suggests that the abundant As(III) (aq) in floodplain tube wells is more likely attributable to downward transport or desorption of As(III)-species, than to contemporary reductive dissolution of As-bearing Fe oxides in sediments at well-screen depth. In contrast with the lower Gangetic plain, where large-scale floodplain sedimentary features are a key control on solid-phase As and Fe speciation, in these alluvial sediments solid-phase As and Fe speciation appears to be controlled mainly by physiographic properties (elevation; depth; seasonal water table) that influence local redox conditions.
    Chemical Geology 11/2015; 419. DOI:10.1016/j.chemgeo.2015.10.035 · 3.52 Impact Factor
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    • "In addition, Toscani et al. (2007) suggested that Fe concentrations in Fe-rich groundwater in the Po Plain are controlled by iron sulfides and siderite precipitation. The sequestration of As by co-precipitation in iron sulfides was identified as a mechanism of As attenuation in other alluvial systems (Buschmann and Berg 2009; Fendorf et al. 2010; Lowers et al. 2007; Root et al. 2009). "
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    ABSTRACT: Groundwater As concentrations > WHO limit (10 μg/L) are frequently found in the Po Plain (N. Italy). Although several hypotheses on As mobilization exist (i.e., reductive dissolution driven by peat degradation), the mechanisms of As release and subsequent attenuation acting in the multilayer aquifer in the Po Plain were poorly understood. The present work aims at implementing a reactive transport modeling of the aquifer system in Cremona, affected by As < 183 μg/L, in order to quantify and test the feasibility of As release by the reductive dissolution of Fe-oxides driven by the degradation of peat contained in leaky aquitards and As attenuation downstream by the co-precipitation in iron sulfides. The model, based on a partial equilibrium approach, revealed that the observed As, Fe and Mn chemistry could be mostly explained by the simultaneous equilibrium between Fe-oxide and sulfate reduction and FeS precipitation and by the equilibrium of rhodochrosite precipitation/dissolution. Model results, together with litholog analysis, supported the assumption of peat as the likely source of organic matter driving As release. The model fitted to measured data showed that the peak in the organic carbon degradation rate at 20-40 m below surface (average of 0.67 mM/y), corresponding to the shallow peaty aquitard and the upper portion of the underlying semiconfined aquifer, is associated with the peak of net release of As (average of 0.32 μM/y) that is followed just downstream by a net precipitation in iron sulfides at 40-60 m below surface (average of 0.30 μM/y). These results support the assumptions of peaty aquifers as drivers of As release and iron sulfides as As traps. The model also outlined the following aspects that could have a broad applicability in other alluvial As affected aquifers worldwide: (a) shallow peaty aquitards may have a greater role in driving the As release since they likely have young and more reactive organic matter; (b) the occurrence of Fe-oxide reduction and FeS precipitation, that represent the As source and sink, together with sulfate reduction occurring simultaneously close to equilibrium may restrict the As mobility limiting the extent of contamination just downstream the source of organic matter that drives its release.
    Applied Geochemistry 08/2015; DOI:10.1016/j.apgeochem.2015.07.001 · 2.27 Impact Factor
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    • "Hydrogeological occurrence of carcinogenic As is widespread in many regions of the world, posing a serious health impact on an estimated 150 million people (Ravenscroft et al. 2009; Fendorf et al. 2010). Source, migration, and transformation of As in groundwater are controlled by a number of factors (Smedley and Kinniburgh 2002). "
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    ABSTRACT: Effects of pH, As species, and Fe/Mn minerals on the fractions of adsorbed As in aquifer sediments were evaluated. Kinetic data showed that As adsorption was controlled by diffusion through the external film. Isothermal data of both As(III) and As(V) fitted the Langmuir isotherm well, revealing a monolayer adsorp-tion process. Sequential extraction demonstrated that water-soluble As and non-specifically sorbed As were the major fractions of adsorbed As. Assessing the relationship between the Freundlich K F and the increases in the amounts of As fractions showed that the pH played a key role in weakly adsorbed As, especially water-soluble As. Although inorganic As species converted each other during the adsorption processes, more non-specifically sorbed As was adsorbed in As(V)-treated sediment than in As(III)-treated sediment, showing that the electrostatic selectivity controlled the non-specific adsorption. Additionally, specifically sorbed As and As associated with the amorphous phases were predominated by Fe/ Mn minerals, especially Fe(III) (hydr)oxides. These results suggested that pH, As species, and Fe/Mn minerals would regulate the As fractions in aquifer sediments, and therefore control As cycling in aquifer systems.
    Water Air and Soil Pollution 07/2015; 226(8). DOI:10.1007/s11270-015-2524-1 · 1.55 Impact Factor
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