Article

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

ABSTRACT

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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    • "Arsenic contamination of groundwater has been a focus for environmental studies due to its global scale of distribution and harmful effect on the ecosystem and human health [1]. It is estimated that tens of millions of people worldwide are under exposure to toxic concentrations of arsenic in groundwater as a source of drinking water supply [2]. "
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    ABSTRACT: In situ treatment of high-arsenic groundwater cost-effectively is still challenging. We proposed an in situ treatment approach which utilizes O2 produced from groundwater electrolysis to increase the redox potential for oxidative removal of arsenic. A sand column was configured to simulate groundwater flow in an aquifer, and a stable anode, a stable cathode and an iron anode were arrayed in an upward mode in the column to evaluate the performance on arsenic removal from the groundwater induced by the oxidative precipitation of Fe(2+) by O2. As(III) at 500μg/L was efficiently oxidized to As(V) by the stable anode followed by the reactive oxidants produced from Fe(II)-O2, and total As were completely removed by the newly formed amorphous iron hydroxides. Quantitative models for the dependence of As(III) oxidation, total As removal and Fe(II) oxidative precipitation on the flow rate and the current density applied to Fe anode were developed. The presence of humic substance promoted the oxidation of As(III) on the stable anode but inhibited the oxidation and removal induced by Fe(II) oxidative precipitation. A stable performance on As(III) oxidation and removal was observed in a 10-day continuous operation. Results from this study prove that groundwater electrolysis could be applicable for oxidative removal of As(III) in porous media, with a controllable and lasting treatment efficiency.
    Full-text · Article · Mar 2016 · Journal of hazardous materials
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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.
    Full-text · Article · Nov 2015 · Chemical Geology
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    • "The question needs attention, because tens of millions of villagers throughout the affected region continue to rely on shallow wells as their main source of drinking water. Without a quantitative understanding of the mechanisms as to how and why As increases with depth, doubts will remain concerning how long a shallow well that once tested low for As can continue to be used for drinking and cooking (McArthur et al., 2010; Fendorf et al., 2010). "
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    ABSTRACT: The spatial heterogeneity of dissolved arsenic (As) concentrations in shallow groundwater of the Bengal Basin has been attributed to transport of As (and reactive carbon) from external sources or to the release of As from within grey sand formations. We explore the latter scenario in this detailed hydrological and geochemical study along a 300 m transect of a shallow aquifer extending from a groundwater recharge area within a sandy channel bar to its discharge into a nearby stream. Within the 10–20 m depth range, groundwater ages along the transect determined by the 3H–3He method increase from <10 yr in the recharge area to a maximum of 40 yr towards the stream. Concentrations of groundwater As within the same grey sands increase from 10 to 100 to ∼500 μg/L along this transect. Evidence of reversible adsorption of As between the groundwater and sediment was obtained from a series of push–pull experiments, traditional batch adsorption experiments, and the accidental flooding of a shallow monitoring well. Assuming reversible adsorption and a distribution coefficient, Kd, of 0.15–1.5 L/kg inferred from these observations, a simple flushing model shows that the increase in As concentrations with depth and groundwater age at this site, and at other sites in the Bengal and Red River Basins, can be attributed to the evolution of the aquifer over 100–1000 years as aquifer sands are gradually flushed of their initial As content. A wide range of As concentrations can thus be maintained in groundwater with increases with depth governed by the history of flushing and local recharge rates, without external inputs of reactive carbon or As from other sources.
    Full-text · Article · Nov 2015 · Applied Geochemistry
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