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Over the past few decades, groundwater welts 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. doi: 10.1126/science.1172974

Tens of millions of people in south and southeast Asia routinely consume ground water that has unsafe arsenic levels (1,2). Arsenic is naturally derived from eroded Himalayan sediments, and is [...]

Arsenic poisoning: Sources of contamination identified What may be the largest mass poisoning in history is occurring now, across southeast Asia, where millions of people are drinking arsenic-tainted well water. Arsenic is naturally derived from eroded Himalayan sediments, but the processes controlling passage of arsenic release into pore water remain unclear. This is partly attributed to a poor understanding of groundwater flow paths altered by extensive irrigation pumping. Polizzotto et al. now use hydrologic and biogeochemical measurements to show that on the minimally disturbed Mekong Delta of Cambodia, arsenic is released from near-surface, river-derived sediments and transported, on a centennial timescale, through the underlying aquifer back to the river. These results represent a model for understanding pre-disturbance conditions for other major deltas in Asia. Tens of millions of people in south and southeast Asia routinely consume ground water that has unsafe arsenic levels.sup.1,2. Arsenic is naturally derived from eroded Himalayan sediments, and is believed to enter solution following reductive release from solid phases under anaerobic conditions. However, the processes governing aqueous concentrations and locations of arsenic release to pore water remain unresolved, limiting our ability to predict arsenic concentrations spatially (between wells) and temporally (future concentrations) and to assess the impact of human activities on the arsenic problem.sup.3,4,5,6,7,8,9. This uncertainty is partly attributed to a poor understanding of groundwater flow paths altered by extensive irrigation pumping in the Ganges-Brahmaputra delta.sup.10, where most research has focused. Here, using hydrologic and (bio)geochemical measurements, we show that on the minimally disturbed Mekong delta of Cambodia, arsenic is released from near-surface, river-derived sediments and transported, on a centennial timescale, through the underlying aquifer back to the river. Owing to similarities in geologic deposition, aquifer source rock and regional hydrologic gradients.sup.11,12,13,14,15, our results represent a model for understanding pre-disturbance conditions for other major deltas in Asia. Furthermore, the observation of strong hydrologic influence on arsenic behaviour indicates that release and transport of arsenic are sensitive to continuing and impending anthropogenic disturbances. In particular, groundwater pumping for irrigation, changes in agricultural practices, sediment excavation, levee construction and upstream dam installations will alter the hydraulic regime and/or arsenic source material and, by extension, influence groundwater arsenic concentrations and the future of this health problem., Author(s): Matthew L. Polizzotto [sup.1] , Benjamin D. Kocar [sup.1] , Shawn G. Benner [sup.2] , Michael Sampson [sup.3] , Scott Fendorf [sup.1] Author Affiliations: (1) School of Earth Sciences, [...]

Naturally occurring Cr(VI) has recently been reported in ground and surface waters. Rock strata rich in Cr(III)-bearing minerals, in particular chromite, are universally found in these areas that occur near convergent plate margins. Here we report experiments demonstrating accelerated dissolution of chromite and subsequent oxidation of Cr(III) to aqueous Cr(VI) in the presence of birnessite, a common manganese mineral, explaining the generation of Cr(VI) by a Cr(III)-bearing mineral considered geochemically inert. Our results demonstrate that Cr(III) within ultramafic- and serpentinite-derived soils/sediments can be oxidized and dissolved through natural processes, leading to hazardous levels of aqueous Cr(VI) in surface and groundwater. manganese oxide | oxidation | serpentine | ultramafic | chromite | lophiolites

Arsenic is a contaminant in the groundwater of Holocene aquifers in Bangladesh, where [approximately equal to] 57 million people drink water with arsenic levels exceeding the limits set by the World Health Organization. Although arsenic is native to the sediments, the means by which it is released to groundwater remains unresolved. Contrary to the current paradigm, ferric (hydr)oxides appear to dominate the partitioning of arsenic in the near surface but have a limited impact at aquifer depths where wells extract groundwater with high arsenic concentrations. We present a sequence of evidence that, taken together, suggest that arsenic may be released in the near surface and then transported to depth. We establish that (i) the only portion of the sediment profile with conditions destabilizing to arsenic in our analysis is in the surface or near-surface environment; (ii) a consistent input of arsenic via sediment deposition exists; (ii) retardation of arsenic transport is limited in the aquifers; and (iv) groundwater recharge occurs at a rate sufficient to necessitate continued input of arsenic to maintain observed concentrations. Our analyses thus lead to the premise that arsenic is liberated in surface and near-surface sediments through cyclic redox conditions and is subsequently transported to well depth. Influx of sediment and redox cycling provide a long-term source of arsenic that when liberated in the near surface is only weakly partitioned onto sediments deeper in the profile and is transported through aquifers by groundwater recharge. redox | Ferric (hydr)oxides

This study was used to examine direct (enzymatic) and indirect (metabolite) reduction processes of CoIIIEDTA by Desulfovibrio vulgaris, a sulfate-reducing bacterium, in surface water and groundwater. The study is broken down into an introduction, a discussion of the experiment and a discussion of its results. Various tables and graphs accompany the article.

This study elucidates the use of dissimilatory iron reduction as a primary pathway to sequester and detoxify chromate in anaerobic water and soils. The Department of Geological and Environmental Sciences at Stanford University and the Soil Science Division of the University of Idaho conducted the study.

Studies were held to see how reactions between arsenic and sulfide influenced water quality in arsenic-contaminated systems. Kinetics and reactions pathways of arsenate with sulfide were investigated. The article breaks down how the study took places and what its results were.

Researchers investigated the relationship between iron and arsenic in assessing the environmental risks that arsenic pose in fragile wetlands such as the Coeur d'Alene Wetlands in northern Idaho

Researchers from Stanford University and the University of Idaho studied the reduction of uranyl(VI) by the metal reducing bacteria Shewanella alga strain (BrY) in the presence of amorphous ferric hydroxide.

This article examines pyrite oxidation caused by cadmium sorption.

Lake Coeur d'Alene, Idaho is contaminated by run off from local silver and lead mining. Researchers studied deposition of iron, arsenic, and other minerals. Core samples taken from the lake showed a consistent deposition. Lead, and zinc deposited low in the sediment. Arsenic remained at the sediment-water interface. This implied that some means was keeping arsenic from setting out. Shewanella alga, is known to be able to mobilize arsenic. Researchers studied this process to see if the bacteria used HFO (ferric oxide) as part of the process of reducing arsenic. For their experiment, they used synthetic scorodite (a FE-As mineral) to confirm their hypothesis.

Arsenic V biotic reduction experiments in groundwater sediments shows that sediments of the Coeur d'Alene Lake (CDAL) have the capacity for As(V) reduction. As(III) generation within As(V) amended organisms was measured to determine As mobility and biotic degradation was measured using an inhibitor of SRB metabolism (molybdate). Results show that amended sediment organisms displayed attenuated As (V) reduction capabilities.

Arsenite adsorption and stability in goethite is investigated and results show that arsenite surface complex formation on goethite oxides may play an important role in the environmental behavior of arsenic. A combination of standard batch techniques and X-ray absorption spectroscopy and results show that the arsenite reactivity at different pH and arsenite concentration provides strong macroscopic evidence for strong surface complexation.

A study aimed at determining the stability and oxyanion sorption mechanisms of arsenic and chromium is presented. Chromate and arsenate are present in the environment in high levels and prove to be very harmful to living organisms. It is deemed important to identify the reactions that will remove these contaminants from the aqueous phase, thereby limiting their mobility and bioavailability. The study utilized extended X-ray absorption fine structure (EXAFS) spectroscopy to deduce the local coordination environment of the two contaminants on the mineral goethite (alpha-FeOOH).

A stopped-flow kinetic approach monitored by UV-VIS spectrophotometry was applied to determine the rate constant and the rate dependence of each reactant in the removal of chromate Cr(VI) from suboxic and anoxic waters and soils. These kinetic information are necessary in determining the rate of Cr(VI) reduction and to investigate the role of oxygen in limiting such reaction. The rate of Cr(VI) removal and that for the oxygenation of Fe(II) indicate that Cr(VI) reduction should not be influenced at all by oxygen, except at pH values exceeding 8 even at micromolar concentrations.

The effects of pH, solution chromium concentrations and surface coverage on the surface structure of chromium(III) sorbed on amorphous silica were studied to determine how various reaction conditions affect the stability of chromium(III) bound to a sorbent. The study protocol involved diffuse reflectance infrared Fourier transform spectroscopy and high-resolution transmission electron microscopy. Results showed that the extent of surface coverage alone affected the atomic structure of chromium sorbed on silica.

The surface structure of chromium(III) (CrIII) sorbed on silica was studied using extended X-ray absorption fine structure spectroscopy to improve the understanding of metal ion interactions at the solid-solution interface. The structural environment of metal ions at the solid-solution interface determines ion remobilization to the solution and the physical and chemical changes of the sorbent. Results showed that Cr(III) forms a monodentate surface complex on silica. Cr-Si and Cr-Cr distances were obtained, with the latter indicating edge-sharing Cr octahedra.

Millions of people living in south and southeast Asia routinely drink groundwater contaminated with dangerous levels of arsenic derived from natural Himalayan sediments. Thousands die every year from cancers caused [...]