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High arsenic concentrations in groundwater are causing a humanitarian disaster in Southeast Asia. It is generally accepted that microbial activities play a critical role in the mobilization of arsenic from the sediments, with metal-reducing bacteria stimulated by organic carbon implicated. However, the detailed mechanisms underpinning these processes remain poorly understood. Of particular importance is the nature of the organic carbon driving the reduction of sorbed As(V) to the more mobile As(III), and the interplay between iron and sulphide minerals that can potentially immobilize both oxidation states of arsenic. Using a multidisciplinary approach, we identified the critical factors leading to arsenic release from West Bengal sediments. The results show that a cascade of redox processes was supported in the absence of high loadings of labile organic matter. Arsenic release was associated with As(V) and Fe(III) reduction, while the removal of arsenic was concomitant with sulphate reduction. The microbial populations potentially catalysing arsenic and sulphate reduction were identified by targeting the genes arrA and dsrB, and the total bacterial and archaeal communities by 16S rRNA gene analysis. Results suggest that very low concentrations of organic matter are able to support microbial arsenic mobilization via metal reduction, and subsequent arsenic mitigation through sulphate reduction. It may therefore be possible to enhance sulphate reduction through subtle manipulations to the carbon loading in such aquifers, to minimize the concentrations of arsenic in groundwaters. [ABSTRACT FROM AUTHOR]

The contamination of groundwater by geogenic arsenic affects the health of tens of millions of people, especially in South and Southeast Asia. The primary cause of the contamination in highly reducing alluvial and deltaic aquifers of South and Southeast Asia, is thought to be microbial reduction of Fe(III) and As(V) to (more mobile) Fe(II) and As(III), respectively. Key to understanding the Fe(III)/As(V) reduction process in aquifers, is the identity and source of electron donors driving the system – this has been a key area of research within arsenic biogeochemistry in the last two decades. Studies in this research area can broadly be divided into three approaches: 1) field-based, which include the organic geochemical analysis of aquifer samples to deduce composition and potential sources (by a variety of techniques and proxies), and/or analysis of multiple biogeochemical parameters (microbes, genes, inorganic and organic geochemistry) combined with multivariate statistical analysis; 2) microcosm-based, where the reductive dissolution mechanism can be studied in laboratory-scale incubation experiments, where key parameters (including electron donor type and availability) can be manipulated, and the effects monitored; 3) in-situ experiments using mineral phases deployed into aquifers. Here we review studies of the three aforementioned approaches to address three key questions: (i) What potential electron donors are present in the aquifers? (ii) What are the environmental sources of the electron donors? (iii) Which electron donors have been directly implicated in arsenic release in aquifers? We find that the potential electron donors available (and utilised), depend on a number of site-specific factors, including geomorphological setting, sediment lithology, and the metabolic potential of the microbial communities present. A wide range of electron donors are implicated, including dissolved labile organic matter, methane and ammonium. In conclusion, we highlight the need for further multidisciplinary investigations that combine state-of-the-art organic geochemical analyses, with mineralogical and microbial analyses in future studies, ideally conducted in-situ. [ABSTRACT FROM AUTHOR]

Abstract: High concentrations of geogenic As in the groundwaters of south and SE Asia, which are used as drinking waters, are causing severe health impacts to the exposed human populations. It is widely accepted that As mobilisation from sediments into these shallow reducing groundwaters requires active metal-reducing microbes and electron donors such as organic matter (OM). Although OM in such Holocene aquifers has been characterised, there is a dearth of data on Pleistocene aquifers from the same areas. Reported here are preliminary studies of OM and microbial communities present in two aquifers, one of Pleistocene and one of Holocene age, with contrasting concentrations of As (viz. Pleistocene: low As <10μg/L; Holocene: high As up to 600μg/L) from Van Phuc village in the Red River Delta, Vietnam. Results revealed OM inputs from multiple sources, including potential contributions from naturally occurring petroleum seeping into the shallow aquifer sediments from deeper thermally mature source rocks. Although concentrations vary, no noticeable systematic differences in biomarker distribution patterns within the OM were observed between the two sites. Microbial analyses did not show a presence of microbial communities previously associated with As mobilisation. All clone libraries were dominated by α-, β-, and γ-Proteobacteria not known to be able to reduce Fe(III) or sorbed As(V). Furthermore, representatives of the Fe(III)-reducing genus Geobacter could only be detected at very low abundance by PCR, using highly selective 16S rRNA gene primers, supporting the hypothesis that metal reduction is not a dominant in situ process in these sediments. No correlation between As concentration in groundwater and OM composition nor microbial community in the host sediments was found. This suggests that either (i) As is not being significantly mobilised in situ in these sediments, instead As appears to be mobilised elsewhere and transported by groundwater flow to the sites or (ii) sorption/desorption processes, as implicated by geochemical data from the cores, play a critical role in controlling As concentrations at these sites. [Copyright &y& Elsevier]