How paleosols influence groundwater flow and arsenic pollution: A model from the Bengal Basin and its worldwide implication

Abstract

[1] In the Bengal Basin, the land surface exposed during the last lowstand of sea level around 20 ka, and now buried by Holocene sediment, is capped by an effectively impermeable clay paleosol that we term the Last Glacial Maximum paleosol (LGMP). The paleosol strongly affects groundwater flow and controls the location of arsenic pollution in the shallow aquifers of our study site in southern West Bengal and, by implication, in shallow aquifers across the Bengal Basin and As-polluted deltaic aquifers worldwide. The presence of the LGMP defines paleointerfluvial areas; it is absent from paleochannel areas. A paleosol model of pollution proposed here predicts that groundwater in paleochannels is polluted by arsenic, while that beneath paleointerfluvial areas is not: paleointerfluvial aquifers are unpolluted because they are protected by the LGMP from downward migration of arsenic and from downward migration of organic matter that drives As-pollution via reductive dissolution of As-bearing iron oxyhydroxides. Horizontal groundwater flow carries arsenic from paleochannels toward paleointerfluvial aquifers, in which sorption of arsenic minimizes the risk of pollution.

Microbially mediated reduction of FeIII and AsV in Cambodian sediments amended with 13C-labelled hexadecane and kerogen

“…[19,[25][26][27] Nevertheless, the identification of the active members of the microbial communities in these complex sedimentary environments is challenging, especially when investigating specific microbe-substrate interactions. Thus, in some studies, the use of 13 C-labelled organic carbon substrates (mainly acetate) in combination with molecular microbiology techniques (DNA stable isotope probing (DNA-SIP), followed by DNA fractionation and 16S rRNA gene cloning and sequencing) have allowed the identification of the active members of the microbial community during As III mobilisation. [25,26,28] Alternatively to DNA-SIP, the incorporation of 13 C in phospholipid fatty acids (PLFAs), which are the major components of the cell membranes and remain intact in viable cells, [29] can be used to investigate environmental microbial communities.…”

“…Thus, in some studies, the use of 13 C-labelled organic carbon substrates (mainly acetate) in combination with molecular microbiology techniques (DNA stable isotope probing (DNA-SIP), followed by DNA fractionation and 16S rRNA gene cloning and sequencing) have allowed the identification of the active members of the microbial community during As III mobilisation. [25,26,28] Alternatively to DNA-SIP, the incorporation of 13 C in phospholipid fatty acids (PLFAs), which are the major components of the cell membranes and remain intact in viable cells, [29] can be used to investigate environmental microbial communities. [30] This is because the concentration and distribution of PLFAs has the capacity to reflect rapid changes in the microbial populations and link specific biomarkers to a phylogenetic or functional group of organisms.…”

“…[10] Owing to the catastrophic effect of groundwater arsenic pollution on the health of millions of people around the world, the factors controlling arsenic release into groundwater have been the subject of numerous studies, which have shown that various processes can contribute to increased As III mobilisation. [11][12][13][14][15][16][17][18] One of these processes, microbial reduction of As V , is widely accepted as being an important mechanism of As III release in shallow reducing aquifers. [19][20][21][22][23] Bioavailable organic carbon serves both as a carbon source and electron donor for the microbial respiratory reduction of sedimentary As V , which serves as an electron acceptor, resulting in the release of As III , which is more mobile, to the aqueous phase.…”

“…We investigated these aims by establishing a series of anoxic microcosms, using sediments collected from an arsenic-affected aquifer in Cambodia, and supplemented with 13 C-labelled substrates. 13 C-Hexadecane was used as a representative of long chain n-alkanes, which are part of the extractable OM (i.e. the part of the OM that can be extracted using common organic solvents).…”

“…the part of the OM that can be extracted using common organic solvents). We also artificially matured 13 C-labelled algal biomass to generate a labelled kerogen analogue to represent the non-extractable part of the OM. Following anaerobic incubation for 8 weeks, degradation of the 13 C-labelled substrates was determined by gas chromatographymass spectrometry (GC-MS), and their incorporation into the microbial biomass was detected by DNA-SIP fractionation.…”