ORGANIC MATTER (INCLUDING FROM THERMALLY MATURE SOURCE ROCKS) IN CONTRASTING (HIGH ARSENIC) HOLOCENE AND (LOW ARSENIC) PLEISTOCENE SHALLOW REDUCING AQUIFERS, RED RIVER DELTA, VIETNAM

High concentrations of geogenic arsenic (As) in shallow reducing groundwaters of circum-Himalayan Asia are widely considered to be related to the activity of metal-reducing microbes, which utilise electron donors such as organic matter (OM) [1-3]. However, although OM in some such Holocene aquifers have been partly characterised [4-7], there is a dearth of data on Pleistocene aquifers from the same areas.

We report here studies of OM and microbial communities present in two aquifers, one of Pleistocene age (with low As <10 μg/L ) and one of Holocene age (with high As up to 600 μg/L) from Van Phuc village in the Red River Delta, Vietnam [8,9]. GC-MS analysis 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. No systematic differences in OM biomarker distribution patterns were observed between the two sites.

Microbial analyses did not show the presence of microbial communities previously associated with arsenic mobilization. All clone libraries were dominated by α-, β-, and γ-Proteobacteria not known to reduce Fe(III) or sorbed As(V) [3]. 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 arsenic 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 mobilized in situ in these sediments instead As appears to be mobilized elsewhere and transported by groundwater flow to the sites and/or (ii) sorption/desorption processes plays a critical role in controlling As concentrations at these sites.

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