Characterisation of organic matter in a shallow, reducing, arsenic-rich aquifer, West Bengal

Elevated arsenic in groundwaters extensively exploited for irrigation and drinking water in West Bengal and Bangladesh is causing serious impacts on human health. A key mechanism for the genesis of arsenic (As) in these waters is microbially mediated reductive transformation of arsenic-bearing Fe(III) hydrated oxides. The role of organic C in this process, whether from in situ organic matter (OM), i.e. OM from within the sediment, or from other sources, is widely recognised. Despite this, there is a paucity of data about the characteristics of OM in these As-rich aquifers. Extraction and analysis of the polar and apolar fractions from seven different sediments from a known groundwater As “hotspot” in West Bengal revealed OM characteristic of the original terrestrial depositional environment. However, this was overprinted by abundant hydrocarbons with thermally mature (e.g. petroleum) distributions. These hydrocarbons included abundant high molecular weight n-alkanes and unresolved complex mixtures (UCMs), as well as thermally mature distributions of hopanes and steranes. Additionally, at certain depths (surface sands: 8 and 13 m, deeper sands: 25 m) the OM appeared to be biodegraded, with the preferential removal of petroleum-sourced n-alkanes, suggesting that indigenous microbes within the aquifer can utilise this carbon source. The presence of this previously unreported source of bioavailable organic carbon is of importance as it has the potential to promote microbial activity and subsequent As release in the aquifers.