Contrasting dissolved organic matter quality in groundwater in Holocene and Pleistocene aquifers and implications for influencing arsenic mobility

Abstract

The discontinuous nature of elevated arsenic (As) in drinking water wells of West Bengal and other regions in the Bengal Basin has led to increased interest in the role that sediment-derived organic matter may play in enhancing reductive dissolution and As mobilization. Higher As concentrations have been observed in groundwater in reduced Holocene (grey) aquifers when compared to oxidized Pleistocene (orange) aquifers. In order to evaluate if the differences in the chemical character of dissolved organic matter (DOM) are present in groundwater in the Holocene and the Pleistocene aquifers that may influence dissolved As concentrations, shallow groundwater and surface water samples were collected from four study sites in Murshidabad district, West Bengal, India, and analyzed for water chemistry parameters and characteristics of DOM. For wells known to typically contain high As concentrations (in Holocene sediments) in Beldanga (10–4622 μg/L, at 35–45 m depth) and Hariharpara (5–695 μg/L, at 6–37, depth) sites, as well as wells characterized by low As concentrations (Pleistocene sediiments) in Nabagram (0–16 μg/L, at 20–45 m depth) and Kandi (5–50 μg/L, at 20–55 m depth), detailed DOM characterization was carried out using fluorescence spectroscopy and parallel factor analysis (PARAFAC). Results from statistical analysis of a variety of optical (absorbance and fluorescence) DOM properties revealed that the DOM in groundwater in the Holocene aquifer had high humification index (HIX) and low freshness index (β:α) values, whereas groundwater in the Pleistocene aquifer comprised more labile and microbial DOM sources. Consistent with the more labile nature of DOM in groundwater in the Pleistocene aquifer, two ratios 1) humic-like to protein-like components (humic:protein) and 2) terrestrially-derived to microbially-derived components (terr:microb) obtained from a four-component PARAFAC model were 1.9 and 2.9 times greater, respectively, in groundwater in the Holocene aquifer than in that of the Pleistocene aquifer, which suggests that the absence of humic-like DOM may be an important limitation to As mobility.