INVESTIGATING GEOCHEMICAL PROCESSES IN SEDIMENTS OF HIMALAYAN TRIBUTARIES TO ARSENIC-CONTAMINATED FLOODPLAIN AQUIFERS

Arsenic (As) contamination of shallow south Asian alluvial aquifers formed by major rivers such as the Ganga, Brahmaputra, Meghna, Sutlej and Indus is primarily attributed to microbially mediated reductive dissolution of As-laden iron (Fe)-oxides. These Fe-oxides form during weathering and transport from the Himalayas. This reduction is fueled by labile sedimentary and dissolved organic matter (OM). A limited number of studies have investigated the interactions between Fe, As and DOC or OM in the Himalayan region upgradient from the broad floodplains where dissolved As contamination occurs widely. We hypothesize that the sediments transported by the Himalayan rivers contain elevated concentrations of Fe and As compared to average crustal abundance. To test this, we collected and analyzed riverbank sediments and pore-waters, and river water from six locations along the Beas River in Himachal Pradesh (India), a major contributor to Sutlej-Indus river delta which has endemic As contamination. X-Ray Fluorescence (XRF) revealed that river sediments contained 12±3 g/kg of total Fe., Of this, 3±1 g/kg (21%) and 0.3±0.1 g/kg (2.2%) total Fe and Fe(II) were extractable by 1M HCl In contrast, only 5±3 mg/kg (0.05%) total Fe was water-extractable. Approximately 4±1 mg/kg of total As was measured by XRF in river sediments, and almost 50% (2±1 mg/kg) of that was HCl-extractable, whereas 2.5% (0.1±0.03 mg/kg) was water-extractable. The river water contained only 98±47 µg/L and 3±1 µg/L of dissolved Fe and As, respectively which is consistent with the oxic environment of the fast-flowing river. Riverbank pore waters contained relatively higher concentrations of Fe (277±344 µg/L) and As (5±4.5 µg/L) than the river water. This could possibly be explained by longer reaction times between river water and sediments along the riverbank. The Beas River sediments contained As concentrations slightly above the crustal abundance of As (2 mg/kg). Analyses of sedimentary and dissolved OM in these samples is currently underway. Lower As concentrations in the river water are expected, whereas lower dissolved As in the porewaters may be attributed to lack of labile organic matter in these highly oxygenating environments to initiate the reductive dissolution processes. The findings of this study will advance our understanding of how As is mobilized from the source to the delta.