Arsenic (As) contamination in the deltaic aquifers of south and southeast Asia threaten the health of millions who rely on this groundwater for drinking purposes. Elevated groundwater As concentrations in Ganga-Meghna-Brahmaputra (GMB) delta spanning Bangladesh and West Bengal (India) have been widely studied. A few recent studies have shown the As enrichment in the delta formed by Sutlej-Indus rivers in the northwestern India and Pakistan. The source of As in these regions is believed to be As- and iron (Fe)-rich sediments eroded from Himalayas and transported to the deltas. However, very few studies have identified these sources and quantified the mass of As being transported to deltas. Microbially mediated reductive dissolution of these As bearing Fe (oxy)hydroxide rich sediments is generally referred to as the main mechanism of As mobility. Whereas the sedimentary and dissolved concentrations of As and Fe have been widely studied in deltas, specific occurrence and distribution of these elements along the rivers in Himalayas remain uncertain.

We hypothesize that although elevated groundwater As concentrations are rare in upper Himalayas (possibly due to oxic environment created by fast-flowing rivers and lack of extensive alluvial aquifers), the sediments transported by these rivers shall contain substantial amounts of Fe and As. We collected and analyzed riverbank sediment, river water, and sediment pore water samples from six locations along the Beas river in Himachal Pradesh (India) which is a tributary of Sutlej river in Sutlej-Indus river basin. Elemental composition including Fe and As in Beas river sediment samples was determined using X-ray fluorescence spectroscopy, HCl-extractable and water-extractable fractions analyzed on ICP-MS. Concentrations of Fe(II) in sediments were also measured in HCl-extracts a Ferrozine method and a spectrophotometer. The sediment-water extracts, river- and pore water samples were analyzed for pH and specific conductivity (in-situ), major and trace elements (ICP-MS), cations and anions (ion chromatography). All the analyses was performed at IIT Mandi Advanced Materials Research Center.

Our results showed that the river sediments contained 11.5±2.6 g/kg of total Fe measured by XRF, of which 2.5±0.7 g/kg was HCl-extractable (21%), 0.26±0.06 g/kg (2.2%) was in the form of HCl-extractable Fe(II), and only 5.3±2.9 mg/kg was water-extractable (0.05%) total Fe. A total of 3.9±1.1 mg/kg of As was measured by XRF in river sediments, and almost 50% (1.96±0.84 mg/kg) of that was HCl-extractable, and 2.5% (0.1±0.03 mg/kg) was water-extractable. Minimal concentrations of Fe (98±47 µg/L) and As (2.8±1 µg/L) in the river waters consistent with the oxic environment of the fast-flowing Beas river were observed. The pore water samples contained relatively higher concentrations of Fe (277±344 µg/L) and As (5±4.5 µg/L) than the riverwater indicating higher reaction time between riverwater and sediments along the riverbank. Whereas the pH did not vary much between river (7.14±0.1) and porewater (7.07±0.1), the specific conductance of pore water (494±349, 80-1,051 µS/cm) was higher than the riverwater (118±42, 75-164 µS/cm) which also indicates the higher reaction time. It appears that the Beas river sediments contained As concentrations slightly above the crustal abundance of As (2 mg/kg). 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 to initiate the reductive dissolution processes. Further investigations are needed to fully understand the preservation of sedimentary As in the sediments of these fast-flowing rivers. Analyses of dissolved organic carbon and matter in the pore water as well as sediments is underway, which will help in further elucidating these processes. The findings of this study will advance our understanding of how As is mobilized from the source to the delta.