INFLUENCE OF GEOMORPHIC TERRAINS AND WATER-SEDIMENT INTERACTIONS ON FATE OF GROUNDWATER ARSENIC IN SHALLOW AQUIFERS OF THE BRAHMAPUTRA RIVER BASIN ADJOINING EASTERN HIMALAYAS

The present study focuses on understanding the influence of geomorphology on groundwater geochemistry and groundwater arsenic (As) distribution in the shallow aquifers in parts of the Brahmaputra river basin of India. The study area extends from northern bank of Brahmaputra river to the foothills adjoining the India-Bhutan international boundary, in vicinity of the eastern Himalayas. Geomorphological units in the area can be broadly classified as piedmont deposits (PD), older alluvials deposited by river Brahmaputra and its tributaries (OA), younger and active alluvial deposits of river Brahmaputra and its tributaries (YA) and river channel deposits (RCD), and were identified by coupled remote sensing and field observations. Stable-isotopic (δ¹⁸O-δ²H) analyses indicate groundwater in these units have been recharged by meteoric precipitation and surface water that has generally undergone some evaporation. The hydrochemical facies are generally Ca–Na–HCO₃ and Na–Ca–HCO₃ type. Most solutes in groundwater of RCD and YA terrain were derived from both silicate weathering and carbonate dissolution, while silicate weathering processes dominate the solute contribution in OA groundwater. Redox conditions of all terrains are in postoxic subgroup of anoxic environment between Fe (III)-Fe(II) and As(V)-As(III) reductive transition zones. More than 65 % of all groundwater samples have dissolved As concentrations ≥ 0.01 mg/L. At least ~ 50 % groundwater samples of RCD, ~ 35 % of YA, ~ 87 % of the OA, and ~ 33 % of PD terrain samples have As concentration >0.01 mg/L, with the highest percentage of groundwater As enrichment occurring in the OA aquifers, which is a distinct difference scenario from the adjoining Gangetic and other southeast Asian As-polluted mega-river aquifers. Arsenic is mostly mobilized by reductive dissolution of Fe–Mn (oxyhydr)oxides in all terrains, however, in OA and PD aquifers, As is probably also mobilized by combined effect of pH dependent sorption along with competitive exchange of anions (e.g. HCO₃).