Brahmaputra river basin groundwater: Solute distribution, chemical evolution and arsenic occurrences in different geomorphic settings

Study region: Brahmaputra River basin, India. Study focus: The present study deciphers the groundwater solute chemistry and arsenic (As) enrichment in the shallowaquifers of the study region. Four different geomorphologic units, e.g. piedmont (PD), older alluvium of river Brahmaputra and its tributaries (OA), active alluvium of river Brahmaputra and its tributaries (Upsilon A) and river channel deposits (RCD) were identified. More than 62% of all groundwater samples collected have dissolved As >0.01 mg/L, whereas about 87% of groundwater samples in OA terrain are enriched with As, which draws a distinct difference from the adjoining Gangetic aquifers. New hydrological insights for the region: Most growundwater solutes of RCD and VA terrains were derived from both silicate weathering and carbonate dissolution, while silicate weathering process dominates the solute contribution in OA groundwater. Groundwater samples from all terrains are postoxic with mean pe values between Fe(III) and As(V)-As(III) reductive transition. While, reductive dissolution of (Fe-Mn)OOH is the dominant mechanism of As mobilization in RCD and VA aquifers, As in OA and PD aquifers could be mobilized by combined effect of p H dependent sorption and competitive ion exchange. The present study focuses on the major ion chemistry as well as the chemistry of the redox sensitive solutes of the groundwater in different geomorphic settings and their links to arsenic mobilization in groundwater. (C) 2015 The Authors. Published by Elsevier B.V.