Arsenic fate in the Brahmaputra river basin aquifers: Controls of geogenic processes, provenance and water-rock interactions

The influence of aquifer sediments provenance and geochemical processes on groundwater solute chemistry, including the fate of groundwater arsenic (As) in aquifers for three distinct tectono-morphic zones (northwestern, northern, and southern [NW, N, and S] in the basin of Himalayan Mega Rivers, i.e., Brahmaputra river basin (BRB), and adjoining the Himalayas and Indo-Burmese ranges, have been delineated. The presence of fluvial re-worked quartz, feldspar, and mica along with orogeny-sourced ferromagnesian minerals contained aquifers sediments in the NW and N regions are hypothesized to be derived from the Himalayan system. The S-aquifers mineralogy differs from northern aquifers by a higher proportion of Fe/Mg aluminosilicates, phyllosilicates/clay minerals, derived from the Indo-Burmese ranges. The dissolved As distribution is highly variable among the different alluvial aquifers. The S-region is highly enriched in groundwater As (bdl to 5.53 mu M or 415 mu g/L, mean 1.77 mu M) compared to NW and N regions (bdl to 1.8 mu M or 134 mu g/L, mean 0.28 mu M [60% As-contaminated samples]; bdl to 2.45 mu M or 184 mu g/L, mean 0.68 mu M [65% As-contaminated samples] respectively). More than 92% of groundwater samples in the S-region are enriched with As ( > 50 mu g/L), which draws a distinct difference from the NW and N regions of BRB aquifers. Reaction-path models suggest intense chemical weathering of S-aquifer matrix due to the higher equilibrium (more stable) of secondary mineral phases with the solution (groundwater), as compared to NW and N regions. The present study proposes a model for the geological control and effect of weathering/water-sediment reaction on As-fate and mobilization mechanism in the groundwater system of the barely studied Brahmaputra river basin aquifers.