Hydrogeochemical factors affecting the mobilization of As into the groundwater of the Brahmaputra alluvial plains of Assam, Northeast India.

Groundwater in the Brahmaputra river basin is known to contain an elevated concentration of naturally occurring Arsenic (As). To better understand the mobilization processes responsible for the As enrichment in the groundwater of the alluvial plains of the Brahmaputra river, the hydrochemical characteristics of the groundwater were studied in two districts, namely the Darrang and Jorhat districts of Assam. A total of 217 groundwater samples were analyzed for 20 water quality parameters. A lower As concentration was observed in the Darrang district with a mean concentration of 0.02 μg l(-1), while the Jorhat district had a higher As concentration (mean 100 μg l(-1)). The groundwater in the study areas is characterized by high concentrations of dissolved Fe, Mn, HCO3(-) and PO4(3-) and low concentrations of NO3(-) and SO4(2-). The redox potential and the groundwater composition showed reducing conditions in the groundwater. A difference in the groundwater geochemistry between the two districts was observed and was mainly controlled by the redox conditions in the subsurface, which control the mobility of As in the floodplains. The saturation indices of various As associated minerals were calculated using the geochemical code PHREEQC. The mobilization of As into the groundwater was anticipated to be largely controlled by the reductive dissolution of Fe-oxides and Mn-oxides and partly by competitive anions viz. PO4(3-).