Anthropogenic dominance on geogenic arsenic problem of the groundwater in the Ganga-Brahmaputra floodplain: A paradox of origin and mobilization

The Ganga-Brahmaputra floodplains constitute a complex system that is vulnerable to arsenic recycling owing to its geomorphic sensitivity, aquifer profiles, high meandering scars and extreme sediment deposition, along with extreme monsoonal disturbances; and are subjected to significant alterations in arsenic recycling. We have put an effort to delineate the similarities and dissimilarities pertaining to the arsenic prevalence, origin and mobilization in the two hotspots, namely the Mid-Gangetic Floodplains (MGFP) and the Brahmaputra Floodplains (BFP). Pertaining to this, we collected 384 representative water samples for hydrogeochemical investigations, multivariate analyses, and saturation status based predictive modelling, with BFP having a maximum concentration of arsenic (As) reaching to almost 97.9 μgL^-1 and MGFP having a maximum concentration of 50.1 μgL^-1. Seasonality impelled changes and conforming riverine recharges are leading major ionic differentiations in both the floodplains across seasons. Meandering and aquifer dynamics control As prevalence in the MGFP and BFP, respectively. Non-interdependent HCO₃^- recharge mediated As-recycling was found in the BFP. Carbonate weathering is dominant in the MGFP, while both carbonate and silicate weathering take precedence in the BFP. Multivariate analysis hints at fertilizer influence on As mobilization in the MGFP. Reductive hydrolysis of Fe-OOH mediated As-release is more prominent in the BFP. Seasonal arsenic fluctuations are going to have more climatic dependency in near future owing to the increasing erratic rains, pumping and recharge events. Erratic precipitation will provoke immediate response in both floodplains in terms of As mobilization which urgently needs attention to counter increasing arsenic vulnerability.