Role of carbon and sulfur biogeochemical cycles on the seasonal arsenic mobilization process in the shallow groundwater of the Bengal aquifer

The bacterial reduction process of As-coated Fe(III)-OOH and SO42- in the sediment plays an important yet complex role in contributing to groundwater arsenic (As) concentration. The process utilizes dissolved organic carbon in the aquifer, which varies seasonally. The present observation documented seasonal variability (between dry pre-monsoon and post-monsoon periods) in the potential methanogenesis and intensity of bacterial reduction activity in the shallow and a few deep groundwaters from the West Bengal region, India (Nadia district). The dataset from the present study includes physical parameters (i.e., pH, Eh, electrical conductivity), multiple redox-sensitive solutes (i.e., dissolved total As, Fe, SO42- concentration), dissolved inorganic carbon (DIC) or HCO3 concentration, major element ratio (HCO3/Na, Ca/Na), and stable isotopes in the dissolved inorganic carbon (delta C-13-DIC) and dissolved sulfate (delta S-34-SO42-) phases in the groundwater samples. Multiple years of observations documented a seasonal pattern of microbially-mediated reduction of As-coated Fe(III)-OOH and potential methanogenesis with limited bacterial sulfate reduction (BSR), causing excess As mobilization in the groundwater registering low sulfate concentration. The process was documented predominantly during the dry pre-monsoon period compared to the post-monsoon period. However, a few seasonal shallow groundwater samples showed abundant sulfate concentration and a signature of potentially active BSR, with low dissolved As concentrations. We showed here that dissolved SO42- concentration and delta S-34-SO(4)(2-)isotope enrichment factor can be reliable indicators of the intensity of BSR using the Rayleigh fractionation model simulation.