Natural recharge transcends anthropogenic forcing that influences arsenic vulnerability of the quaternary alluviums of the Mid-Gangetic Plain

We evaluated the anthropogenic forcing and exceedance probability of arsenic vulnerability in the Quaternary Alluviums of the Mid-Gangetic Plain through the coupled application of hydrogeochemical analyses, inverse modelling, probability analyses, kriging, bivariate and multivariate statistical analysis (MVA). Fifty-seven groundwater samples (total 171) each were collected during the pre-monsoon (April 2015), monsoon (July 2015) and post-monsoon (January 2016). Seasonal cyclicity of ion exchange and reverse ion exchange was observed, with the former being common during pre-monsoon while the latter was dominant during post-monsoon. PHREEQC modelling showed the involvement of the agrochemicals such as calcium nitrate and calcium phosphate and other isolated incidences of chemical usage like bleaching powder as well as the probability of wet deposition of the anions like SO<inf>4</inf> ²⁻ and NO<inf>3</inf> ⁻. Kriging-based arsenic mapping revealed that rainfall recharge-led dilution plays the most dominant role in its mobilization. Owing to natural recharge in the alluvium plain, the exceedance probability of arsenic concentration above 5 μg L⁻¹ falls drastically from more than 0.8 in the pre-monsoon to 0.5 during the post-monsoon. Study implies that pre-monsoon pumping of groundwater must be regulated in combination with proper disposal of pH and ORP affecting chemicals so that the high natural recharge should not significantly induce arsenic mobilization. Further, we recommend that vulnerability estimation should not be based solely on the present scenarios but must include the features of probable exceedance and saturation status of groundwater in this belt. We put forward a comprehensive model to explain the operative processes governing arsenic mobilization.