COMPARATIVE UNDERSTANDING OF ARSENIC ENRICHMENT AND MOBILIZATION IN THE AQUIFERS OF THE RIVER GANGES AND BRAHMAPUTRA: A PROVENANCE, PREVALENCE AND HEALTH PERSPECTIVE

The hydrogeochemistry especially with respect to arsenic (As) in, the Brahmaputra flood plain (BFP) of Assam and the eastern part of the Ganga flood plain (GFP) in the state of Bihar, India were compared in this study. Since most part of the Ganga and Brahmaputra flood plains are affected by the devastating effects of flood and the after effect of such incidences get enhanced by many fold with the presence of any contaminant present in the aquifer system of the area. The study involved the systematic sampling of groundwater samples over a period of three years (2011-2013) in three different seasons (pre-monsoon, monsoon and post-monsoon) in both the BFP and the GFP. Major ionic components, pH, oxidation-reduction potential (ORP), As and iron (Fe) were analyzed. An integrated approach utilizing hydrochemical and statistical techniques was utilized to study the As release mechanisms and the factors promoting them in both flood plains. Weathering and dissolution due groundwater recharge was found to be a major factor controlling the hydrochemistry of both the regions along with carbonate weathering. Hierarchical cluster analysis (HCA) and principal components analysis (PCA) revealed that agricultural chemicals like fertilizer usage were more common in the GFP than in the BFP. In BFP the groundwater environment was found to be strongly reducing and the main pathway of As release could be reductive hydrolysis of Fe (hydr)-oxides which is also supported by PCA and HCA. Bivariate analyses however revealed that Fe in the BFP behaved non-conservatively. The process of As release in the BFP was also found to be influenced by an enhancement in pH. The As release mechanism in GFP was found to be more complex as it showed more dependence on groundwater recharge rather than the prevailing ORP of the aquifers. Groundwater As was found to spike suddenly in the post-monsoon season of the GFP, which could be due to influence of pH assisted desorption of As from Fe (hydr)oxides which is also suggested from the statistical relations observed between the two. Hazard indices (HI) revealed that children (3-8 years) were at greater health risk due to drinking As- contaminated groundwater than adults in both floodplains. The use of shallow aquifers containing higher As levels for drinking water resulted in higher HIs.