OCCURRENCE OF ARSENIC-ENRICHED GROUNDWATER BY SEDIMENT-WATER INTERACTIONS IN ALLUVIAL AUIFERS OF BRAHMAPUTRA RIVER BASIN, INDIA

The present study delineates the aquifer geochemistry and arsenic (As) enrichment in groundwater along the northwestern (NW), and northern (N) parts (located near foothills of the Eastern Himalayas) of the Brahmaputra river basin, India. The major-ion composition shows that the groundwater of NW and N region are dominated by Ca²⁺-HCO₃⁻ and Ca²⁺-Na⁺-HCO₃ hydrochemical facies, and molar ratios suggest that most groundwater solutes were derived from both silicate weathering and carbonate dissolution. They have not been affected by cation exchange. The two sites show differences in minor solute concentration indicating varying pathways of hydrogeochemical evolution. Stable isotopes (δ²H and δ¹⁸O) in groundwater indicate that some evaporation may have taken place through recharging water in the study areas. More than 60% of groundwater samples in northwestern part and 65% in northern part are enriched with As (maximum concentration 0.13 mg/L; 0.18 mg/L respectively). However, both sites have similar reducing, postoxic environments with high concentrations of total organic carbon (TOC), and saturation index calculations suggest that As is probably liberated primarily by reductive dissolution of metal oxides. Positive correlations of As with Fe, Mn and HCO₃ were observed in NW and N region aquifers. Shallow alluvial aquifers of the study sites are primarily composed of grey/brown sand (fine, medium and course). Major mineralogical composition of the aquifer sediments are analyzed by FESEM/EDX and XRD, which indicate the major presence of Fe/Mn-oxyhydroxides and aluminosilicate minerals. Thin section study with optical microscopy shows the presence of ferromagnesian minerals like biotite, olivine, pyroxene, amphiboles along with abundance of quartz, feldspars, chlorite and iron oxides. Therefore, similar mechanisms of As mobilization by reductive dissolution of (Fe-Mn)OOH with combined effect of pH dependent sorption and competitive ion exchange are observed in both tectono-morphic settings.