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Paper No. 283-12
Presentation Time: 11:15 AM
CHARACTERIZING THE CONTROLLING GEOCHEMICAL PROCESSES FOR SPATIAL, VERTICAL AND TEMPORAL VARIATION OF ARSENIC IN SHALLOW GROUNDWATER OF THE BENGAL BASIN
This study combined the sediment geochemistry to the high resolution hydrogeochemical monitoring study over 20 months at the two sets of piezometers (2 × 5) from the areas with high (average: 146 µg/L, n = 5) and relatively low (average: 53.3 µg/L, n = 10) dissolved As concentration in groundwater at Chakdaha Block of Nadia, West Bengal, India, to refine the understanding of the geochemical processes that regulate the spatial, vertical and temporal variation of As in shallow groundwater of the Bengal Basin. Results indicated that the lithology and sediment geochemistry were very much similar at both high and low As sites. The determination of isotopic composition of δ2H and δ18O in groundwater of the two sites indicated the recharge of evaporative surface water to the aquifer. The concentrations of major aqueous solutes and electrical conductivity were considerably higher at high As site compared to low As site. Furthermore, the stratification of groundwater composition in the aquifer was evident at high As site. This study supported the view that reductive dissolution of Fe oxyhydroxides followed by competitive ion adsorption by PO43- to the aquifer sediment is the complete processes, responsible for As enrichment in groundwater of the Bengal Basin. However, the decoupling of As and Fe enrichment in groundwater was observed in the shallowest part of aquifer (<24 m bgl) at high As site due to the Fe enrichment by weathering of silicate minerals, the precipitation of secondary mineral phases like siderite and vivianite and the incomplete reduction of Fe oxyhydroxides. The vertical distribution of As in groundwater was possibly related to the redox zonation within the aquifer. The seasonal variations of As and other aqueous solutes were limited within the upper portion of aquifer only (<30 m bgl) and might be linked to the seasonal cycling of redox status, aggregation and dispersion of As scavenging colloids, local groundwater abstraction and monsoonal recharge.