REGIONAL HYDROCHEMICAL STUDY OF GROUNDWATER ARSENIC CONTAMINATION ALONG TRANSECTS FROM THE HIMALAYAN ALLUVIAL DEPOSITS TO THE INDIAN SHIELD, CENTRAL GANGETIC BASIN, INDIA

Although intensive studies of groundwater arsenic (As) contamination have been conducted in the Bengal (Ganges-Brahmaputra) Basin, information on the regional As distribution in adjoining upstream regions of the Ganges, which could provide insights into provenance and mobilization mechanisms, are extremely limited. Groundwater (66) and surface-water (12) samples were collected along two transects in the central Gangetic plain (states of Bihar and Jharkhand), which extend from the Quaternary Himalayan alluvial fans and plains (Indo-Nepal border) to the pre-Quaternary (Precambrian to Cretaceous) Indian cratonic, igneous and metasediment shield provinces.

Groundwater samples were collected from shallow aquifers (well depths £ 137 m, but mostly 8 to 60 m) and surface-water was collected from the Ganges and its Himalayan (northern) and shield- (southern) originated tributaries. Preliminary results indicate that As concentrations are much higher in groundwater collected from the youngest alluvial terraces (~50% samples have ³0.01 mg/L [WHO Maximum Contaminant Level] As, maximum 0.52 mg/L), than those associated with the shield (all £ 0.005 mg/L). Most (87%) of the As is present as As(III). Redox conditions are highly spatially variable (oxic to methanic, dominated by metal-reduction), with no systematic depth variation. Coexistence of As(III), Fe(II), NH_3(dis), elevated HS^- and CH_4(dis), and other redox sensitive parameters delineate reduced environments, although As is not linearly correlated with these individual solutes. Dissolved oxygen (DO) concentrations were generally <0.5 mg/L in reduced groundwater, however, some of the highest As concentrations were detected in groundwater with DO ~1 mg/L. These initial results suggest much higher groundwater As contamination in aquifers with sediment derived from the Himalayas than in aquifers with sediment derived from the cratonic shields.