ARSENIC IN GROUNDWATER OF THE AQUIFERS OF CENTRAL GANGETIC PLAIN OF UTTAR PRADESH, INDIA

Arsenic (As) has recently been detected in groundwater of Central Gangetic Plains (CGP) of northern India in the states of Uttar Pradesh, Bihar and Jharkhand. The levels of As in groundwater exceed the WHO drinking water guideline (10 µg/L) and the limits of the drinking water standard (50 µg/L) prescribed by Bureau of Indian Standards (BIS) resulting in a severe health risk for a population of ca 100 million in the region. In this paper, we present the preliminary results from our ongoing studies on the salient hydrogeochemical characteristics of the groundwaters of the sedimentary aquifers of the CGP from the districts of Ballia and Ghazipur in eastern Uttar Pradesh, India and their implications for mobilisation of As in groundwater. A total of 61 water samples were collected from wells placed in shallow (< 35 m), intermediate (35-50m) and relatively deep (> 50 m) aquifers and analyzed for major ions and trace elements including As and Fe. Groundwater pH (7.5-8.2) indicates a circum-neutral to alkaline character with moderately high redox status (+0.12 to +0.70 V). The major ion characteristics reveal that the groundwaters are of Ca-Mg-HCO₃-type with considerable variations in the concentrations of HCO₃^- (25-551 mg/L), Cl^- (5-500 mg/L), NO₃^- (<1-120 mg/L) and SO₄^2- (15-379 mg/L). The total As concentrations are high in the wells of both Ballia and Ghazipur districts, ranging between 13-259 µg/L and 6-164 µg/L respectively. The concentrations of total Fe is low, 0.01-0.27 mg/L in Ballia and 0.04-1.94 mg/L in Ghazipur, and the corresponding PO₄^3- levels are low (below detection levels to 0.09 mg/L). Distinct peaks of As concentrations are observed in the samples at depths of 10-20 m, and 30 m in Ballia, whereas in Ghazipur the affected wells seem to be placed at depths of 10-20 m as well as 30-60 m. These aquifers seem to be particularly in risk, due to the prevailing geochemical conditions in which oxidized and reduced waters mix. The redox status is fairly elevated that limits the microbial reduction of sulfate, nitrate and Fe. Detailed studies are in progress to delineate the the various geochemical processes controlling the As concentration in groundwaters in the CGP.