Ground water geochemistry of Ballia district, Uttar Pradesh, India and mechanism of arsenic release.

Threat to human health worldwide due to the natural contamination of arsenic in ground waters has led to extensive studies on factors controlling the distribution of arsenic and conditions leading to arsenic mobilization in different arsenic contaminated areas. Another aspect of the arsenic crisis, especially in South Asia, is the degree of spatial variability of ground water arsenic concentrations. Thus it becomes necessary to study the source and the processes involved in arsenic mobilization into ground water under such conditions. An arsenic contaminated area namely, Ballia district of UP was chosen for this study. A set of 56 samples were collected from India Mark II hand pumps (30-33 m depth) thrice in a year namely pre-monsoon (April '07), monsoon (July '06) and winter seasons (December '06). Nine samples were also collected from deep bore well hand pumps (66-75 m) to study the difference in geochemistry with the shallow pumps. Various water quality parameters like As(III), As(V), sulfate, nitrate, phosphate, bicarbonate, ammonia, were determined. Arsenic concentrations ranged from 0 to 468 microg L(-1) in ground water collected from depths of 30-33 m. In the deeper wells (66-75 m), arsenic concentrations ranged from 12 to 20 microg L(-1). Most samples contained both As(III) and As(V) and the concentration of As(III) was generally equal/higher than As(V). Not much variation of arsenic concentration was observed when sampled in summer, monsoon and winter seasons. Correlation studies among various water quality parameters revealed that reductive dissolution of FeOOH was the most probable mechanism for release of arsenic.