Theme 3-16-Isotope hydrological investigation in arsenic infested areas of West Bengal (India)

Abstract

The presence of arsenic in groundwater is an environmental problem of considerable importance. About a million people living over an area of 37,000 km2 in West Bengal, which is located in Indo-Gangetic plain, consume arsenic laced drinking water. In this area, arsenic levels in groundwater have been found to be 0.01 mg/L to 3.0 mg/L. Environmental isotope techniques have been applied to study the origin, source, residence time and the dynamics of arsenic contaminated and uncontaminated aquifers in Murshidabad, Nadia and North and South 24-Parganas districts in West Bengal. Groundwater samples from different depth and a few surface water samples have been collected, during post and pre-monsoon period in 1996 - 1999 and analysed for 2H, 18O, 13C, 34S, 3H and 14C isotopes as well as arsenic concentration. Results show high arsenic content (0.01 to 1.3 mg/L) in shallow aquifer (Depth: <100m ) compared with deep aquifer in Murshidabad and Nadia districts. stable isotopic (2H and 18O) contents of these groundwaters show contribution of local precipitation as well as river water. They have modern tritium and carbon -14 content. In deep aquifer (Depth: >100m), arsenic concentration is within the permissible limit and their stable isotope, tritium and carbon-14 contents indicate possible interconnection between shallow and deep aquifer.

In North and South 24-Parganas districts, shallow aquifer shows very high arsenic (0.01 to 3.0 mg/L) concentration. In South 24 - Parganas, shallow groundwaters are more depleted in stable isotopic content(2H &18O) compared with deep groundwaters. In both cases, tritium and carbon -14 values of the shallow groundwaters indicate the precipitation input, and possibly they are a mixture of young and old groundwater. In deep aquifer, arsenic is within the permissible limit. They have low tritium and carbon -14 contents indicating that they are old waters.

The overall isotope hydrological studies indicate that in Mursidabad and Nadia districts, there is a possibility of interconnection between shallow and deep aquifers and receive recharge from River Ganga as well as local precipitation. Surface and deep groundwaters are free from arsenic. Over-exploitation of shallow aquifer may make deep aquifer also vulnerable to arsenic contamination. In North and South 24-Parganas districts, the chances of interconnection between shallow and deep aquifers are remote. Shallow aquifer is a mixture of old and young water, whereas deep aquifer is old water. The elevated arsenic concentration in groundwaters of Murshidabad, Nadia, and North and South 24 Parganas is possibly released from the sediment under anoxic condition in the aquifer.