Assessing Hazards of Arsenic Leakage in Multi-layered Aquifer System in a Part of Middle Ganga Plains in Northern India

In view of dwindling groundwater reserve and arsenic contamination in the shallow aquifers, indiscriminate pumping from the deeper aquifers is taking place at several parts of the Ganga river basin of Northern India. The severity of this uncontrolled pumping has disturbed the multi-layered aquifer hydrodynamics. The major concern is the inter-aquifer leakage through the intervening clay layers of varying thickness which has been deduced by dense geophysical measurements. The heavy pumping from the deeper aquifer (largely uncontaminated) and reduced thickness of the confining clay layer at places between the two aquifers offer a possible threat of leakage, and the arsenic-contaminated groundwater entering into the deeper aquifer, deteriorating the groundwater quality seriously. We present a groundwater flow model of a multi-layered aquifer system from a part of the Middle Ganga Plains (MGP). The simulation presented here is for understanding the hydrodynamics under stress conditions, and for assessing groundwater leakages. The model utilizes a previously 3-D aquifer geometry obtained from geophysical investigations and aquifer properties from in-situ measurements. The results reveal that the groundwater dynamics between two aquifers with increasing pumping from the deeper zone is leading to rapid depletion of the piezometric head, and possible mixing of arsenic-contaminated water to that from the shallow aquifer. This reversal in the inter-aquifer leakage has been simulated in both regional and micro-scales, and the areas vulnerable to the leakage reversal have been indicated as an indicator of arsenic hazard. These results provided safe pumping rates at a micro-scale for the marginal farmers and sites for the decision-makers to provide an alternate supply of water.