MODELING OF ARSENIC TRANSPORT IN GROUNDWATER USING MODFLOW

Abstract

Groundwater plays an important role in India’s economy. According to World Bank study, the contribution of groundwater to India’s GDP is estimated to be 9%. This resource has gained importance as a source of drinking water and food security. Dependence on this resource has also increased due to its availability and less risk to pollution than the surface water. Groundwater development has been intensive in alluvial areas of Indo Gangetic plains. This intensive development in many areas has given rise to a number of problems including social and public health. One such grave problem is arsenic menace in groundwater of Gangetic plains in West Bengal. The arsenic pollution in West Bengal has been described as world’s biggest calamity. Numerical modeling has emerged as an effective tool for managing groundwater resources and predicting future responses, especially when dealing with complex aquifers systems and heterogeneous formations. Groundwater flow models solve for the distribution of heads, whereas solute transport models solve for concentration of solute as affected by advection, dispersion and chemical reactions. A MODFLOW and MT3D model has been simulated to understand the movement of water and Arsenic in English Bazar block of Malda district, West Bengal. A 3-dimensional groundwater flow and contaminant transport model has been developed considering semi-confined unconfined aquifer system. vi Reduction of iron oxyhydroxide (FeOOH) and release of its sorbed arsenic load to solution is an important mechanism by which Arsenic enters groundwater. In the Bengal Basin of Bangladesh and West Bengal (India), it is the main mechanism by which arsenic pollutes groundwater. Arsenic is present in very high concentrations in certain pockets of the study area with respect to average concentration of the panchayat. These high concentration pockets have been taken as the point source of arsenic for the study area. Simulation was performed for the movement of arsenic for every 120 days for a stretch of 1080 days. The time concentration outputs of the model were recorded by observation wells placed at a distance of 1000 m for all the localized pockets. Distribution of arsenic in the different soil layers were presented in the form of contour maps for the study area at the end of every year for a time period of three years. It was observed that the distribution of arsenic in the sand and coarse sand layers was more uniform in comparison to the sandy clay layer. Arsenic concentration recorded at observation boreholes showed a marginal increase over time for the coarse sand layer. It is known that the potential aquifers in the study domain are present in the depth range of 44 - 69 m and 73 -89 m where coarse sand and gravel is encountered. Therefore, even marginal rise in arsenic concentration for the bottom most layer might be a major cause of concern for people surrounding high arsenic concentration pocket areas. Areas along the flow direction are at much greater risk to the rise in arsenic concentration than the lateral ones.