Mass transport modelling to assess contamination of a water supply well inSabarmati river bed aquifer, Ahmedabad City, India

Drinking water supply wells were constructed in the Sabarmati river bed aqu ifer of Ahmedabad city using radial pipes and are known as French Collector wells. Contamination of groundwater from one of the French wells near Saba rmati railway bridge was noticed in 1992. The suspected pollution sources a re Duff-nala of Shahibaug and two other sources from slum dwellings on eith er side of Sabarmati river. A combined groundwater flow, pathlines and a ma ss transport model was constructed covering an area of 9 km(2) to analyse t he capture zone of the French well under two different scenarios. Aquifer p arameters of the river bed aquifer were available. Dry river bed condition was simulated under scenario I and controlled flow in the river bed was sim ulated under scenario II. The groundwater velocity and migration of contami nant particles from sources was analysed in the pathline model. Total disso lved solids (TDS) concentration contours originating from sources in the ma ss transport model (MT3D) were computed by solving an advection-dispersion equation. The computed pathlines and TDS concentration contours indicate li kely migration of contaminant plume from pollutant sources to the French we ll during 365 days under two scenarios. The model results confirm the trace r injection studies carried out to know the likely migration of contaminant s towards the French well. The modelling study emphasised the necessity of controlled release of surface water in Sabarmati river bed from Dharoi rese rvoir throughout the year.