Modelling the impact of a subsurface barrier on groundwater flow in the lower Palar River basin, southern India M. Senthilkumar & L. Elango Abstract Groundwater modelling is widely used as a management tool to understand the behaviour of aquifer systems under different hydrological stresses, whether induced naturally or by humans. The objective of this study was to assess the effect of a subsurface barrier on ground- water flow in the Palar River basin, Tamil Nadu, southern India. Groundwater is supplied to a nearby nuclear power plant and groundwater also supplies irrigation, industrial and domestic needs. In order to meet the increasing demand for groundwater for the nuclear power station, a subsurface barrier/dam was proposed across Palar River to increase the groundwater heads and to minimise the subsurface discharge of groundwater into the sea. The groundwater model used in this study predicted that groundwater levels would increase by about 0.1–0.3m extending out a distance of about 1.5– 2km from the upstream side of the barrier, while on the downstream side, the groundwater head would lower by about 0.1–0.2m. The model also predicted that with the subsurface barrier in place the additional groundwater requirement of approximately 13,600m 3 /day (3 million gallons (UK)/day) can be met with minimum decline in regional groundwater head. Keywords Numerical modelling . India . Coastal aquifer . MODFLOW . Subsurface barrier Introduction Groundwater is a major source of freshwater that is widely used for domestic, industrial and agricultural purposes in most parts of the world. Increasing demand for ground- water due to ever-increasing population has highlighted the importance of using scientific techniques to properly manage this precious resource. Demand has created a need for proper and effective management of available ground- water resources. Groundwater modelling is a powerful management tool that can serve multiple purposes such as providing a framework for organising hydrological data, quantifying the properties and behaviour of the systems and allowing quantitative prediction of the responses of those systems to externally applied stresses. Three-dimen- sional groundwater models are usually very effective groundwater management tools. Many groundwater mod- elling studies have been carried out around the world for effective groundwater management (Corbet and Bethke 1992; Gnanasundar and Elango 2000; Senthilkumar and Elango 2001 and Strom and Mallory s1995). Groundwater modelling had already been carried out for the lower Palar River basin, southern India (Fig. 1a), by Senthilkumar and Elango (2004). The purpose of this earlier study was to develop a groundwater flow model to effectively manage the aquifer system, as a remedy for over-dependence on groundwater for domestic, irrigation and industrial purposes. Industrial abstraction includes the water supply for Madras Atomic Power Station (MAPS), which is located 1 km north of the study area, on the coast. The earlier study indicated that the current level of abstraction from the aquifer would be sustainable. How- ever, there are plans to expand MAPS, which would involve an increase in groundwater pumping by approx- imately 13,600 m 3 /day, 3 million gallons (UK) per day (MGD), in this area. Senthilkumar and Elango (2004) showed that this would lead to a decline in the ground- water levels in this area, which would in turn affect farmers’ irrigation wells and other pumping stations. They also showed that even if the abstraction increase were limited to pumping 2 MGD (∼9,000 m 3 /day) this would still affect the region to the east of the MAPS pumping station (Fig. 1a), which may also lead to seawater intrusion. Furthermore, the regional groundwater levels indicate that groundwater flows into the sea for most of the year. In order to meet the increased demand for groundwater, it was proposed that a subsurface barrier/dam be built across Palar River (Fig. 1b) in this area, with the aim of increasing the groundwater availability and minimising Received: 4 November 2009 /Accepted: 1 April 2011 * Springer-Verlag 2011 M. Senthilkumar Central Ground Water Board, South Eastern Coastal Region, Chennai, 600090, India L. Elango ()) Department of Geology, Anna University, Chennai, 600025, India e-mail: elango34@hotmail.com e-mail: elango@annauniv.edu Hydrogeology Journal DOI 10.1007/s10040-011-0735-0