Aquifer characterization and hydrogeological modeling for devising groundwater management strategies for the Chennai aquifer system, southern India

The Chennai aquifer system, which occupies an area of 6629 km2, is one of the most stressed aquifer systems in southern India and is under severe threat of over exploitation and quality deterioration. This is due to the increasing groundwater abstraction for irrigation, domestic, industrial purposes and for drinking water supply to the ever-expanding Chennai city. To offset the effect of this heavy extraction a paradigm shift towards groundwater management was imperative. A multidisciplinary integrated approach was used to map the aquifers, delineate their geometry, to determine the hydraulic behavior of the aquifer system, and to formulate an aquifer management plan through the development of a groundwater flow model. The main aquifers in the area include weathered and fractured crystalline rocks and recent alluvial formation. Alluvium is the most significant aquifer system in the study area, and this aquifer contains potable quality groundwater except in the eastern part of the study area that has been affected by seawater intrusion. A two-layered groundwater flow model was developed using Visual MODFLOW classic version 4.6 with a 1 km2 grid pattern to simulate groundwater flow for a period of 9 years. The model was calibrated under steady and transient state conditions and allowed components of the water balance of the system to be determined at a regional scale. The simulated results indicate that this aquifer system is under tremendous stress at the prevailing groundwater withdrawal rate of 899 million cubic meter (mcm)/year and would become unstable with the predicted 25% increase in groundwater withdrawal by 2025. However, the interventions to recharge an additional 54 mcm of water could help mitigate the current decline in potentiometric heads and could partially help to arrest the further advancement of seawater intrusion. A scenario of maintaining flow in rivers for a period of 120 days each year coupled with the construction of an unlined canal shows increase in groundwater head and development of the groundwater mounds, which are positive signs for arresting the decline of the water table and pushing saline groundwater in a seaward direction. As a result of the high rate of groundwater depletion in the area, management strategies need to be implemented urgently in the region. These strategies should include the regulation of groundwater abstraction and maintaining an extended flow period in the rivers. These measures are required to improve the sustainability of the available groundwater resources of the region.