Groundwater Contamination and Mitigation Strategies in Coastal Aquifers: A Modeling Approach for the Varahanadhi Sub-Basin

Coastal aquifers serve as crucial freshwater reserves for domestic, agricultural, and industrial use; however, rapid urbanization and overexploitation have disrupted their natural equilibrium and accelerating seawater intrusion and deteriorating groundwater quality. The study area, Varahanadhi sub-basin is itself a witness; although it is very critical for their socio-economic and hydrogeological significances. This study utilized bromide distribution to understand the extent and dynamics of seawater intrusion by the numerical groundwater flow and solute transport modeling by using MODFLOW and MT3DMS software with the integration of hydrogeological, groundwater level, and water quality data from 2020 and 2023. Simulation results revealed higher bromide concentrations in the coastal zones, confirming active seawater intrusion and the variable-density flow modeling approach, effectively captured the influence of pumping, recharge, and aquifer properties on solute migration. Therefore, to mitigate this problem three artificial recharge strategies; recharge ponds, recharge borewells, and recharge shafts were tested through simulation scenarios. Among these, the recharge shaft emerged as the most effective intervention, reducing bromide levels from 10 mg/L to 0.1 mg/L in shallow aquifers and from 10 mg/L to 1 mg/L in deeper layers. This indicates its strong capacity to restore hydraulic balance and restrict the lateral migration of saline water. The comparative performance of recharge structures demonstrates that the efficiency of mitigation strongly depends on the recharge rate, infiltration depth, and hydraulic connectivity of the aquifer system. These findings establish a scientific framework for coastal aquifer restoration, demonstration that recharge shafts are the most efficient and scalable solution to mitigate seawater intrusion in the Varahanadhi sub-basin. Thus, the numerical modeling effectively elucidates aquifer behavior under saline stress and guides sustainable management strategies.