Hydrogeological and Hydrochemical Investigations for Coastal Saline Aquifer Management: A Field-Based Managed Aquifer Recharge Study

Abstract

Amid intensifying water scarcity and heightened vulnerability of coastal aquifers to saltwater intrusion, the prudent adoption of Managed Aquifer Recharge (MAR) surfaces as a promising approach for effective water management in coastal zones. This study aimed to evaluate the applicability of MAR in the coastal aquifer to better understand the dynamic freshwater-saline water interactions. For this, a series of field experiments were conducted on the coastline of the Arabian Sea located in Kerala State of India. A comprehensive Hydrogeological and Hydrochemical approach is employed to discern salinity patterns within coastal aquifers, with the aim of facilitating sustainable water resource management through MAR. Initially, four vertical electrical soundings (VES) were conducted to get the resistivity variation along the coastline. The results unveiled distinct subsurface conditions. Towards the north, a narrow freshwater zone was identified overlying saline water. Conversely, in the southernmost region, signs of saltwater intrusion were observed, with highly saline groundwater present at shallow depths. Subsequently, groundwater characterization within a 5-kilometre stretch revealed total dissolved solids (TDS) concentration ranging from 80 to 18810 ppm with an average of 4244.74 ppm. While electrical conductivity (EC) ranged from 0.17 to 38.37 mS/cm, with an average value of 8.66 mS/cm. These findings reflect the non-uniform distribution of salinity due to saltwater intrusion in the region, posing significant challenges to the utilization of freshwater resources. Based on these observations, three piezometer wells were drilled in proximity to the high-salinity zone within the unconfined aquifer. These comprised one injection well and two observation wells.  Three different volumes viz. 2500L, 5000L and 7500L were injected in the injection well at a similar recharge rate. The results indicated that higher injection volumes led to significantly increased freshwater storage duration: 178% for 5000L and 500% for 7500L compared to 2500L. Results also indicate that MAR has the potential to customize the groundwater gradient, mitigating saltwater intrusion in freshwater zones to some extent in the short term. However, concerns arise about the long-term viability of MAR in highly saline coastal areas, particularly due to higher diffusion, buoyancy effects, and mixing of freshwater and saltwater observed in the sandy aquifer.