Enhancing groundwater quality in a saline coastal aquifer through managed aquifer recharge: A comprehensive study by long-term groundwater level and hydrochemical monitoring

Abstract

Safe drinking water has become scarce in many parts of the world, especially along the coast. Despite adequate rainfall in many coastal regions, over-exploitation has led to the threat of seawater intrusion. Induced recharge enhances groundwater storage and can mitigate seawater intrusion, especially in coastal areas with over-exploited freshwater resources. This study investigates the hydrogeochemical processes due to induced recharge by managed aquifer recharge (MAR) structure and its impact on improving groundwater quality in a saline coastal aquifer. As part of this study, an extensive analysis of hydrogeochemical processes was conducted within a combined MAR structure featuring a percolation pond and recharge shaft, along with the nearby observation piezometers from 2012 to 2019. Over 300 water samples from the MAR structure and the piezometers were analysed to assess the enhancement in groundwater storage and quality in the nearby area. The electrical conductivity of groundwater was reduced from 1801 to 149 μS/cm in the MAR structure and from 2627 to 183, 41,212 to 9127 and 70, 345 to 2933 μS/cm, respectively, in the piezometers P1, P2 and P3 drilled up to different depths. The major hydrogeochemical facies in MAR structure and piezometers are Na⁺-Cl^- and Ca²⁺-Cl^- types, indicating the influence of seawater mixing and base exchange of Na⁺ and Ca²⁺ ions. The principal component analysis identified three significant components in pond and piezometers, contributing to a corresponding total variance of 90.85 % and 97.82 % with higher factor loadings for Na⁺, Cl⁻, and K⁺. Considering the benefits and the economic feasibility of these MAR structures, they are strongly recommended in other regions with sufficient rainfall and salinity-related challenges.