Multi-isotopic and hydrochemical evidence of water resources evolution and recharge estimation in the tropical coastal aquifer

The study integrates hydrochemical and isotopic (518O, 52H, 3H, and 513C) techniques to investigate water evolution in the North Kelantan River basin. Groundwater facies were mainly classified as Ca-Mg-Cl, Ca-Mg-Cl-HCO3, and Na-Cl during the rainy season and shift toward Ca-Mg-HCO3 and Na-Ca-HCO3-Cl during the dry season in surface water and shallow aquifer, while facies in intermediate and deep aquifers were classified as Na-HCO3 and Na-Cl. The 518O and 52H compositions of most groundwater samples have not deviated significantly from NKMWL (52H = 8.4 518O + 11.5) with slightly depleted isotopic values due to the humid climate. Isotopically, evaporation does not impact recharged water significantly. However, shallow aquifers are slightly enriched than deep aquifers of the study area. The estimated recharge was 20.17% and 22.52% of annual rainfall based on the CMB and 518O values, respectively. The recharge mainly occurs during the wet season and is influenced mostly by the amount of rain. Aquifers clustered in distinct groups based on their isotopic signatures and hydrochemical results. The decomposition of organic matter is the primary carbon source in the study area. Building on this, by utilizing isotope hydrology to study water resources, we can gain a deeper insight into the susceptibility of groundwater in coastal aquifers of monsoonal tropical humid regions.