Geochemical and isotope characterization of groundwater and assessment of surface water mixing in the coastal karst aquifer basin in northwestern Sri Lanka

Sinkholes play a critical role in groundwater systems by facilitating both recharge and discharge of groundwater, serving as indicators of underlying geologic processes, shaping subsurface hydrology, and influencing water quality. Therefore, studying karst groundwater systems is essential for effective groundwater management, especially in areas with limestone sequences. Spatial variations of water isotopes (δ²H, δ¹⁸O), dissolved inorganic and organic carbon (DIC and DOC), including their isotopes alone with pH, EC, alkalinity and chloride were determined in surface- and groundwater of the Mulankavil karst aquifer basin in the northwest of Sri Lanka. Twenty-seven groundwater wells, one offshore spring and two sinkholes were selected for sampling. The contribution of sinkhole water and seawater to karst aquifers was calculated using δ¹⁸O and chloride as tracers. In addition, deuterium excess (d-excess) values were used to determine groundwater evaporative loss before infiltration. Near the Mulankavil sinkhole area, the mixing of surface water and infiltrating rainwater was found to vary between 26 and 60%, with an average of 56%. In contrast, surface water mixing in the Nagapadduvan and Vellankulam sinkholes was about 41–59% and 33–70%, respectively. In near coastal wells, these values fell to 20% due to the influence of seawater intrusion. During the dry season, groundwater d-excess varied between −5.0 and + 10.8‰, while surface water in sinkholes showed values between +1.8 and −9.4‰. Groundwater wells near the Mulankavil sinkhole showed lower DIC (<5.5 mmol/L) with enriched δ¹³C_DIC (>-14‰). The karst aquifer systems in northern Sri Lanka exhibit two main recharge mechanisms: (a) selective recharge by surface water at sinkholes and (b) diffuse recharge by direct rainfall over the study area. Seawater intrusion near the coast can be prevented by artificially recharging rainwater during the monsoon season.