Influences of seawater intrusion and anthropogenic activities on shallow coastal aquifers in Sri Lanka: evidence from hydrogeochemical and stable isotope data

Abstract

Water supplies in coastal aquifers throughout the world are often threatened by salinization due to seawater intrusion and anthropogenic activities. In the Kalpitiya Peninsula in Sri Lanka, agricultural and domestic water supplies entirely depend on groundwater resources extracted from unconfined Holocene sandy aquifers. To differentiate the effects of seawater intrusion and agriculture on the coastal aquifers of this 160 km² peninsula, 43 groundwater samples were collected. These samples were analyzed for major ions, trace elements, and stable isotopes of water (δ¹⁸O and δ²H). The solute compositions were dominated by Cl⁻, ${\mathrm{SO}}_4^{2-}$, and ${\mathrm{HCO}}_3^{-}$, which were mostly balanced by Ca²⁺, Na⁺, and Mg²⁺. Among the four main water types, Na⁺-Cl⁻ and Ca²⁺-${\mathrm{SO}}_4^{2-}$ classifications were predominant in the investigated aquifers. Modifications of the groundwater due to evaporation during irrigation activities, but also due to seawater intrusion seem most plausible as indicated by the correlation of δ¹⁸O with δ²H (δ²H = 5.51 * δ¹⁸O-3.08, r = 0.93) deviating from the local meteoric water line. Particularly in the southern part of the peninsula, Mg²⁺/Ca²⁺ ratios and stable isotopes of water attributed salinization of groundwater to agricultural activities. However, especially in the north, seawater intrusions were also evident. Established mass balance calculations revealed that local groundwater had seawater admixtures of up to 12%. Our results indicate that integrated water management is essential and water resources should critically monitor in the Kalpitiya Peninsula in order to avoid over-exploitation and further seawater inflows.