[A-HW34] Isotope Hydrology 2020

[AHW34-04] Natural and anthropogenic impact of groundwater in sandy aquifer systems in Kalpitiya Peninsula, Sri Lanka

★Invited Papers

*Rohana Chandrajith¹, Kaushalya Jayathunga¹ (1.Department of Geology, University of Peradeniya, Sri Lanka)

Keywords:water isotopes, coastal aquifers, seawater intrusion, mass balance calculations

Coastal aquifers are important sources of freshwater but highly vulnerable to salinization, mainly due to seawater intrusion and anthropogenic activities. We investigated the Kalpitiya Peninsula in Sri Lanka, with an area of 160 km², to identify groundwater intrusion and effects of agriculture on coastal aquifer systems. Groundwater in the region is an important resource since no other freshwater sources are available. Major and trace element content in 43 groundwater samples were measured alone with δ¹⁸O, δ²H and δ¹³C_DIC contents. The groundwater in the peninsula was dominated by Na⁺, K⁺, Ca²⁺, Mg²⁺, NO₃-N, HCO₃⁻, Cl⁻ and SO₄²⁻. The nitrate concentrations recorded up to 186 mg/L which is well above the WHO recommended limits, providing evidence for anthropogenic impact, most likely from agriculture activities. Among the toxic trace elements, As and Se levels in groundwater were also higher compared to that of the dry zone metamorphic aquifers in Sri Lanka. The solute and water isotope contents modified by irrigation return flows and associated evaporation. This was confirmed by the stable isotope composition of groundwater that deviated from the local meteoric water line (LMWL) and formed its own regression line denoted as the local evaporation line (LEL) denoted by δ²H= 5.3δ¹⁸O - 3.75. The corresponding δ¹³C_DIC values varied from −13.8‰ to −8.0‰. These isotopes are more typical for carbonate dissolution and equilibration of CO₂ in the aquifer. These factors were further confirmed by multivariate statistical analyses. Seawater intrusions were more prominent in the northern part of the peninsular, while agricultural contributions were more obvious in the southern part. Established mass balance calculations revealed that local groundwater had saltwater admixtures of up to 12 %. Our results indicate that integrated water management should critically monitor water resources in the Kalpitiya peninsular in order to avoid overexploitation and further seawater inflows.