Recharging Mechanisms and Seawater Intrusion in Karst Aquifers in Northwest Sri Lanka, Based on Hydrogeochemistry and Water Isotopes

• Published in: ACS ES&T water 2023-06, Vol.3 (6), p.1678-1686
• Main Authors: Bandara, U.G. Chaminda, Diyabalanage, Saranga, Barth, Johannes A.C., Chandrajith, Rohana
• Format: Article
• Language: English
• ISSN: 2690-0637; 2690-0637
• DOI: 10.1021/acsestwater.2c00454

Geochemical and isotope data of karstic aquifer systems in the Mulankavil area in northwest Sri Lanka were investigated to constrain recharging and discharging mechanisms, and possible seawater intrusions. The electrical conductivity (EC) of groundwater was measured in the dry (n = 139) and rainy (n = 78) seasons to assess possible saltwater intrusions, from which 25 wells were selected for detailed geochemical analyses. Water isotope analyses (expressed as δ2HH2O, δ18OH2O) were measured in 32 wells for five months. The mean EC of groundwater during both seasons was 2,435 μS/cm, and higher values from the coastal stretch indicated seawater intrusion. Increasing EC values in the dry period indicated a temporary inland saltwater wedge. High hydraulic gradients with rapidly discharging groundwater partially removed seawater intrusions during the monsoon season. Water isotopes in the region defined an individual evaporation line with δ2HH2O= 6.69*δ18OH2O −0.77 (r 2 = 0.989), indicating a long enough residence time to allow evaporation before infiltration. Groundwater near sinkhole systems was found to be predominantly of the Ca–Mg–HCO3 type and changes to the Na–Cl type toward the coast. The results of this study revealed conduit systems in the karst subsurface as essential pathways for controlling seawater intrusion.