The impact of hydrogeological settings on geochemical evolution of groundwater in karstified limestone aquifer basin in northwest Sri Lanka

摘要

The geochemical and isotope analysis of groundwaters from the Murunkan basin in north western Sri Lanka was carried out to examine their evolution and recharge sources in order to shed light to enhance the current knowledge of the hydro-geochemical processes in a karst geological setting. A total of 40 water samples from ground and surface water bodies were collected from the Miocene limestone terrain, nearby metamorphic and from unconsolidated Quaternary terrains for major anions, cations and environmental isotopes ratios (delta O-18(VSMOW) and delta H-2(VSMOW)). Distinct geochemical differences were noted between waters from limestone terrain and nearby metamorphic terrain indicating the modification of groundwater flow paths. Bicarbonate-chloride rich water is dominated in the limestone terrain in which water flows through a less mineralized aquifer system and is modified by the sea water intrusion. Groundwater in the metamorphic terrain is modified by dissolving of Ca-Mg rich mineral phases and subsequent ion exchange processes. The environmental isotopes of groundwater from both limestone and metamorphic terrains vary from -0.38 to -6.68 aEuro degrees and -2.41 to -42.3 aEuro degrees for delta O-18(VSMOW) and delta H-2(VSMOW), respectively. However, slightly enriched isotope signatures and low d-excess values from limestone terrain indicate an excessive evaporation compared to that of the metamorphic terrain where rapid infiltration occurs through the overlying highly permeable grumusols soil layers.