Deep groundwater recharge mechanism in the sedimentary and crystalline terrains of Sri Lanka: A study based on environmental isotope and chemical signatures

Abstract

Springs discharge groundwater that travels along flow paths of different spatial and time domains. In many instances, it is difficult to describe its involved groundwater movement, availability as well as geochemical evolution due to its hidden nature. The present study has assessed the deep groundwater in the form of thermal and non-thermal springs in the Palavi and Vanathavillu basins of sedimentary terrain as well as the Southern, Eastern, and Eastern-North Central lowlands of the crystalline terrain of Sri Lanka, using isotope and geochemical approaches. The results revealed that cation exchange in the sedimentary aquifers, whereas rock-water interaction in hard rock and fairly deep aquifers, are the important factors that control the groundwater chemistry. Artesian wells in the sedimentary terrain are mainly recharged by meteoric water, moving through fractured basement rocks at elevations from 100 to 200m above sea level, and have a long residence time. Despite its proximity to the West coast, the data acquired do not indicate that there is seawater mixing. Mixing of percolated water through an unconsolidated geological matrix appears to promote an increase in the salinity of groundwater. Non-mixed, non-evaporated, and young groundwater with a recharge elevation of more than 1000 m above mean sea level can be found in the Southern lowlands of the crystalline terrain. In the Eastern North Central lowlands, groundwater is lowland originated, fast replanished and significantly mixed with evaporated surface water. Thermal waters do not contain tritium and therefore are older than 50 years and have deeply percolated. Artesian water in the Eastern region is high altitude originated and intensely rock-water interacted. Some hot springs in the same area emerging through weathered overburden have water significantly mixed with recent precipitation. Thermal springs that emerge through quartzite bedrock have a non-mixed, non-evaporated, and less rock-water interacted nature. Both thermal and non-thermal water with artesian conditions have clearly indicated that they originate from a common recharge source but with different flow paths, penetration depths, and travel distances, resulting in different chemical characteristics. Non-thermal springs are recharged by recent precipitation and have percolated only to shallow depths.