Spatial and seasonal variation of groundwater geochemistry in high-grade metamorphic aquifers- implications for CKDu in Sri Lanka

Abstract

Two-thirds of the island of Sri Lanka is considered dry land due to limited rainfall. At the same time, these regions are predominantly underlain by high-grade metamorphic rocks with limited water storage capacity. Widespread chronic kidney disease of unknown etiology (CKDu), which is considered to be influenced by groundwater geochemistry and related factors, is reported in this part of the island. This study, therefore aimed to investigate the geochemical composition and seasonal variations of groundwater in metamorphic aquifers to elucidate possible relationships with the occurrence of CKDu in the dry climatic region. For this study, samples from sixty-six groundwater wells, five surface water sources and one natural spring were analyzed for major anions, cations, trace elements and stable isotopes (δ²H and δ¹⁸O) during pre- and post-monsoon periods. The results showed that the major ions in groundwater mainly derived from the weathering of silicate and carbonate minerals, while seasonal fluctuations significantly influence the geochemical composition. High ionicity in groundwater was found in the pre-monsoon period compared to the post-monsoon. Groundwater isotope compositions confirmed extensive evaporation and possible mixing with surface water during the dry period. Compared to non-CKDu clusters, EC, HCO₃^– and TH were significantly higher in CKDu hotspots, where over 70 % of samples exceeded safe drinking water limits in both sampling seasons. The co-occurrence of significantly high dissolved-Si (mean = 44 mg/L) and F⁻ (mean = 0.90 mg/L) levels in hard water (mean = 340 mg/L) showed the potential risk of their combined effect for CKDu incidence in hotspots. The study showed that groundwater in CKDu hotspots is not suitable for direct consumption due to exceeding geochemical contaminants compared to their standards. Therefore, it is recommended to improve groundwater quality and make it suitable for consumption, possibly through artificial aquifer recharge and water treatment methods. This study highlights the potential groundwater-related risk factors for CKDu in the dry zone, demonstrating that groundwater geochemistry is significantly influenced by seasonal fluctuations and underlying geological characteristics in the region.