Hydrological interactions between surface water and groundwater in ancient manmade village tank cascade systems (VTCSs) in the dry zone of Sri Lanka.

Sri Lanka, a tropical island, confronts climate-driven water scarcity and is of great concern to building climate-resilient water management to achieve UN SDGs 6 and 13. This study explores the dynamic interaction between surface water (SW) and groundwater (GW) in the dry zone of Sri Lanka, employing a multi-tracer of dual stable isotopes (18O and 2H) and chloride. Two basins, Mahakanadarawa (MK) basin with numerous village tanks and tank cascade systems (VTCSs/TCSs) and Kawudulla (KW) basin serving as a control with a lack of tanks, were selected in this study. Results show that the presence or absence of TCSs emerges as a pivotal factor influencing regional water dynamics. Water quality (TDS and Cl-) in natural waters between the two basins was significantly different (ANOVA: p < 0.05) in both seasons. Statistically uniform dissolved mineral content in tanks and shallow groundwater in the MK basin contrasted with significant regional deviation in the KW basin. ANOVA and Tukey tests showed significant seasonal differences (p < 0.05) in mean values of ẟ2H and ẟ18O compositions of shallow groundwater and tank water within the MK basin, while non-significance in the KW basin. Isotopic mass balance calculations revealed that tank water mixing in the shallow groundwater (fT: mixed Tank water mass fraction) ranged from 0.9 % to 77.8 % across the MK basin depending on the regional soil characteristics. Lack of tank-aquifer interconnection observed in the shallow soil in rock knob plain results in groundwater depletion while enhanced interconnections within alluvial sediment regions encourage stable and pollution-resistant shallow groundwater bodies. The consistency of the groundwater flow system in TCS regions throughout the year ensures a stable water supply, highlighting the vital synergy between TCSs and regional groundwaters in these arid regions. These results shed insights for policymakers and water managers to implement effective conservation strategies for rehabilitation and restoring these ancient VTCSs/TCSs.

Chemical characteristics and water stability evaluation of groundwater in the CKDu Zone of Sri Lanka.

Groundwater is the main source of drinking water for the rural population in the chronic kidney disease of unknown etiology (CKDu) zone of the North Central Province (NCP) in Sri Lanka. In this study, a total of 334 groundwater samples (311 dug wells, 21 tube wells and 2 springs) during the wet season from two aquifers in the NCP were collected, and investigated their chemical characteristics and evaluate their water quality, including groundwater chemistry, main ion sources, the corrosion and scaling potential of groundwater. The results showed that the two hydrochemical types of groundwater in the NCP were mainly of the Ca-HCO3, Na·Ca-HCO3 types, with the main HCO3-, Na+ and Ca2+ ions in both types of groundwater originating from silicate and evaporite salt dissolution and influenced by alternating cation adsorption, while the presence of NO3- was mainly anthropogenic. Evaluation of water stability using namely Langelier saturation index (LSI), Ryznar stability index (RSI), Puckorius scaling index (PSI) and Larson-Skold index (LS), indicated that most groundwater presents corrosion potential and has corrosion behavior tendency of metals to some degrees. The water quality of Polonnaruwa was better than that of Anuradhapura in the NCP, and when the groundwater was worse than the "good" grade, which must be properly treated before it is used as drinking water.

Fluorescence characteristics and source analysis of DOM in groundwater during the wet season in the CKDu zone of North Central Province, Sri Lanka.

The dissolved organic matter (DOM) should be purified for safe drinking water due to disinfection by-products (DBPs) produced by disinfectants reaction with DOM. Current research on groundwater in the chronic kidney disease with unknown etiology (CKDu) zone of the North Central Province (NCP) in Sri Lanka has focused mainly on aquatic chemistry, with limited attention paid to the spatial distribution, compositional sources and factors of DOM. Therefore, the structure, composition, source and spatial distribution of the DOM of two kinds of groundwater samples collected from dug well and tube well in the NCP during the wet season were determined, compared and analyzed by analytical tools such as parallel factor analysis (PARAFAC). Results show that the average concentrations of TOC in these two groundwater samples are generally higher than 5.0 mg/L, and the concentration of TOC in the groundwater of the shallow weathered aquifer is higher than that of the deep hard rock aquifer, while its distribution of the two aquifers are on contrary. The DOM in the dug well has three types and four components, including humus-like component C1 (33.36%) and C2 (38.60%), protein-like component C3 (13.09%) and heterogeneous organic component C4 (14.95%). In the tube well, two types and two components of the DOM are determined, including humus-like component CⅠ (69.80%) widely existing in natural water and soluble microbial by-product CⅡ (30.20%) produced by microbial community activities. In the dug well, DOM is mainly exogenous input, the higher ion concentration in water affected the fluorescence intensity of humus and protein components. And in the tube well, DOM has obvious endogenous characteristics, and higher pH value may inhibit the production of protein like fluorescent substances to a certain extent.

Evaluation of Performance of Existing RO Drinking Water Stations in the North Central Province, Sri Lanka.

Reverse osmosis (RO) drinking water stations have been introduced to provide safe drinking water for areas with prevailing chronic kidney disease with unknown (CKDu) etiology in the dry zone of Sri Lanka. In this investigation, RO drinking water stations established by community-based organizations (CBO) in the North Central Province (NCP) were examined. Water samples were collected from source, permeate, and concentrate in each station to determine water quality and performance. Furthermore, the operators of the systems were interviewed to evaluate operational and maintenance practices to identify major issues related to the RO systems. Results show that the majority (>93%) of RO systems had higher salt rejection rates (>92%), while water recovery varied from 19.4% to 64%. The removal efficiencies of hardness and alkalinity were averaged at 95.8% and 86.6%, respectively. Most dominant ions such as Ca2+, Mg2+, K+, Na+, Ba2+, Sr2+ Cl-, F-, and SO42- showed higher rejections at averaged values of 93.5%, 97.4%, 86.6%, 90.8%, 95.4%, 96.3%, 95.7%, 96.6%, and 99.0%, respectively. Low recovery rates, lower fluoride levels in product water, and membrane fouling were the main challenges. Lack of knowledge and training were the major issues that could shorten the lifespan of RO systems.