Assessment of Water Quality Parameters in the Cascade Systems that Feed Peraru Reservoir

Abstract

The water resources of the Peraru reservoir are treated by the National Water Supply and Drainage Board, Vavuniya and distributed to the public for potable and non-potable purposes. However the water hardness fluctuate with season and exceeds the standards in dry season. Therefore, this study aims to analyze the water quality, primarily focusing on hardness of the water in the tanks and channels along the cascade systems that feed the Peraru reservoir in the Vavuniya District. The Peraru Reservoir is fed by a cascade system made up of many tanks and channels connected one after another. Water samples were taken from these tanks and channels, and pH, electrical conductivity (EC), chloride, alkalinity, hardness, and fluoride were tested. The data was processed using ArcMap spatial platform to develop Inverse Distance Weighting interpolation maps.The temporal and spatial analysis of water quality parameters from 2022 to mid-2025 reveals that pH and chloride levels generally remain stable within permissible limits, while hardness, alkalinity, electrical conductivity (EC), and fluoride show significant variations influenced by seasonal changes and geological factors. Hardness and alkalinity frequently exceed Sri Lanka Standards (SLS) limits in key tanks and the Peraru reservoir, especially during dry seasons. Fluoride levels exceed safe limits at several sites, notably in Peraru surface and channels connected to Charnockite and amphibolite formations. Statistical tests confirm a strong correlations between EC, hardness, alkalinity and fluoride highlight their common mineral sources related to the different type of geology of the cascade system. From that it is found that there is a significant variation of key water quality parameters in different locations. EC, alkalinity, hardness, and fluoride show a strong positive relationship across the cascade system. Elevated levels of these parameters occur at several locations, particularly near the Nochchimoddai Bridge and along the Madukanda–Peraru cascade line. In June and July, there was more variation in water quality in channels than in tanks. Hornblende-biotite gneiss and Biotite–rich amphibolite zones are mostly contribute for the hardness.