Spatial variation of water quality in Nilwala river of the Southern Province of Sri Lanka

Abstract

This study addresses the urgent need for comprehensive water quality assessments of the Nilwala River, a vital water source in Sri Lanka’s Matara district, to understand the impacts of anthropogenic activities on regional water resources. The research, conducted over an eight-month period encompassing both dry and wet seasons (March–October 2019), systematically analyzed water samples from eight strategic locations to assess spatial variations in key water quality parameters, including pH, electrical conductivity (EC), temperature, chemical oxygen demand (COD), biological oxygen demand (BOD), alkalinity, hardness, chloride, nitrate, and phosphate. Significant spatial differences were detected in parameters such as pH, EC, COD, BOD, hardness, alkalinity, and chloride (p < 0.001), with COD levels exceeding permissible limits at the Bandaththara power plant and urban Matara, highlighting the influence of industrial and urban discharges. The Weighted Arithmetic Water Quality Index (WAWQI) suggested that most sites require general treatment for drinking water, except Wellathota, which met acceptable standards. Principal Component Analysis (PCA) revealed that alkalinity, chloride, hardness, and COD were the primary contributors to water quality variations, while BOD was negatively correlated with these factors. The analysis identified rural, agricultural, and urban-industrial sites as key areas where water quality is significantly impacted by land use practices, including sand mining, agriculture, and urbanization. The study underscores the necessity for targeted monitoring and management interventions to protect the Nilwala River, emphasizing the importance of implementing mitigation strategies to address pollution sources. The findings provide critical insights into the dynamics of river water quality and highlight the need for sustainable resource management through identifying the pollution hotspots and the factors driving water quality deterioration.