Impact of rainfall on the water quality of a tropical river: based on the Nilwala River in the southern province of Sri Lanka between March and October 2019

The study addresses the pressing need for an effective monitoring system to assess aquatic water quality, given the exacerbating impact of climate change on surface water bodies. Focusing on the Nilwala River, the research pursues three main objectives: comparative analysis of the Weighted Arithmetic Water Quality Index (WAWQI) values across different river locations, exploration of correlations between monthly rainfall and various physicochemical parameters at distinct sampling sites, and examination of notable disparities in the WAWQI between dry and wet months. Data collected from eight river locations from March to October 2019 revealed significant temporal variations in pH, temperature, chemical oxygen demand (COD), electrical conductivity (EC), alkalinity, chloride (Cl−), and nitrate (NO3-) content. The WAWQI increased across all sites during the drought, with only Site 4 (Wellathota) deemed suitable for drinking. Statistical analyses using an one-way ANOVA and multiple linear regressions unveiled significant relationships between rainfall and pH, biological oxygen demand (BOD), EC, and total alkalinity. The total alkalinity is the most affected parameter by the rainfall. Rainfall positively predicted BOD, COD, and NO3- concentration while negatively predicting other parameters.