Assessment of nutrient loads and physicochemical properties of river Nyakomisaro for water quality management in Kisii County Kenya

The Nyakomisaro River, a critical tributary within the Gucha River sub-basin of the Lake Victoria catchment, serves as a vital resource supporting millions of people in its upper and lower parts. This study, therefore, aims to assess the nutrient loads and physicochemical properties of the river to facilitate water quality management. In this study water samples were collected from ten sites along the river from upstream to downstream in triplicates for accuracy and reliability. Pre-cleaned and properly preserved sampling containers, cool box to store the samples, and adherence to standard protocol methods were used to ensure the quality assurance and control of water samples and then transported to the laboratory for analysis. Spectrophotometry was used to analyze the nutrients and physicochemical parameters were determined in situ using a multiparameter probe. The river water quality was determined using the water quality index (WQI), contamination factor (CF), nutrient pollution index (NPI), and pollution load index (PLI). The results revealed that phosphate values ranged from 0.41 ± 0.02 mg/L (Bobaracho) to 0.92 ± 0.04 mg/L (Denmark). The nitrate concentrations ranged from 1.58 ± 0.05 mg/L (Kisii University) to 3.68 ± 0.26 mg/L (Denmark), which were well below the World Health Organization (WHO) levels. The concentration of temperature, pH, EC, TDS, and salinity where also below the WHO limits. The water quality presented a critical duality: while the CF for phosphates and nitrates ranged from 0.83 ± 0.03 (Bobaracho) to 1.85 ± 0.08 (Denmark) and 0.03 ± 0.00 (Bobaracho, Jerusalem, and (Kisii University) to 0.07 ± 0.01(Denmark), respectively, indicated only low pollution upstream, the overall water quality was severely compromised. This deterioration was driven primarily by turbidity, which ranged from 61.5 ± 0.2 to 261.8 ± 0.1 NTU above the WHO standards. Consequently, WQI ranged from 168.08 ± 0.56 (Bobaracho) to 604.04 ± 1.39 (Denmark), confirmed the water’s immediate unsuitability for drinking without significant treatment and highlighting the urgent need for integrated watershed management to prevent degradation and ensure the sustainability of river resources.