Exploring seasonal variability in water quality of Nyabarongo River in Rwanda via water quality indices and DPSIR modelling

Understanding seasonal variations in water quality is crucial for effective management of freshwater rivers amidst changing environmental conditions. This study employed water quality index (WQI), metal index (MI), and pollution indices (PI) to comprehensively assess water quality and pollution levels in Nyabarongo River of Rwanda. A dynamic driver-pressure-state-impact-response model was used to identify factors influencing quality management. Over 4 years (2018–2021), 69 samples were collected on a monthly basis from each of the six monitoring stations across the Nyabarongo River throughout the four different seasons. Maximum WQI values were observed during dry long (52.90), dry short (21.478), long rain (93.66), and short rain (37.4) seasons, classified according to CCME 2001 guidelines. Ion concentrations exceeded WHO standards, with dominant ions being \({\text{HCO}}_{3}^{-}\) ${\text{HCO}}_3^{-}$ and \({\text{Mg}}^{2+}\) ${\text{Mg}}^{2+}$ . Variations in water quality were influenced by factors such as calcium bicarbonate dominance in dry seasons and sodium sulfate dominance in rainy seasons. Evaporation and precipitation processes primarily influenced ionic composition. Metal indices (MI) exhibited wide ranges: long dry (0.2–433.0), short dry (0.1–174.3), long rain (0.1–223.7), and short rain (0.3–252.5). The hazard index values for Cu²⁺, Mn⁴⁺, Zn²⁺, and Cr³⁺ were below 1, ranging from 8.89E − 08 to 7.68E − 07 for adults and 2.30E − 07 to 5.02E − 06 for children through oral ingestion, and from 6.68E − 10 to 5.07E − 07 for adults and 6.61E − 09 to 2.54E − 06 for children through dermal contact. With a total carcinogenic risk of less than 1 for both ingestion and dermal contact, indicating no significant health risks yet send strong signals to Governmental management of the Nyabarongo River. Overall water quality was classified as marginal in long dry, poor in short dry, good in long rain, and poor again in short rain seasons.