Seasonal Climate Variability and Water Quality Dynamics in The Osun River Basin, Southwestern Nigeria

Abstract

Seasonal variability strongly influences river water quality in tropical regions, where alternating wet and dry seasons shape hydrological and ecological processes. This study examined the effects of seasonal climate variability on the physicochemical and microbial quality of the Osun River Basin in Southwestern Nigeria. Water samples were collected from twenty georeferenced sites during both wet and dry seasons and analyzed for dissolved oxygen (DO), biochemical oxygen demand (BOD), chemical oxygen demand (COD), total dissolved solids (TDS), turbidity, ammonia, nitrate, pH, total coliforms, and Escherichia coli. Standard laboratory procedures were employed, and results were compared against international drinking water standards. The findings revealed marked seasonal contrasts, with significantly higher BOD, COD, turbidity, and microbial loads during the wet season, indicating runoff-driven pollution and surface wash-off of organic and microbial contaminants. In contrast, ionic parameters such as pH and TDS remained relatively stable across seasons, suggesting limited fluctuation in the base water chemistry. Elevated ammonia concentrations and microbial contamination across several sites point to ongoing ecological stress and potential public health hazards. Overall, the study demonstrates that the Osun River Basin is highly vulnerable to seasonal climatic dynamics, with rainfall-driven runoff and reduced dry-season dilution acting as key drivers of pollution variability. These results emphasize the urgent need for seasonally adaptive water quality management, stricter effluent regulation, and improved community-level sanitation measures to protect both ecosystem integrity and human health.