SEASONAL HYDRODYNAMIC CONTROLS AND MULTIVARIATE POLLUTION SIGNATURES IN IKWU STREAM, A TROPICAL COMMUNITY WATER-SUPPLY SOURCE IN SOUTHEASTERN NIGERIA

Summary

Small tropical streams used for community water supply experience simultaneous dilution, sediment mobilisation and oxygen-balance responses during seasonal transitions. This study quantified dry- and wet-season hydrodynamic and physicochemical variation along four reaches of Ikwu Stream in southeastern Nigeria and evaluated the resulting source-water implications. A structured longitudinal design generated 72 matched observations across Umu-Ire, Umuegwu-Okpuala, Umukabia and Amaogwugwu. Width, depth, velocity, discharge, temperature, pH, dissolved oxygen (DO) and electrical conductivity were measured in situ; turbidity, colour, total dissolved solids, total suspended solids (TSS), total solids, alkalinity and five-day biochemical oxygen demand (BOD₅) were determined using standard water-analysis procedures. Seasonal effects were evaluated with paired log₂ response ratios and bootstrap confidence intervals. Pearson relationships were examined after adjustment for season, reach, lateral position and downstream distance, and principal component analysis (PCA) was used to resolve process signatures. Mean discharge increased from 0.817 to 2.251 m³/s in the wet season, DO increased by 12.8%, and BOD₅ decreased by 14.2%; however, TSS increased by 61.1% and turbidity remained above 5 NTU in most observations. Adjusted associations identified coupled discharge-TSS, turbidity-TSS and alkalinity-BOD₅ signals, while PCA separated a hydrodynamic suspended-solids axis from an organic-mineral axis. Umu-Ire exhibited the lowest BOD₅ and remained the most defensible reach for detailed abstraction assessment; however, this physicochemical ranking does not establish microbial safety or suitability for untreated consumption, and treatment must address persistent particulate loading and unmeasured microbial risk. The results demonstrate that increased wet-season flow does not uniformly improve source-water quality and support season-specific monitoring, source protection and multi-barrier treatment.

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