Impact of car-wash wastewater discharge on urban streams: a case study from Jimma city, southwestern Ethiopia

Abstract

Freshwater ecosystems in rapidly urbanizing regions are increasingly threatened by effluent discharges. Car-wash stations are a significant source of pollution, releasing effluents rich in detergents, oils, and organic matter directly into streams. Despite global evidence demonstrating the effects of car-wash effluent on receiving water bodies, it remains overlooked in Ethiopia. This study assessed the impact of car-wash effluent on water quality and habitat condition of urban streams in Jimma city, Ethiopia. The study was conducted at seven car-wash stations in April 2023. Forty-two water samples were collected from upstream, discharge, and downstream sites during weekdays and weekends and analyzed for different physicochemical quality parameters. Habitat integrity was evaluated using the USEPA rapid bioassessment protocol. Principal component analysis (PCA) was applied to identify water quality variation among stream sites. This study revealed deteriorated water quality at discharge points, with significantly higher mean values for turbidity (483.3 ± 193.5 NTU), total suspended solids (TSS) (561.0 ± 460.9 mg/L), and 5-day biochemical oxygen demand (BOD₅) (333.6 ± 104.5 mg/L) and with lower dissolved oxygen (DO) record (4.7 ± 1.5 mg/L) compared to upstream sites (p < 0.05). Higher BOD₅, orthophosphate, nitrate, and water temperature were recorded during weekends compared to weekdays. PCA results showed that discharge points were associated with elevated turbidity, TSS, BOD₅, and orthophosphate, which are inversely correlated with DO. Habitat scores classified most discharge points as poor and several downstream sites as marginal, suggesting ecological stress consistent with car-wash effluent impacts. These findings indicate a need for targeted monitoring and regulatory attention to uncontrolled car-wash wastewater discharge.