Hydrological modulation of heavy metal co-transport and natural attenuation in an industrial urban river (Mukuvisi)

Industrial effluent discharge combined with climate-driven hydrological variability poses increasing risks to urban rivers in semi-arid regions. This study quantifies how seasonal hydrological forcing regulates heavy metal transport, natural attenuation and co-transport in the Mukuvisi River, an industrially impacted urban catchment in Zimbabwe. Water samples were collected at four sites spanning industrial, wetland and residential zones during dry low-flow and rainy high-flow seasons, and analyzed for As, Hg, Cd, Cr and Pd alongside contamination factors, pollution load index, correlation statistics and two-way ANOVA. The dry concentrations of As, Hg and Cd exceeded WHO guideline values at most sites, whereas rainy-season concentrations were reduced but remained elevated near industrial sources. The downstream attenuation efficiencies ranged from 40%–60% in the dry season and 50%–65% in the rainy season, with strong inter-metal correlations (r > 0.90, p < 0.01), indicating a dominant common point-source input and synchronized co-transport behavior. These results show that seasonal flow conditions strongly modulate contaminant persistence and the natural attenuation capacity. This study provides novel, process-based insight into the hydrological modulation of heavy metal co-transport and attenuation in an industrial urban river under current and climate-stressed flow regimes.