Characterisation of Urban Runoff Quality in a Residential Catchment in Jos, Plateau State

This study characterises the quality of urban stormwater runoff from a medium-density residential catchment in Jos, Plateau State, North Central Nigeria, examining the pollutant concentrations, dominant contaminant sources, and seasonal variation in runoff quality. Urban stormwater is an underregulated pollution pathway in Nigerian cities, and characterising its quality is a necessary precursor to assessing its impact on receiving streams and designing sustainable urban drainage solutions. A monitoring programme was established at the outlet of a 3.2-hectare residential sub-catchment in the Rantya district, capturing flow samples from twelve storm events over the rainy season. Runoff samples were analysed for total suspended solids, chemical oxygen demand, oil and grease, nitrate, phosphate, total and faecal coliform, and zinc. First-flush effects were examined by collecting sequential aliquot samples at the beginning and end of each storm event. Results show high event mean concentrations of total suspended solids averaging 420 milligrams per litre and chemical oxygen demand averaging 185 milligrams per litre, substantially above receiving stream quality standards. First-flush ratios indicate that the initial 25 percent of runoff volume carries 60 to 70 percent of the total suspended solid and chemical oxygen demand load, confirming pronounced first-flush behaviour. Faecal coliform counts in runoff are high, attributable to pet waste and poorly maintained soakaway pit overflow. Zinc and oil concentrations suggest contribution from vehicle maintenance activities. The study recommends installing first-flush diversion structures and vegetated bioswales at catchment outlets. Keywords: urban stormwater, runoff quality, first flush, residential catchment, Jos.