Urban Surface Run-Off as a Vector for Soil Contamination in Rapidly Growing Cities in Rivers State, Nigeria

Abstract

Urban surface runoff remains a significant but often underappreciated vector in the transport and deposition of contaminants in soil, especially in rapidly expanding cities such as Port Harcourt, Nigeria. Urbanization and industrialization in Port Harcourt metropolitan city, Nigeria, have intensified environmental concerns, especially relating to the degradation of soil quality through contaminated surface run-off. This study assessed the physicochemical and heavy metal characteristics of surface runoff-impacted soils in Aluu and Eleme—two distinct yet environmentally vulnerable zones within the city. Soil samples were analyzed for key parameters, including pH, electrical conductivity (EC), moisture content, total organic matter (TOM), total organic carbon (TOC), total nitrogen (TN), available phosphorus (P), sulfate (SO₄²⁻), total hydrocarbon content (THC), total petroleum hydrocarbon (TPH), and heavy metals such as iron (Fe), arsenic (As), cadmium (Cd), lead (Pb), zinc (Zn), copper (Cu), chromium (Cr), and nickel (Ni).The results revealed alarming exceedances of permissible limits across most parameters. Soil pH values ranged from 5.88 in Eleme to 6.42 in Aluu, indicating slightly acidic conditions below the WHO/FAO acceptable range (6.0–8.5). Electrical conductivity was higher in Eleme (265.1 µS/cm) than Aluu (203.4 µS/cm), suggesting elevated ionic concentrations in runoff-receiving soils. Moisture content values were appreciable at 21.83% in Eleme and 19.21% in Aluu. High TOM (6.95% in Eleme) and TOC (4.03%) levels exceeded typical natural soil benchmarks, signifying significant organic loading, likely due to hydrocarbon and waste inflow. Similarly, TN and available phosphorus levels were elevated (0.57% and 15.07 mg/kg respectively in Eleme), pointing to nutrient enrichment that could disrupt native microbial and vegetation communities. More critically, THC and TPH levels far exceeded the NESREA threshold of 50 mg/kg, reaching 1602.9 mg/kg and 1521.8 mg/kg in Eleme, respectively, indicating severe hydrocarbon pollution. The heavy metal profile further confirmed contamination, with As (2.01 mg/kg), Cd (1.44 mg/kg), and Pb (54.62 mg/kg) all surpassing WHO safety limits. Although Zn, Cu, Cr, and Ni were within or near acceptable ranges, their cumulative presence adds to the overall ecological burden. These findings suggest that surface runoff is a major conduit for both organic and inorganic pollutants, especially in densely populated and industrially active areas like Eleme. The elevated concentrations pose serious ecological risks, with implications for groundwater contamination, agricultural productivity, and public health. It is recommended that integrated urban runoff management strategies, regular monitoring, and remediation interventions be prioritized to safeguard soil health in Port Harcourt and similar urban environments.