Abstract
With increasing urban population, attention had been focused on environmental degradation of urban drain-age system with respect to trace/heavy metal contaminations. Such concerns underlie the ever-increasing impacts of urbanization and industrial activities on urban watershed in the developing regions of the world, especially in areas with inadequate land-use plan and poor waste disposal and management practices. Hence, this study highlights the hydrogeochemical assessment of surface water and bottom-sediment samples from an urban drainage system in Osogbo Township, SW-Nigeria with respect to trace metals contaminations. The results show that the surface water samples have generally low TDS with average value of 362mg/l, while the average dissolved concentrations of the trace metals (Cu, Pb, Zn, Ni, As and Cr) vary from 0.01 to 0.5mg/l. Cu, Cr and As exhibit concentrations similar to the local background concentrations (LBC) in the pristine stream water with low single metal contamination factor (CF ≈ 1). Pb, Zn and Ni are 5 folds enriched with contamination factor (CF) of >5 indicating moderate to high contamination. For the sediment phase, the adsorbed concentrations of the trace metals (Cu, Pb, Zn, Ni, As, Cr and Co) vary between 0.1 to 3.1mg/kg. These represent about 1 to 3% of the respective total metal concentrations with average values of 18.2– 533.4mg/kg. Also low anthropogenic factor, AF (0.002 to 0.08) and mostly negative values (–5 to –15) of Mueller’s geo-accumulation index (Igeo) for adsorbed metal contents in the sediments suggest dominant geo-genic controls. However, the total metals concentrations in the sediment phase have high estimated AF of 1.1 to 9.3 and positive values of the estimated Igeo (0.9–2.0) and metal contamination index (MCI) of 2.5–8.3. All these suggest a medium to high level enrichment (of 2 to 10 factor) for most of the metals with respect to the local background concentration (LBC) in the basement bedrock units (with the exception of Cr and Ni). This is consistent with the preferential metal enrichment in the sediment phase as indicated by the estimated parti-tioning/distribution coefficient, Kd of >1 exhibited by the total metal concentrations in the stream sediment. Nonetheless, the correlated high peaks of electrical conductivity of the stream water samples and adsorbed concentrations of some trace metals within the urban stretches are indications of point source inputs of un-treated sewage into the drainage system.
Highlights
Hydrogeochemical surveys of drainage systems have long been employed as geochemical exploration tool but to provide information on contaminant metals sources in relation to weathering and erosion transport processes within the catchment area [1,2,3]
This study presents the contamination assessment of an urbanized drainage catchment in SW-Nigeria, and highlights possible impacts of urbanization and associated anthropogenic activities on the distribution of trace metals in both water and sediment phases of the drainage networks in the study area
The adsorbed trace metals in the sediment phase are considerably lower representing about 1 to 3% of the respective total metal concentrations in the sediments with average concentrations of 18.2–533.4mg/kg
Summary
Hydrogeochemical surveys of drainage systems have long been employed as geochemical exploration tool but to provide information on contaminant metals sources in relation to weathering and erosion transport processes within the catchment area [1,2,3]. The quality of water in surface drainage system is a function of anthropogenic influences (urbanization, agricultural and industrial activities) as well as natural processes (weathering and catchment erosion) [9]. There had been increasing concerns about degradation of urban drainage systems with respect to trace/heavy metal contaminations, in the recent past [5]. This environmental concern is apparently due to the toxicity and perceived persistency of trace metals within the drainage/aquatic systems. Such concerns underlie the ever-increasing impacts of urbanization, agricultural, mining and industrial activities on surface drainage systems. This is more so in the developing regions of the world, especially in areas with inadequate land use plan and lack of proper waste disposal and management practices [10,11]
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