Assessing the impact of anthropogenic influences on the sediment quality of Owalla Reservoir, Southwest, Nigeria

The study aims to investigate the impact of anthropogenic influences within the catchment areas on the sediment physico-chemical quality of Owalla Reservoir in Osun State, Nigeria with the primary aim of creating baseline information on the aspect of limnology. Twenty sampling stations were established along the reservoir representing, its three different sections and two zones. The sampling was conducted for a period of two years to cover both seasons of the year in the area. The sediment samples were collected using a Van-Veen grab of 0.04 m2 area (0.2m × 0.2m) from the waterbed at each sampling locations; labelled, kept in cooler at 4°C and transported to the laboratory for the analysis. The reservoir sediment was mainly clayey-sand in texture, while the particle size distribution was in this order: sand (73.1%) > clay (17.4%) > silt (9.4%). The exchangeable cations order of dominance was in this order: Mg2+ > Ca2+ > H+ > Al3+ > K+ >Na+ and anions in this order: PO34- > SO42- > NO3-> Cl-. The highest mean values for water depth, transparency, air temperature, water temperature and sediment temperature (5.28 ± 0.40 m, 1.60 ± 0.05 m, 31.90 ± 0.29°C, 30.6 ± 0.18°C and 27.6 ± 0.23°C) were recorded during the dry season compared with the rainy season. The results obtained from this study, shows that Owalla Reservoir is fairly clean and not contaminated with toxic pollutants. The sediment pH is within acidic range, and enriched with nutrients due to the anthropogenic activities around this reservoir catchment areas. However, if the organic enrichments and siltation in this reservoir is not controlled, it could lead to the future eutrophication of this waterbody, which can cause water quality degradation, alter the aquatic biota and natural ageing process of this reservoir making it unsuitable for its purposes.