Linkages between catchment characteristics and river water chemistry at the locality scale in the Ikpoba River, Southern Nigeria

This study explored the linkages between catchment characteristics (topography, land cover, soil and geology), average water chemistry (pH, calcium, magnesium, sulphate, nitrate, ammonium, orthophosphate, iron, zinc and lead) and rainfall in the Ikpoba River catchment, southern Nigeria, using statistical and locality-based GIS analyses. The results show that sites with high rainfall and percentage cover of arable and sand-gravel-clay lithologies were characterized by high acidity. There were strong links between average nutrient (𝑃𝑂43−-P, 𝑁𝐻4+-N and 𝑁𝑂3−-N) concentrations and diffuse agricultural sources in the catchment. Rainfall was strongly related to 𝑆𝑂42−, 𝑃𝑂43−-P and 𝑁𝐻4+-N suggesting that atmospheric deposition may influence their riverine concentrations. Results also suggest that decomposition of organic matter from forest stands was a significant driver of nutrient concentrations. Although metals (Fe2+ and Zn2+) were positively related to bedrock geology of sand-gravel-clay, there was no clear link between Pb2+ and the catchment characteristics investigated. Wetlands was found to be attenuating river water chemistry especially 𝑆𝑂42−, 𝑃𝑂43−-P and 𝑁𝐻4+-N concentrations. To underpin current environmental protection strategies, there is need to integrate a GIS-based analysis approach with monitoring data to fully identify the variability patterns in river water chemistry dynamics at local and multiple scales of water resource management in Nigeria. Key words: river catchment, water quality, metals, nutrients, GIS.