NITRATE IN A GROUNDWATER AQUIFER OVERLAIN BY POROUS STRATA VIS-A-VIS ANTHROPOGENIC EFFECTS ON EFFECTIVE GROUNDWATER GOVERNANCE IN A SUB-SAHARAN URBAN SETTING

This study establishes synergy among hydrogeology, public health and policymaking via a multi-disciplinary approach in the risk valuation, categorical and spatial mapping of contaminants in groundwater. An extensive field sampling campaign that generated chemical datasets on groundwater conditions in 386 domestic wells was carried out between 2015 (rainy season) and 2017 (dry season) in the city of Owerri, southeast Nigeria. Samples were analysed for ions and DOC in groundwater using ion chromatography and Shimadzu TOC-V CPN series analyser. Result showed nitrate concentration above the WHO threshold of 50 µg/mL in ca. 13% of sampled wells (up to 148 µg/mL), and provided evidence linking seasonality to variability of nitrogen species and DOC, implying biochemical reactions. Consequently, time series measurements of nitrate and ammonium concentrations, and proximity analyses of potential pollution sources to selected wells were carried out, with values suggesting that nitrate is derived from nitrification of ammonium from dumpsites and septic tanks. Geographically Weighted Regression (GWR) analysis was used to explore spatial relationships in the evaluation of factors responsible for observed spatial patterns. Empirical Bayesian Kriging (EBK) technique helped to model and predict values at unmeasured locations while eliminating the bias often associated with the underestimation of prediction errors. The study recommends creation of an Integrated Vulnerable Zone Management (IVZM) framework as an effective tool for groundwater governance. It also shows a broader significance since urban congestion and unsewered domestic waste can pose hindrance to the global safe water projects aimed at averting health risks associated with nitrate.