Geospatial mapping and suitability classification of groundwater quality for agriculture and domestic uses in a Precambrian basement complex

Abstract

Application of groundwater for domestic and agricultural uses has increased in sub-Sahara Africa. However, geology and anthropogenic activities can impact groundwater quality. But information on quality evaluation and suitability classification of groundwater based on water quality index (WQI) is limited. Thus, this study evaluated the spatial variability of groundwater quality and its suitability for potable and irrigation purposes in a Precambrian complex of southwest Nigeria. Water samples were collected from 40 wells across the study area and analyzed for physicochemical properties. Suitability for domestic and irrigation uses were determined. Maps showing spatial variations of pH, electrical conductivity (EC), total dissolved solid (TDS), total hardness (TH), cations (Ca2+, Mg2+, Na+, and K+), anions (SO42−, Cl−, NO3−-N, PO3−-P, B−, F−, and HCO3−) and irrigation water quality index (IWQI) and drinking water quality index (DWQI)) were produced using ArcGIS. The average values of pH, EC, TDS and TH were 6.6, 0.3 dS m−1, 152.4 mg L−1, and 208.7 mg L−1, respectively. The concentrations of Ca2+, Mg2+, K+ and Na+ were 28.7, 32.8, 2.49, and 7.13 mg L−1, respectively, whereas SO42−, Cl−, NO3−-N, PO3−-P, B−, F−, and HCO3− concentrations were 20.4, 0.32, 0.46, 0.04, 0.39, 0.42, and 230.7 mg L−1, respectively. The quality indices for irrigation (IWQI) and drinking (DWQI) were 0.22 and 0.75, respectively. The IWQI significantly correlated with Mg2+, TH, SO42−, pH and TDS (P ≤ 0.05, r = 0.34–0.66), whilst DWQI significantly correlated with TDS, SO42−, Cl−, NO3−-N, PO3−-P, Ca2+, Mg2+, pH, TH, and F− (P ≤ 0.05, r = 0.38–0.93). The suitability classification of the groundwater quality in the area shows that 100% and 78% of the water are safe for irrigation and domestic purposes, respectively. However, regular quality monitoring of the groundwater is recommended in order to avert likely deterioration indicated by some quality parameters.