Integrated Hydrochemical and Geospatial Modelling of Nitrate Contamination, Groundwater Suitability and Public-Health Risk in Hand-Drilled Wells of Iree, Southwestern Nigeria

Abstract

Groundwater from hand-dug wells is the primary source of domestic water for residents of Iree Township in Osun State, Nigeria. This study integrates hydrochemical analysis and geospatial modelling to evaluate groundwater suitability for domestic use and quantify public-health risks associated with nitrate contamination. Twenty-five hand-drilled wells were sampled and analysed for key physicochemical parameters, including pH, temperature, turbidity, electrical conductivity (EC), total dissolved solids (TDS), total hardness (TH), total suspended solids (TSS), dissolved oxygen (DO), biochemical oxygen demand (BOD), nitrate (NO₃⁻), sulphate (SO₄²⁻), chloride (Cl⁻), carbonate (CO₃²⁻), alkalinity and acidity. Water Quality Index (WQI) scores were calculated using the weighted arithmetic method, and the spatial distributions of selected parameters and WQI were interpolated using the Inverse Distance Weighted (IDW) method in ArcGIS 10.5. Results showed that pH values ranged from 6.8 to 7.8 (mean 7.27), EC from 128 to 509 µS/cm (mean 327.4), and TDS from 122 to 333 mg/L (mean 185.8). Nitrate concentrations were elevated across all samples (200–325 mg/L; mean 258.9 mg/L), exceeding WHO and NSDWQ limits (50 mg/L) by more than fivefold. WQI values ranged from 17.7 to 47.0 (mean 32.7), indicating mostly excellent to good overall quality. However, despite the favourable WQI classification, high nitrate levels indicated localised contamination with severe public-health implications for groundwater consumption. This contradiction shows an inherent limitation of the WQI approach, as the index aggregates all physicochemical parameters into a single value and may therefore mask the influence of nitrate when other parameters remain within maximum permissible limits. Correlation analysis linked elevated BOD, turbidity and TSS to higher WQI values and suggested anthropogenic sources such as agricultural runoff, septic leachate and poor sanitation. Spatial maps identified northeastern and some central pockets as nitrate hotspots. Immediate interventions are recommended, including targeted nitrate remediation and source-control measures (improved sanitation, safe waste disposal and fertiliser management), regular monitoring and community awareness campaigns. The study highlights the importance of integrating hydrochemical and geospatial analyses to inform local water-safety planning and meet SDG 6 ambitions for safe water access.