Statistical evaluation of geology-controlled groundwater quality variations in southern Ebonyi district, Nigeria

Groundwater chemistry is essentially controlled by lithology and water–rock interaction processes, which collectively govern its evolution and suitability for use. This study evaluates the hydrogeochemical characteristics and quality of groundwater in two contrasting geological settings underlain by sandstone (Afikpo zone, AZ) and shale (Ohaozara Zone, OZ), with the aim of understanding the role of lithology in controlling water quality. A total of 60 groundwater samples (30 from each zone) were analysed for physicochemical parameters, major ions, and trace metals. Cluster analysis, were employed to delineate hydrogeochemical evolution patterns, while the Mann–Whitney U test was used to assess significant differences between the two aquifers. Water quality index (WQI) and irrigation indexes were used to map water quality. The results revealed four distinct hydrogeochemical clusters representing progressive evolution from acidic, low-total dissolved solids (TDS) recharge waters to more mineralized and near-neutral groundwater. Mann–Whitney U test results showed statistically significant differences ( p < 0.05) in pH, TDS, and major ions between the two zones, with higher concentrations generally observed in the shale aquifer. The WQI revealed a spatial disparity. In the AZ, groundwater quality was physicochemically Excellent (16.7%) and Good (80%), and Poor (3.3%). Conversely, the OZ showed a wider range: physicochemically Excellent (36.7%), Good (43.3%), Poor (10%), Very Poor (6.7%), and Unsuitable (3.3%). This geological control was further evidenced by irrigation water quality, which consistently rated the OZ as poorer than the AZ. This study provides valuable insight into groundwater evolution processes and has implications for water resource management in heterogeneous geological terrains.