Integrated hydrogeochemical and microbiological assessment of groundwater quality in a fractured volcanic aquifer, Ibb City, Yemen

Abstract Groundwater quality assessment in fractured volcanic aquifers presents significant challenges due to structural heterogeneity that may influence groundwater flow and contaminant transport. This study presents an integrated hydrogeochemical and microbiological assessment of groundwater from twenty production wells tapping the fractured volcanic aquifer of the Tertiary Yemen Volcanic Group in Ibb City, Yemen. Groundwater samples were collected during the dry season (November–December 2025). Hydrochemical characterization identified the Ca–Mg–HCO₃ facies as the dominant groundwater type, consistent with recharge-driven water–rock interaction and predominantly weathering of Ca- and Mg-bearing volcanic silicates, with carbonate-bearing phases representing a possible but unresolved contribution. PHREEQC saturation index modeling indicated generally near-equilibrium to oversaturated conditions with respect to carbonate minerals (calcite SI = 0.12–1.17) and persistent undersaturation with respect to halite, gypsum, and anhydrite, indicating limited thermodynamic control by these evaporite and sulfate mineral phases. Water Quality Index (WQI) values (38–95) classified all groundwater samples as excellent to good for drinking purposes, whereas irrigation indices indicated generally low sodium hazard (mean SAR = 1.59). However, fecal-coliform indicator detection occurred in 25% of the sampled wells (1–5 CFU/100 mL), indicating localized sanitary concerns. Mann–Whitney U tests showed no statistically detectable differences ( p > 0.05) in the investigated hydrochemical parameters between fecal-coliform-positive and fecal-coliform-negative wells. Given the small number of fecal-coliform-positive wells ( n = 5), these comparisons were considered exploratory and were not statistically powered to establish independence or equivalence between fecal-coliform indicator detection and groundwater physicochemical characteristics. These findings demonstrate that physicochemical compliance alone cannot ensure drinking-water safety in fractured volcanic aquifers and highlight the need for integrated groundwater quality assessment frameworks that combine hydrochemical and microbiological monitoring.