Hydrochemical assessment and monitoring priorities in anthropogenically affected arid urban groundwater systems of Riyadh, Saudi Arabia

Study Region This study investigates arid urban groundwater in metropolitan Riyadh, Saudi Arabia, where limited recharge, intense evaporation, urbanization, agriculture, treated-wastewater reuse, industry, and landfills may alter carbonate and alluvial aquifers. Study Focus Forty-one groundwater samples from accessible wells were assessed to characterize dry-condition hydrochemistry and identify spatial response domains for targeted monitoring. Physicochemical parameters, major ions, trace metals, carbon fractions, and water-table depth were evaluated using inferential statistics, multivariate ordination, pollution and exposure indicators, bootstrap confidence intervals, and hydrochemical process screening. The analysis considered natural mineralization, aquifer context, and human pressures without assigning a single contamination source. New Hydrological Insights for the Region Multivariate analysis separated salinity–metal and carbon-related gradients and confirmed group differences (PERMANOVA R² = 0.388, p = 0.0001). Wadi Sulay showed the strongest salinity and trace-metal enrichment and the highest pollution-index and exposure-indicator values. Wadi Hanifa exhibited a carbon-fraction-dominated carbonate-system signature, whereas deeper Southern Riyadh displayed a buffered urban-mixed response. Process indicators suggested a transition from rock weathering to evaporation-controlled conditions, with stronger Ca–Cl–SO₄ mineralization in Wadi Sulay and lower chloride dominance with higher HCO₃⁻calc/Na⁺ ratios in Wadi Hanifa. Mean total hazard quotients remained below 1 under the adopted scenario. Monitoring should therefore follow hydrochemical responses, aquifer vulnerability, and land-use pressures rather than a uniform parameter set. This framework supports targeted monitoring in data-limited arid aquifers but requires independent seasonal validation.