<p>Groundwater plays a vital role in drinking and daily supply in the Islamabad-Rawalpindi metropolitan areas. The current study is designed to comprehensively address the problem of groundwater quality using a multi-methodological approach with water quality indices (WQI). For this purpose, 122 observations were recorded and analyzed following standard procedures. Piper and Gibbs diagrams demonstrated groundwater characterization. The GIS and multivariate statistical analyses were employed for vulnerability assessment and source apportionment. The findings of the present study revealed that certain parameters (pH, alkalinity, bicarbonates, and potassium) were within the desirable range stipulated by the WHO and PSQCA. However, groundwater quality impairment is related to toxicities of EC, TDS, turbidity, TH, calcium, magnesium, sodium, chlorides, sulfates, nitrates, fecal, and total coliform. The WQI indicated that the study area exhibited poor to very poor groundwater quality. Irrigation indices explained that it is suitable or marginal at most studied sites, and only a few sites displayed unsuitable quality. Piper and Gibbs diagrams suggested that groundwater belongs to Ca-HCO₃ and Ca-Mg-HCO₃ or mixed types influenced by rock-water interactions and evaporation. Statistical analysis deciphered that anthropogenic and geogenic factors are the key determinants of water quality in the study area, including Lei recharge, domestic, agricultural, and industrial effluents, improper waste disposal, poor maintenance, and weathering processes. The study provides benchmark groundwater quality data that decision-makers can utilize to take appropriate measures for groundwater monitoring and pollution risk management in the twin cities.</p>