Assessing urban stormwater runoff quality and impact on shallow groundwater in suburb of Urmia city

Rapid expansion of Urmia City in recent decades has caused significant changes in land use and subsequent contamination of water resources. This study aimed to investigate the quality of urban runoff and stormwater from different urban surfaces and assess the impact of stormwater infiltration on groundwater quality in suburban areas. For this purpose, 35 urban stormwater runoff samples and 18 groundwater samples were collected in May 2015 and analyzed for organic matter, nutrients, and heavy metals. Results showed that TSS, COD, BOD, PO₄, Fe, Pb, and Cr concentrations reached 1,100 mg/L, 181 mg/L, 48 mg/L, 3 mg/L, 2,800 µg/L, 50 µg/L, and 240 µg/L, respectively, in parking lot and road runoff—significantly higher than other land uses. Due to sewer overflow, TSS, NO₃, and total coliform increased to 2,050 mg/L, 123 mg/L, and >1,100 per 100 mL in stormwater at the city outlet. Hydrochemical analysis revealed groundwater in suburban areas is hard to very hard (TH: 167–527 mg/L), fresh (TDS: 358–832 mg/L), and of Ca-HCO₃ type, controlled by natural geochemical processes and anthropogenic activities. Ion exchange and reverse ion exchange were identified as dominant mechanisms. Due to shallow groundwater (1–5 m), infiltrated runoff has elevated nitrate concentrations up to 67.3 mg/L, exceeding the WHO guideline of 45 mg/L in 38.8% of samples. Groundwater Quality Index (GWQI) showed 89% of samples were of good quality and 11% poor, with poor-quality samples associated with infiltration areas. Effective urban runoff quality control is essential to protect receiving water resources.