In many arid urban regions, increasing reliance on inter-basin water transfers and inadequate wastewater management have led to serious threats to groundwater quality and sustainability. Zahedan, located in southeastern Iran, is a prime example of such challenges. This study presents a comprehensive hydrogeological and environmental assessment of groundwater quality in Zahedan, where pressures from imported water and wastewater infiltration are intensifying. A total of 191 samples from diverse water sources were analyzed for major ions, stable isotopes (δ18O, δ2H), and microbial contaminants. Groundwater types ranged from Na-SO4 in the west to Na-Cl in the east. Isotope analysis confirmed recharge from inter-basin transferred originating in Zabol, which raised the urban water table by 2-6 m during 2002 to 2021. Over 80% of groundwater samples exceeded WHO nitrate guidelines for nitrate (50 mg/L as NO3⁻), with a strong correlation between nitrate and fecal/total coliform contamination, highlighting the critical impact of urban wastewater. A mixing model identified contributions from multiple sources, especially water transferred from Zabol and local aquifers. Elevated fluoride, ammonia, and microbial loads were observed in shallow and low-discharge wells, suggesting an urgent need for targeted mitigation. These findings emphasize the complex interactions between urbanization, water transfer infrastructure, and aquifer vulnerability in arid regions.
Keywords: Fecal coliforms; Groundwater quality; Hydrogeology; Nitrate contamination; Stable isotopes; Urban water management; Water mixing model.
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