Integrated assessment of drinking water quality and child nitrate health risk using indices and Monte Carlo simulation.

This study presents an integrated assessment of the physicochemical quality and nitrate-related health risks associated with drinking water in Zanjan City, northwestern Iran. In 2024, fifty-six water samples were collected from groundwater wells, treatment plant outlets, and distribution networks, and analyzed for key parameters (pH, turbidity, EC, TDS, major ions, hardness, nitrate) following APHA (2017) standards. Water quality was evaluated using the water quality index and the Schoeller classification scheme to concurrently assess regulatory compliance and usability. WQI results indicated that 46.4% of samples were "Excellent," 44.6% "Good," and 5.4% "Moderate," reflecting generally acceptable water quality from a health-based perspective. In contrast, the Schoeller classification revealed widespread usability concerns, with many samples classified as "Unsuitable" to "Non-potable" due to elevated total hardness, bicarbonate, and sodium of predominantly geogenic origin. Nitrate concentrations (5.3-45 mg/L) remained below WHO and national guidelines. However, probabilistic health risk assessment using Chronic Daily Intake, Target Hazard Quotient, and Monte Carlo simulation indicated a potential non-carcinogenic risk for children, whose mean THQ (1.04) slightly exceeded the safety threshold, while adults remained at low risk (THQ = 0.304). Monte Carlo analysis confirmed a significant probability of exceedance for children, attributable to higher water intake relative to body weight. The study concludes that although Zanjan's drinking water largely meets health-based standards, its geogenically influenced mineral content impairs usability and poses a measurable nitrate risk to children, underscoring the need for continuous monitoring, targeted management, and preventive public health measures to ensure sustainable water resource safety.