Toxicological and Carcinogenic Risk from Fluoride and Arsenic in Drinking Water of the Mezquital Valley, Mexico, Using Monte Carlo Simulation

Groundwater is a primary source of drinking water in the southern Mezquital Valley, Mexico. This region has documented high rates of dental fluorosis in children and cancer incidence above the national average, likely due to environmental factors. This study assessed the non-carcinogenic and carcinogenic risks associated with groundwater consumption contaminated with fluoride (F⁻) and arsenic (As) using a tiered approach. For this study, 30 samples were collected from wells and springs, and were analyzed for temperature, electrical conductivity, pH, TDS, redox potential, alkalinity, Cl⁻, SO₄²⁻, F⁻, K⁺, Ca²⁺, Mg²⁺, Na⁺ and As. Initial exposure points were identified based on Environmental Media Evaluation Guidelines (EMEG), with conservative thresholds set for children at 0.5 mg/L for F⁻ and 0.003 mg/L for As. A deterministic first-tier evaluation calculated the Hazard Quotient (HQ) and Hazard Index (HI) under the "worst-case scenario," revealing HQ > 1 at multiple exposure points, particularly in children. Subsequently, a probabilistic approach using Monte Carlo simulation (10,000 iterations) was applied to incorporate data variability and uncertainty, resulting in probabilistic distributions of Estimated Exposure Dose (EED). The results indicated that 67% of exposure points exceeded safety thresholds for F⁻, while 100% exceeded those for As, confirming unacceptable risk levels, especially for children due to their higher vulnerability. The R code developed, provided as supplementary material, enables replication and adaptation of this methodology to other scenarios. The highest-risk locations identified in this study include wells W-25, W-24, S-3, and W-6, where both fluoride and arsenic concentrations exceeded health-based thresholds in over 75% of probabilistic scenarios. These points supply primarily rural and semi-urban communities with limited access to treated water, heightening the potential public health impact. Given the magnitude of the exceedances—especially for arsenic, which surpassed safe limits at all sampled sites—there is an urgent need to implement affordable treatment technologies, expand groundwater quality monitoring, and integrate these findings into policy reforms aimed at reducing exposure in vulnerable populations.