National-scale prediction of arsenic, fluoride and its co-occurrence in groundwater of Mexico: Implications for drinking-water.

Arsenic (As) and fluoride (F-) in groundwater pose significant health risks and frequently co-occur in arid and semi-arid settings. We assembled a dataset comprising 3403 As measurements and 2084 F- measurements for Mexico (44% and 24% above the WHO guideline values of 10 µg/L and 1.5 mg/L, respectively) and trained Random Forest (RF), Extreme Gradient Boosting (XGBoost), and Artificial Neural Networks (ANN) classifiers using 69 predictor variables, including climate, hydrogeology, and anthropogenic pressures. Beyond separate single-contaminant models, we developed a co-occurrence classifier that explicitly predicts the joint exceedance of As and F- above WHO drinking-water limits, capturing areas where concurrent exposure to both hazards is most likely. XGBoost performed best (test balanced accuracy/AUC: As 0.82/0.90, F- 0.83/0.94, co-occurrence 0.83/0.92) and generated 1-km probability of exceedance maps for each contaminant and their joint exceedance, providing a national-scale co-occurrence model for Mexico. Model interpretation showed that exceedance probabilities for As and F- are primarily controlled by elevation, climate (precipitation and temperature), and soil pH, with additional contributions from water‑table depth, population density, and volcanic-alluvial settings for co‑occurrence, highlighting coupled geogenic and anthropogenic controls. Risk mapping indicates at-risk areas covering 15.1% (As), 13.5% (F-), and 8.9% (co-occurrence) of the national territory, with hotspots concentrated in north-central Mexico. Population overlays suggest ∼10.4 million people may be exposed to As (8.2%), ∼9.7 million to F- (7.7%), and ∼7.2 million to concurrent exposure (5.7%). These quantitative products provide an operational baseline to prioritize monitoring wells, well management and treatment (well switching and point-of-use technologies), and risk communication in high-burden communities. SYNOPSIS: First national co-occurrence model identifies Mexico's As-F- groundwater risk hotspots, enabling targeted monitoring, well management, and treatment to reduce population exposure.