Monte Carlo Assessment of Heavy-Metal Persistence in Mexican Aquifers Using an Arsenic Environmental Index

Groundwater is essential for drinking-water supply in Mexico; however, the long-term persistence of metal contamination has rarely been assessed comprehensively at the national scale. This study evaluated the spatiotemporal persistence of twelve heavy metals and metalloids (Al, As, Ba, Cd, Cu, Cr, Fe, Mn, Hg, Ni, Pb, and Se) in Mexican aquifers using the National Water Commission (CONAGUA) monitoring database, which comprises 127,424 records from 653 aquifers collected between 2012 and 2022. Based on the maximum permissible limits (MPLs) established by Mexican Standard NOM-127-SSA1-2021, 132 monitoring sites across 18 states were identified as exceeding the regulatory thresholds. Arsenic (As) was the most frequently detected and persistent contaminant, occurring at 64.4% of the affected sites nationwide, followed by Pb (53.0%) and Cd (41.7%). The maximum As concentration reached 0.58 mg/L, equivalent to 23.2 times its MPL. The results revealed a north–central region with a high incidence of contamination, encompassing Durango, Zacatecas, Coahuila, and Guanajuato, where 88 wells exhibited exceedances for six or more years (≥6 years). Of the 108 sites with persistent As contamination, 59 (approximately 55%) were located in overexploited aquifers. The Arsenic Environmental Index (AEI) was developed to integrate indicators of contamination magnitude, duration, and persistence, thereby classifying affected sites into three severity levels: Critical (Level 1; 13 wells, 14.8%), Moderate (Level 2; 15 wells, 17.0%), and Persistent (Level 3; 60 wells, 68.2%). Durango had the highest number of Critical wells (6), followed by Guanajuato (3), Coahuila (2), and Zacatecas (2). Geographic information system (GIS)-based spatial analysis revealed spatial associations between AEI-classified sites and anthropogenic pressure factors, including aquifer overexploitation, metropolitan areas, municipal solid waste (MSW) disposal sites, and mining areas, as well as geological factors such as volcanic structures and land subsidence. Monte Carlo simulations were conducted to project arsenic concentrations in four representative Level 1 (Critical) wells over the 2023–2028 period. The results indicate that contamination is not attributable solely to isolated events. Under current groundwater extraction conditions and in the absence of effective intervention, projected arsenic concentrations (0.28–0.42 mg/L, approximately 11–17 times the MPL) remain above both the Mexican regulatory limit and the World Health Organization (WHO) guideline value, indicating a scenario of sustained exposure and potential chronic health risks. The integrated approach developed in this study provides a scientific basis for prioritizing groundwater management strategies according to the AEI classification, as well as for strengthening monitoring programmes and implementing targeted treatment measures in the regions identified as priorities.