Marco Rodrigo López-López

Water security in rapidly urbanising river basins is increasingly threatened by untreated city effluents, industrial discharges, and legacy agricultural contamination. The Tula River basin in central Mexico illustrates this issue, absorbing the majority of Mexico City’s effluent while sustaining a heavily exploited aquifer beneath one of the nation’s largest irrigation districts. This study provides an integrated assessment of surface water and groundwater quality throughout the basin, including the Endhó Dam and its associated aquifer. Water quality analysis revealed severe surface water contamination (WQI > 300), driven by untreated sewage and inadequate sanitation infrastructure. Elevated COD, BOD, and nutrient concentrations indicate significant organic loading and eutrophication risk. Near Tula City, arsenic, copper, and zinc were detected at levels posing direct risks to human health. Groundwater quality was comparatively favourable, with 71% of wells recording WQI < 100; however, arsenic exceeded permissible limits more than twentyfold in select wells, attributed to geological sources. The detection of SVOCs in both hydrological compartments confirms cross-compartment contamination. Point-source reduction alone is insufficient for aquifer recovery; comprehensive sanitation strategies and long-term monitoring are urgently required. These findings carry direct relevance for water governance in megacity-dependent basins globally, where urban, agricultural, and geological stressors demand integrated management approaches.

This study presents an integrated assessment of surface water and groundwater quality in the Tula River basin, Mexico, encompassing the Endhó Dam and its associated aquifer. Water quality index (WQI) analysis revealed severe contamination along the Tula River (WQI >300), driven primarily by untreated sewage discharges from Mexico City and inadequate regional sanitation infrastructure. Elevated concentrations of COD, BOD, and nutrients indicate significant organic loading and eutrophication risk across aquatic ecosystems. Near Tula City, heavy metals including arsenic, copper, and zinc were detected at levels posing direct risks to human health. Groundwater quality was com-paratively favorable, with 71% of sampled wells recording WQI < 100; however, arsenic concentrations exceeding permissible limits by more than twentyfold were identified in select wells, attributed to geological sources. Semi-volatile organic compounds (SVOCs) were detected in both hydrological compartments, confirming cross-compartment con-tamination and highlighting the need for contaminant transport and fate modelling. The inertial contamination trajectory of the aquifer indicates that point-source reduction alone is insufficient for remediation. Comprehensive sanitation strategies, including pre-discharge treatment of Mexico City effluents, alongside proactive long-term aquifer monitoring and remediation programs, are urgently required to safeguard water sup-plies, public health, and ecological integrity in the Tula Valley.