306-11 Long-term decline in groundwater redox potential reveals heterogeneous mobilization of redox-sensitive trace elements across Mexico

Abstract:

Groundwater redox conditions are fundamental to the mobility of redox-sensitive trace elements (RSEs), yet their long-term evolution and influence on groundwater chemistry remain poorly understood at regional scales. This study evaluated decadal changes in groundwater redox conditions and hydrochemistry using paired observations from 352 monitoring wells across Mexico sampled in 2014 and 2022. Groundwater exhibited a widespread shift toward less oxidizing conditions, with mean oxidation–reduction potential (ORP) declining from 185.8 to 125.8 mV, while 67.4% of monitoring wells showed decreasing ORP. Concurrently, mean Fe, As, Cr, and NO₃⁻ concentrations increased from 0.152 to 0.159 mg L⁻¹, 0.021 to 0.027 mg L⁻¹, 0.0028 to 0.0060 mgL⁻¹, and 4.24 to 5.95 mgL⁻¹, respectively. At the well scale, NO₃⁻ increased in 71.2% of wells, Fe in 41.6%, and Cr in 40.1%, whereas As exhibited heterogeneous behavior, with 39.5% of wells showing no detectable temporal change. Correlation analysis indicated that relationships between ORP and redox-sensitive variables weakened between 2014 and 2022, suggesting that declining redox potential alone did not uniformly regulate trace-element mobility. Principal component analysis resolved two dominant hydrogeochemical controls: PC1 was dominated by major ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, and SO₄^2-), reflecting groundwater mineralization, whereas PC2 was characterized by carbonate chemistry (HCO₃^-, CO₃^2-), electrical conductivity, and total dissolved solids, with secondary contributions from ORP, Fe, and Mn, indicating coupled carbonate–redox evolution. Consistent with these results, R-mode hierarchical cluster analysis revealed a reorganization of hydrochemical associations between 2014 and 2022, with major ions consistently forming a distinct mineralization cluster, while ORP, carbonate species, and redox-sensitive trace elements exhibited changing clustering patterns through time. These findings demonstrate that regional groundwater evolution reflects the interaction of redox processes with groundwater mineralization and localized hydrogeochemical conditions, producing element-specific rather than coordinated responses of redox-sensitive trace elements across Mexico. These results provide a regional framework for understanding long-term redox evolution and groundwater quality in heterogeneous aquifer systems.

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