Hydrochemical redistribution in a stressed aquifer system: Decadal attenuation of salinity extremes and expansion of diffuse agricultural contamination across Mexico.

Groundwater chemistry in large aquifer systems is reshaped not only by gradual degradation but also by subtle reorganization of contaminant stress. We analyze a decade of federal monitoring data (2012-2021) to assess national-scale hydrochemical change in Mexican aquifers using a distribution-based contaminant hydrology framework that explicitly accounts for non-coincident sampling locations. We track shifts in concentration distributions, exceedance probabilities, hydrochemical facies and multivariate association patterns. We find that salinity extremes have retreated, but without commensurate basin-wide freshening. Maximum electrical conductivity declined from 15,160 to 10,640 μS cm-1, accompanied by strong compression of the upper Na+ and Cl- tails, whereas median salinity indicators remained largely unchanged. This pattern indicates attenuation of localized, high-magnitude geogenic salinity hotspots rather than system-wide dilution. In contrast, NO₃-, PO₄3-, K+ and F- exhibit systematic rightward shifts across broad portions of their distributions, revealing the expansion of diffuse, low- to moderate-level enrichment. Hydrochemical facies remain dominated by Ca2+-Mg2+-HCO₃- waters, but the growing occurrence of mixed Ca2+-Na+-HCO₃- and Na+-Cl- types points to intensified cation exchange and mixing under sustained abstraction. Correlation structures show weakening salinity-controlled coupling and a rising influence of nutrient-related associations, while hierarchical clustering indicates that diffuse anthropogenic indicators now exert stronger control on regional groundwater grouping than salinity metrics. Together, these results point to a fundamental shift in contaminant configuration: from conspicuous, spatially concentrated geogenic salinity toward widespread, moderate anthropogenic pressure that is less visible, more pervasive and harder to reverse. Our findings underscore the need for distribution-sensitive monitoring and analysis capable of detecting subtle, system-wide hydrochemical reorganization in heavily exploited aquifer systems.