Arsenic and fluorine in groundwater in northern Mexico: spatial distribution and enrichment factors

North-central Mexico has groundwater contaminated with arsenic (As) and fluoride (F). Based on the dispersion patterns of these solutes, their sources are linked to felsic volcanic rock fragments and secondary minerals (clays, iron oxyhydroxides) within the alluvium fill of the aquifers. However, little is known about the effect of the enrichment factors for F and As in this area. Natural enrichment factors include evaporation, Ca/Na, and competitive adsorption and desorption from solid phases. This study used 1237 groundwater quality data measurements from 305 sampling sites collected between 2012 and 2019 in the state of Durango in north-central Mexico. To determine the contribution of enrichment factors to As and F content, the study area was divided into four sections, two being in the mountainous part of the state and two in the high plateaus. The data were compared among sections and analyzed using Spearman correlation and Piper and Block diagrams. The results indicate that the main solute enrichment mechanisms are evaporation and weathering of silicates and evaporites. Among the four sections, As, pH, and HCO₃ seemed not to vary, F varied slightly, and nitrate and total dissolved solids varied the most. The lack of variation in As among sections is associated to its strong adsorption to clay minerals and iron oxyhydroxides, whereas the diminished F content in the eastern sections is likely linked to the adsorption of F to precipitating calcite (since groundwater is saturated with respect to calcite (SI_calcite = 0.43) and undersaturated for fluorite (SI_fluorite =  - 1.16). These processes shed light on the distribution of F and As in this area, and are likely operating in other states in northern Mexico and in semi-arid areas elsewhere.