Hydrochemical and isotopic characterization of the Region Carbonifera aquifer: An example of hydrogeological systems in the semi-arid climates of northeastern Mexico

Hydrogeological resources in the semiarid regions of Mexico support the economy and essential domestic activities of around 17 million people. However, adverse climatic conditions and overexploitation of aquifers affect the quantity and quality of the resource, added to problems of anthropogenic pollution and the salinity of water bodies. The Region Carbonifera aquifer represents the primary hydrogeological system in the most important coal-mining region in Mexico, located in the state of Coahuila. In this work, we present a complete dataset of 157 samples from surface and groundwaters sites have been used to characterizethe physicochemical and isotopic processes responsible for the composition of circulating waters, clarifying their origin, and to evaluate the water quality in terms of human consumption and irrigation use. The aquifer is mostly represented by Ca2+-Mg2+-SO42- and Ca2+-Mg2+-Cl- type waters, that supports salinization problems in 76% of the samples as well as sulfate excess. The origin of this chemical behavior seems to be the result of three main processes: 1) dissolution of soluble salts (gypsum, anhydrite and halite), 2) high surface evaporation under semiarid climate conditions, and 3) ionic and reverse ionic exchange. Processes 1 and 2 are also supported by the enrichment trends in the delta O-18 and dD signatures. For human consumption, 21% of the samples show high concentrations above the maximum permissible limits of the Official Mexican Standard (NOM-127-SSA1-1994) in total dissolved solids, Cl-, Na+, and SO42-. Additionally, 80% of the waters have some irrigation limitations due to excess sodicity or salinity. The worst quality waters for human or irrigation uses are located south of the aquifer in the municipalities of Juarez and Progreso. This study exhibits the complex hydric situation of the aquifer, raising awareness of the need to seek alternative sources, rational exploitation of resources, the use of crops that better adapt to these semiarid conditions, and intensifying hydrochemical monitoring in the region.