Hydrogeochemistry characterization of an overexploited municipal, agricultural, and industrial aquifer, central Mexico

Abstract

The Valley of Celaya aquifer, located in one of the most important agricultural and industrial areas in central Mexico, has been extremely overexploited for several decades for agricultural, industrial, and municipal uses. The Celaya aquifer is a multilayer, composed of an open upper alluvial (sedimentary deposit) aquifer with inter-layers of late tertiary acid tuffs and basalts (medium semi-confined aquifer) and a downward recharge from River La Laja. The groundwater follows a natural hydrogeochemical evolution, producing low total dissolved solids (TDS) carbonate-type to medium TDS Na-enriched carbonate-type water. The average water temperature is 33 °C, with three thermal groups (low temperature: 16–29 °C, medium temperature: 30–30 °C, and high temperature: 46–60 °C). The electrical conductivity ranges from 645 to 805 μS/cm. Isotopic data (δ2H and δ18O) indicates an evaporation and mixing line with a regression equation of δ2H = 5.48 xδ18O-20.67; R2 = 0.9827. Saturation index geothermometric calculations for the bottom of the well indicate temperatures ranging from 98 to 135 °C. The Cl vs. B relationships indicate the water chemistry is mostly due to local water-rock interaction with a minor magmatic contribution (deep igneous rock). The arsenic and fluoride concentrations are above the limits of the standard on the eastern part of the study area (As = 0.54 mg/L and F = 3.2 mg/L). The arsenic concentration may be due to groundwater residence time, while the fluoride concentration is generally associated with acid and intermediate volcanic rocks. Inverse geochemical modeling indicates a normal evolution of groundwater from meteoric water with some influence from meteoric water to rock-water interactions with sediments and volcanic rock to a deep regional aquifer. This aquifer is considered a low-temperature system (≤150 °C), conductive, and non-magmatic with extensional dominance where local faults produced by overexploitation play an important role in transporting water, and potentially toxic elements to the surface. The hydrogeochemical characterization in this study will help improve the sustainable management of groundwater resource.