Understanding hydrologic connectivity of groundwater and surface water in western Mexico using chemistry and stable isotopes

Surface water (SW) and groundwater (GW) are deeply interconnected and can vary according to different hydrologic conditions, including physical, chemical and biological variations. Understanding the nature and extent of involvement between SW and GW is particularly important under global change, where the alteration of freshwater cycles and transformation of natural landscapes has led to widespread ecosystem degradation, as well as issues regarding availability and quality of water resources.

Therefore, the present work aims to identify relevant hydrological pathways at a regional scale using a combined approach to study GW-SW interaction in western Mexico, considering hydrochemistry, stable isotopes of oxygen (¹⁸O) and hydrogen (²H), and statistical analysis.

Stable isotopes data showed SW undergoing evaporation and becoming enriched in heavy isotopes. Widespread drought showed lake water (LW) isotopes enriched beyond the isotopic range observed in precipitation samples. Spatial differences of LW δ2H and δ18O suggest precipitation and GW as sources for only one of the lakes. Lake samples also exhibited the largest variability, as well as the lowest d-excess values reported for SW samples. Among GW samples, wells showed the most variability and hot springs the least. Major ions data showed strong thermal influence on groundwater processes, related to both tectonic and volcanic processes developed in the region.

Environmental tracers, such as stable isotopes can help us understand complex SW-GW interactions at a broader scale. This is particularly true for arid and semi-arid areas were interactions are becoming more complex under the effects of human activity.