Fault‐Mediated Groundwater‐Surface Water Mixing Amplifies Hydrochemical/Hydrological Dynamics in a Semi‐Arid Volcanic Lake Basin

ABSTRACT Understanding groundwater (GW) to surface water (SW) connectivity (i.e., the exchange of water and solutes between aquifers and surface bodies) is increasingly important under changing climate conditions, where precipitation deficits, water demand increase, and landscape transformations are intensifying water scarcity and ecosystem deterioration. This study examines the spatial distribution of GW and SW solute compositions and stable water isotopes across a temperate, volcanic lake basin in west‐central Mexico to assess the hydrogeological connectivity and impact of anthropogenic activities within GW and the lacustrine system during the dry season. A spatially distributed sampling was conducted for major ions, nitrate, strontium, and δ 18 O and δ 2 H (precipitation, rivers, lakes, wells, and springs). Our results indicate that GW recharge at higher elevations is largely driven by direct infiltration of meteoric water, particularly in forested areas, and is characterised by Ca 2+ –Mg 2+ –HCO 3 − facies. Evidence of GW mixing with deeper sources is reflected in elevated water temperature and δ 18 O values associated with water‐rock interactions. Evapoconcentration exerts a clear influence on isotope enrichment in rivers and lakes. Increasing strontium concentrations toward Lake Cuitzeo highlights the influence of fracture and fault connectivity on the regional GW flow system. High nitrate concentrations in shallow GW sources suggest a potential link to irrigation return flows and wastewater. Our findings provide novel evidence of the climatic and anthropogenic influence on hydrological dynamics of GW‐fed systems in semi‐arid regions of Mesoamerica, underscoring the urgent need for integrated management and protection strategies to sustain regional water resources and fragile lake systems worldwide.