Hydrogeochemical and isotopic evidence for groundwater-geothermal connectivity between Valle de Perote and Los Humeros geothermal field, Mexico

• Groundwater in Valle de Perote shows hydrogeochemical signals of geothermal mixing. • δ¹¹B, δD, and δ¹⁸O isotopes confirm fault-mediated fluid transfer from LHGF. • Conservative tracers reveal regional mixing along the Alchichica fault zone. • Principal Component Analysis identifies geothermal influence in shallow aquifers. • Results refine the conceptual model of Los Humeros geothermal recharge pathways. The Los Humeros Geothermal Field (LHGF), the third most important geothermal power plant in Mexico with an installed capacity of 95 MW, has the potential to double its output. While the hydrological recharge of the LHGF has been previously investigated, refining the systems conceptual model is essential for future development. One recharge zone of particular interest is the Valle de Perote aquifer, where selected wells exhibit elevated temperatures and boron concentrations compared with regional averages, suggesting possible mixing between Perote Valley groundwater and fluids from the LHGF reservoir. The geological framework of the LHGF in the Valle de Perote includes normal faults located outside the Humeros caldera, such as the Alchichica and El Limón faults (Veracruz, Mexico). These structures may facilitate the upward migration of geothermal fluids that interact with the shallow Perote aquifer. This study combines isotopic (δD, δ 18 O, δ 11 B), hydrochemical, and statistical analyses (PCA, Durov, and Piper diagrams) to characterize the origin and evolution of groundwater. The results reveal that most cold and hot water samples are of meteoric origin, with signals of mixing between the sodium-bicarbonate and calcium/magnesium-bicarbonate facies. In contrast, wells located near the geothermal field show enriched boron concentrations and isotopic shifts, suggesting a geothermal contribution. Principal Component Analysis (PCA) and Durov diagrams support the occurrence of geochemical mixing and cation exchange processes, indicating that geothermal fluids migrated into the shallow aquifer of the Perote Valley. This fluid transfer is facilitated by regional fault systems and permeable lithological contacts between volcanic and sedimentary units. The integration of isotopic and hydrochemical tracers provides new evidence for the hydraulic connectivity between the Valle de Perote aquifer and the Los Humeros geothermal system. These findings improve the understanding of fluid flow paths and will support future groundwater management and geothermal resource assessments in the region.