CHEMICAL GEOTHERMOMETRY AND ENTHALPY-CHLORIDE RELATIONSHIPS INDICATE MIXING/DILUTION OF METEORIC AND GEOTHERMAL WATERS, GUANAJUATO, MÉXICO

Groundwater samples were collected from production wells, a thermal well, and springs, including hot springs from the region surrounding the City of León (population ~ 1 million) in the State of Guanajuato, México. Groundwaters were analyzed for major solutes as well as Li and B in order to characterize the hydrochemistry, and to assess any relationships between the well waters, agricultural return flows (i.e., from irrigation), and the geothermal waters. Focus was directed towards the La Muralla well field where water temperatures range from 28 to 38°C (mean=32.5°C). By comparison, the temperature of waters discharging from Comanjilla Hot Springs, roughly 50 km northeast of the well field, exceed 100°C. All groundwaters sampled are dilute Na-Ca-HCO3 type waters with relatively high Li and B concentrations (mean Li and B of 19.3 and 13.7 µmol/kg, respectively). Moreover, the high Li/Na ratio (x10-4) of the La Muralla well waters (58 ± 8.9) suggest these groundwaters have interacted with geothermal fluids or have very long aquifer residence times. Chemical geothermometry, using the Mg-Li geothermometer, indicates that La Muralla well waters attained temperatures between 40°C and 60°C at some point during their history. The Mg-Li geothermometer indicates that water from Comanjilla Hot Springs attained an "equilibration" temperature of roughly 156°C. Enthaply-chloride relationships demonstrate that La Muralla well waters are mixtures of local meteoric water and thermal waters from the Comanjilla Hot Springs system. The proportion of local meteoric water that contributes to the La Muralla well waters is estimated to range between 33% to 100%. Furthermore, enthalpy-chloride relationships for Comanjilla Hot Springs and the Los Charcas thermal well water suggests the existence of a deep, common thermal reservoir for the León Valley. The deep thermal reservoir is predicted to have a minimum temperature of 254°C, and occur between 2.5 km and 7 km depth. Consequently, it is proposed that the heat source for the thermal waters of the León Valley are "shallow" magmatic bodies and/or conductive heating during deep convection of meteoric waters along local faults.