EFFECT OF LOW- AND HIGH-ENTHALPY GEOTHERMAL TEMPERATURES ON ARSENIC AND FLUORIDE LEACHING WITHIN THE INDEPENDENCE BASIN AQUIFER SYSTEM, MEXICO

The Independence Basin Aquifer System (IBAS) exhibits elevated concentrations of elevated arsenic (As) and fluoride (F^-) in groundwater. In the municipal area of San Miguel de Allende, south-central IBAS, values up to 50 μg/L of dissolved As and 3.4 mg/L of F^-are reported, exceeding the WHO maximum contamination limits of 10 μg/L and 1.5 mg/L, respectively. These toxic elements are attributed to mobilization via interactions between recharge water with volcaniclastic sediments, desorption dependent on pH, as well as influence from probable geothermal activities via mixing between hydrothermal fluids and younger shallow groundwater. We aim to investigate the influence of groundwater interactions via upwelling hydrothermal fluids through existing fault systems in the area (e.g., San Miguel de Allende fault).

Sediment analysis from two drill cutting cores taken from the eastern and western areas (away from the fault) involved mineralogical characterization and elemental composition via XRD and XRF. Mineralogy of these two cores indicated F-bearing minerals such as biotite and fluorapatite, and of minerals known to contain trace amounts of As such as ferrosilite and hematite are present. Dissolution experiments of each drill cutting cores completed with DI water and groundwater collected within the IBAS indicated that the local groundwater allowed for higher levels of leaching with respect to F^-. Batch reactor experiments using deeper sediment (Pozo Mirador 300 – 350 m) and groundwater (pH 7.4 and low As/F^-) sampled close to Pozo Mirador were conducted to understand the mobility of these elements under varying conditions such as pH (5, 7, and 9) and temperature (50, 125, and 200˚C). Arsenic showed higher concentrations with increasing temperature in acidic, neutral, and alkaline conditions. Leaching of F^- showed no significant changes between 50 and 125˚C, however its concentrations increased at the high temperature (200˚C). These results indicate that elevated temperatures control the release of higher concentrations of As within the groundwater of the IBAS in relation to both low- and high-enthalpy geothermal systems. Release of F^- into the groundwater is not affected by low-enthalpy and require high-enthalpy geothermal conditions to leach higher concentrations.