HYDROTHERMAL INFLUENCE ON ARSENIC AND FLUORIDE IN GROUNDWATER OF THE INDEPENDENCE BASIN AQUIFER SYSTEM IN CENTRAL MEXICO

Elevated concentrations of arsenic (As) and fluoride (F^-) have been reported from many Latin American regions including Independence Basin Aquifer System (IBAS) located in Guanajuato (central Mexico). Groundwater in San Miguel de Allende, a municipal area located in southern IBAS contains up to 50 μg/L of dissolved As exceeding WHO and national limits of 10 and 25 μg/L, respectively, and 3.4 mg/L of F^- exceeding WHO limit of 1.5 mg/L. Mobility of these toxic elements in IBAS groundwater has been linked to interactions between recharge water and sediments containing volcanoclastic particles and pH dependent desorption. We investigate the influence of hydrothermal activities in this region on mobility of As and F^- in groundwater. A strong correlation was observed between concentrations of As and F^-, and geothermal tracers such as lithium (Li) and boron (B), and groundwater temperature (>30°C). We collected sediment samples from two drill cuttings (56 and 350 m in depths) located on either side of the Taxco-San Miguel de Allende fault for mineralogical characterization and elemental composition. Mineralogical characterization was performed on powdered samples using X-ray diffraction, while elemental concentrations were measured using stand-alone X-ray fluorescence. The results showed that the average As and F^- concentration in two cores were 0.87 and 2.32 mg/kg, and 9.74 and 21.70 mg/kg, respectively. Bench-scale incubation experiments are underway to study the mobility of As and F^- from these sediments under simulated geothermal conditions with varying temperature and pH. The results of this study are important to identify the source and mechanisms of mobilization of As and F^- in hydrothermally influenced groundwater systems and will assist the administrators to provide safe drinking water to the residents of IBAS in Mexico.