Extent and Impact of Arsenic and Fluoride Leaching from Hydrothermally Impacted Sediments of the Independence Basin Aquifer System, Mexico

The Independence Basin Aquifer System (IBAS) exhibits elevated concentrations of arsenic (As) and fluoride (F-) in drinking waters (groundwater). 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 Mexican national limits of 10 and 25 μg/L, respectively), and 3.4 mg/L of fluoride (exceeding WHO limit of 1.5 mg/L). Mobility of these toxic elements in IBAS groundwater has been linked to interactions between recharge water, volcaniclastic sediments, pH-dependent desorption, and geothermal influence. We aim to investigate the influence of hydrothermal activities in this region on mobility of As and fluoride in groundwater. We analyzed sediment samples from two drill cutting cores (56 and 350 m in depths) collected from either side of the Taxco-San Miguel de Allende fault for mineralogical characterization and elemental composition on powdered samples using X-ray diffraction and X-ray fluorescence, respectively. The results displayed higher concentrations of As and F for the deeper drill cutting core, with average concentrations of 9.74 and 21.70 mg/kg, respectively, whereas the shallower core displayed average concentrations of 0.87 and 2.32 mg/kg, respectively. X-ray diffraction identified F-bearing minerals biotite and fluorapatite are likely contributing to release of these trace elements as they are also known to contain trace amounts of arsenic. Dissolution experiments conducted using 10-meter increments of both drill cutting cores at ambient temperatures indicated that there is a higher level of fluoride leaching when using groundwater from the IBAS in comparison to ultrapure water. The results from this experiment also indicated that groundwater chemistry of nonhydrothermal sites display higher concentrations of fluoride sourced from chemical weathering of biotite within the lowest depths of the deeper drill cutting core (340 m). Batch reactor experiments using deeper sediments (300 - 350 m) were conducted to study the mobility of As and fluoride from sediments containing biotite under simulated upwelling of hot water, as well as low-and high-enthalpy geothermal conditions with varying pH and temperature. Results from this batch reactor experiment, in addition to analysis of correlations between As and nitrate (NO3-), indicated that hydrothermal sites are likely influenced by upwelling of hot spring water (50˚C) sourced from a mixture of geothermal fluid and young shallow groundwater. The results of this study are important to identify the sources and mechanisms contributing to mobilization of As and fluoride in hydrothermally influenced groundwater systems and will assist administrators and government to provide safe drinking water to the residents of the IBAS.