USING X-RAY FLUORESCENCE (XRF) AND X-RAY DIFFRACTION (XRD) TO IDENTIFY THE POSSIBLE OCCURRENCES OF FLUORIDE AND ARSENIC IN THE INDEPENDENCE AQUIFER (AI), A SEMI-ARID BASIN IN THE STATE OF GUANAJUATO, CENTRAL MEXICO

The elevated dissolved fluoride and arsenic concentrations in the Independence Aquifer located in the central plains of Mexico in the state of Guanajuato, bounded by the three Sierras acting as a natural boundary to the recharge zones to the aquifer, is a point of concern for its inhabitants. The high relief of the Sierra Santa Barbara, El Cubo and Santa Rosa, is an extension of the Trans-Mexican Volcanic Belt (FVTM). The Guanajuato State Water Commission (CEAG) recently decreed that all the cities in the state should perforate new wells for potable water to depths between 300 to 500 m to access water at greater depths. Two new potable water wells in the communities of Arrastres and Estación de Lourdes, perforated to depths of 504 and 550 m respectively, their core cuttings were analyzed for every 2 m. For both wells, major minerals, accompanied by trace and major elements were evaluated using XRF and XRD to identify the possible geogenic origin in this high F and As containing area reported in the basin. Local geology suggests that both analyzed cores are surrounded by similar lithologies as: Quaternary alluvial (sediments) and basalts (volcanics), Plio-Miocene sandstone/conglomerates (sediments) and rhyolites (volcanics). These soil derivatives of basalts have a very high Fe content which coincides with some of the XRF results containing up to 180000 and ~87000 mg/kg of Fe (communities of Arrastres and Estación de Lourdes respectively). Mineralogical analysis for 6 samples were executed at depths of 24-26, 114-116, 228-230, 300-302, 398-400 and 510-512 m for the community well in Estación de Lourdes. Analysis of sample depth of 398-400 m shows minor presence of Sabinaite (Na₄Zr₂TiO₄(CO₃)₄), Malladrite (Na₂SiF₆), and quartz (SiO₂). Also the presence of mafic volcanics nearby suggests that underground basalts and basalt-derived sediments may contribute to water chemistry. Elemental graphs were constructed using the XRF data for both wells to identify trends between elements and possibly mineralogical phase changes in the borehole lithology. To correlate the XRD and XRF mineral analysis with the surrounding geology of both wells respectively, geochemical water analysis was done on several neighboring wells that contain exceeding levels of As and F, with some concentrations reaching >50 ppb and >4ppm for As and F respectively.