DISTRIBUTION OF LEACHABLE ARSENIC AND FLUORIDE IN THE INDEPENDENCE BASIN AQUIFER

Groundwaters from the volcano-sedimentary Independence Basin Aquifer (IBA) serves as the primary source of drinking water for over ~500,000 citizens in the northeast region of the state of Guanajuato in Mexico. The communities that rely on this water source have been exposed to high concentrations of geogenic arsenic (As) and fluoride (F), but the specific source rocks that release these are to some extent unexplored. To measure the distribution and extent of As and F in the region, twenty-four water samples were collected in Jan 2017 from the rural and urban areas within the IBA. Arsenic and F values ranged from 0 to 80 µg/L and 0.25 to 15.2 mg/L with mean values of 15 µg/L and 2.5 mg/L, respectively. These results agree with prior investigations which indicate the co-occurrence of As and F concentrations above the WHO’s guidelines of 10 µg/L and 1.5 mg/L, respectively. Previous work which surveyed adolescents from the region found the exposure to F has resulted in severe dental fluorosis. An epidemiological study of As in this region is still required to be conducted. Although awareness of the inhabitants about the co-occurrence of As and F in drinking waters must be increased with more systematic studies. The rock distributions of the IBA were analyzed by mineralogical and elemental analysis through drill cuttings from two wells located on the eastern and western sides of the basin, collected by the University of Guanajuato. The western core (500m) is primarily composed of felsic to intermediate lithologies. The eastern core (550m) displayed similar lithologies along with dominant calcite in the upper 300m. Along with usual fluorite and villaumite which are common in such rhyolitic terrains, cuspidine, a fluoride bearing calc-silicate mineral, was located which could represent one source of F. It seems there is no specific mineralogy identified to hint at the presence of As, hence it might for now be postulated that As has more deep hydrothermal fluid sources. A water – rock interaction study via batch experiment was conducted on the <63 µm grain size fraction at 1:5 weight-to-volume ratios in de-ionized water and IBA groundwater. After 200 hours, the western core displayed elevated F from all depths with the highest concentrations occurring in the upper 160 m ranging from 0.7 to 1.9 mg/L. This study will elude for more extensive geochemical fingerprinting studies in this region.