Geochemical Characterization of Sediments from Laguna de Encinillas, Chihuahua, México

Prevailing arid conditions have caused a pervasive mining of groundwater resources, leading to social and environmental issues. This study is a relevant case study of As and F occurrences in an over-exploited granular aquifer, typical of the semi-arid basins found throughout the southwest USA and northern Mexico, areas sharing a similar geology, climate and history. Since the 1980s, high concentrations of arsenic (As) and fluoride (F) have been detected in groundwater samples from El Sauz-Encinillas' aquifer, one of the main groundwater sources to Chihuahua city. Some cases of arsenic-related keratosis have been reported in central Chihuahua and it's common to see persons with dental fluorosis. Different authors have speculated on the origin of these elements in the area, from weathering of the felsic rocks (mostly rhyolites and rhyolitic tuffs) outcropping in the surrounding ranges, or presence of hydrothermal minerals such as fluorite and arsenopyrite, and/or ion exchange reactions within the fluvio-lacustrine deposits in the valley, etc. This project is a long-term effort aimed to perform geological, geophysical, and geochemical studies in the area. It is currently supported by a research grant provided by the Mexican Education Secretary. Representative rock and sediment samples were collected during the field campaign in 2017. To characterize sediments, ten sediment samples were collected in the Laguna de Encinillas, the area showing higher levels of As and F in groundwater in the study area. Soil samples were digested and analyzed (via ICPOES and XRD) in the Mexican Geological Survey labs in Chihuahua to obtain heavy elements concentrations and identify mineral phases present in the granular material. Five column leachate experiments were carried out using sediment samples collected from the study area. The column experiments were carried out in closed system, i.e. utilizing closed plastic columns which did not allow interaction with atmospheric gases (such as CO₂). The test simulates the potential leachability of soluble components during water-rock/sediment interaction processes. Concentrations of As in the sediments, and As and F in leachates were above the Mexican norms for soils and water for human consumption.