GEOCHEMICAL CHARACTERIZATION OF SEDIMENTS WITHIN THE SAUZ-ENCINILLAS VALLEY, CHIHUAHUA, MÉXICO

Prevailing arid conditions in the northern part of Mexico, have caused an excessive dependence on groundwater extraction in its granular aquifers. Since the 1980s, high As and fluoride concentrations in groundwater coming from the Sauz-Encinillas´ aquifer in the central portion of the state of Chihuahua. Different authors have speculated on the origin of these elements within the area, possible explanations range from: 1) the abundance of felsic rocks, 2) clay deposits caused weathering of the igneous rocks, 3) presence of minerals such as fluorite and arsenopyrite in the surrounding ranges, 4) occurrence of clastic sedimentary rocks at depth (¿shales?), etc. Although several studies have been conducted on the groundwater and surface water (Laguna de Encinillas), to our knowledge no previous work have been published on the characterization of geological materials (rocks and sediments). Due to all of this, a long-term project has been carried out in the area and several theses have been obtained from this effort. Rock analysis and geophysical measurements have been performed since 2010. To characterize the granular material making up the aquifer, sediment samples at least 40 cm deep were collected from the valley. Samples were analyzed via digestion analysis and ICPOES in labs from the Mexican Geological Survey (SGM), concentrations of 33 elements were obtained. XRD analyses were later performed at the SGM and CIMAV labs in order to identify mineral phases in the sediments. None of these element levels are above the NOM147 levels, which regulates heavy element content in soils. Once concentrations of analyzed elements were obtained, a geostatistical analysis and kriging were carried out. Maps are still inconclusive, due to the low number of samples (12). Results from 10 other samples are expected and may help explain the behavior seen in maps.