Peralkaline Tuffs, Potential Source of Arsenic and Fluoride in the El Sauz-Encinillas Aquifer, Chihuahua, México

In central Chihuahua, Mexico, water supply to population relies on groundwater wells, some springs, and dams, and is characterized by elevated levels of As and F (above the Mexican norm for human consumption), as well as to other potentially toxic elements such as U, Al, Fe and Mn. The source of this contamination is thought to be geogenic and caused either by hydrothermal mineralization and/or weathering of igneous rocks and ion exchange reactions within clays in the valleys. The NW part of Mexico exposes Mid-Cretaceous to early Tertiary igneous rocks. In central Chihuahua, andesites are scarce, and rhyolites and basalts are abundant, reflecting a bimodal volcanism. Basalts composition is mildly alkaline, probably evolving to peralkaline silicic magmas that produced peralkaline rhyolite ash-flow tuffs. The youngest volcanic rocks exposed in central Chihuahua, are classified by composition, as peralkaline. The tuff glasses are hydrated and had experienced Na loss, presenting lower alkalis values, not all considered peralkaline, but they were higher at the time of eruption. The interacting rocks are mainly glassy rhyolites with relatively few As and F bearing accessory minerals. Comparing abundance and composition of glassy groundmass with other mineral phases, it appears that glass stores most of the fluoride. This glassy material is extremely reactive, and its weathering products (fluvio/lacustrine sediments) seem to concentrate the fluoride. It can be assumed that ion exchange processes are the main controlling mechanisms for the elevated content of fluoride in groundwater; mainly taking place in the water within fluvio/volcano lacustrine sediments. For the As, the source also seems to be the pyroclastic material. During explosive volcanic eruptions, gases such as HCl, HF, and other elements showing affinity for the volatile phases (i.e. As) may probably have been released and trapped within tephra. It is plausibly to assume that during the weathering of these volcanic rocks, As is adsorbed to and/or co-precipitated with metal (Fe, Mn and Al) oxides, especially with iron oxides, and in minor amount adsorbed to clay-mineral surfaces or organic carbon. These phases can be very sensitive to Eh-pH changes, and if destabilized, can release their chemical constituents within the interacting groundwater.