Origin and fate control of F, As and U in groundwater flow systems of the Sierra Madre Occidental and Mesa Central, Mexico

Fluoride (F), arsenic (As) and uranium (U) are naturally occurring trace elements that cause adverse health effects when ingested by humans. The groundwater in the cities of Chihuahua, Zacatecas and Salinas, Mexico, have elevated concentrations of these elements that are linked to felsic magmatic sequences (rhyolites, ignimbrites and granites; comprising the Sierra Madre Occidental and Mesa Central) and sediments derived from these rocks. Recent studies have identified that these elements´ host phases are fluorapatite, biotite, fluorite and glassy matrix. However, the remobilization mechanisms and efficiency of the different bearing phases from the rocks and/or sediments to groundwater are rarely characterized. In a multi-method approach that combines the analysis of groundwater geochemistry, different degrees of rock alteration, and sequential extractions procedure (SEP), the mechanisms of mobilization, as well as the fractions that contribute higher concentrations of As, F and U to the environment are proposed. Based on the present study the presence and distribution of As, F and U in groundwater are associated with fault zones in rhyolitic and ignimbrite volcanic rocks, and basin fill sediments derived from this type of rocks, deposited in the central parts of basins. The concentration of F and As exceed the drinking water limits set by both the World Health Organization (WHO) and Mexican standards by several orders of magnitude in numerous wells (>60 %), which indicates a substantial and general water quality problem. Weathering has been identified as one of the main mechanisms favoring F, As and U mobilization from the bearing phases to groundwater. This process is controlled and benefited by the different bearing phases of F, As and U (F-apatite, biotite and glassy matrix), oxidizing conditions (Eh ≈340), neutral pH (≈7.7), high temperatures (>27 °C) and regional flows. The solid phase F, As, and U fractionations were characterized by means of a modified SEP. The results showed that F is mainly linked to F-apatite and the silicate portion (biotite and glassy matrix), while As and U are strongly related to the silicate fraction associated with the glassy matrix. This research gives insight into F, As and U geochemistry and mobilization processes that can be used to further guide research needs in this area for the protection of groundwater resources. In addition, methodologies and results obtained can be transferred to geogenically impacted zones in Latin America and other parts of the world.