306-14 Redox Processes and Water–Rock Interactions Governing Arsenic and Fluoride Co-Occurrence in a Semi-Arid Volcanic Aquifer

Abstract:

Arsenic (As) and fluoride (F^-) groundwater contamination is a global concern posing significant health risks, especially semi-arid regions where population growth and agricultural demand increase reliance on volcanic aquifers with limited recharge. The Independence Basin Aquifer System (IBAS), Mexico, serves as an ideal study location for this contaminant co-occurrence. Currently, the primary processes in the study area presumed to be causing high dissolved As and F^- are anthropogenic overexploitation of the aquifer and geogenic geothermal influence in fault zones. Recent studies suggest that a chain of geochemical reactions responsible for mobilization begin with silicate hydrolysis, followed by oxyanion release from arsenic-bearing iron-oxides, and dissolution of F-bearing minerals. The influence of groundwater temperature, pH controls on solid-phase materials, and dissolved organic matter (DOM) impact on these contaminants are not well understood. The objective of this dissertation is to establish locations of elevated As and F^- and to investigate potential mobilization drivers such as groundwater temperature, pH influence on leachates of soils and rocks, and DOM from groundwater, soil leachates, and rock leachates. Characterization of major and trace elements of groundwater, soil leachates, and rock leachates established a hydrogeochemical framework and a baseline for present contamination and solid-phase contributions of As and F^-. Characterization of DOM composition and relationships with contaminants delineated the mobilization processes taking place in recharge flow paths and groundwater interface with lithology along fractures. Results demonstrated that elevated arsenic and fluoride are associated with elevated temperatures, alkaline Na-HCO₃ type waters, depleted calcium, prolonged residence times and longer water-rock interactions. This study also revealed that volcanic soils and rocks are viable contaminant sources. DOM facilitates the mobilization of arsenic, whereas fluoride was decoupled from DOM, highlighting that F^- mobilization is primarily controlled by water-rock interactions. These findings provide an integrated conceptual model linking groundwater evolution, soil contents, volcanic lithology, and DOM dynamics within the Independence Basin. This research establishes a framework that is applicable to global analogue settings with semi-arid climates and volcanic aquifers experiencing As and F^- co-occurring contamination.

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