Fluorite and Gibbsite Solubility Controls the Vertical Transport of Fluoride and Aluminum during Rainwater Percolation through Ashfall Deposits in La Palma (Canary Islands, Spain)

This study addresses the in situ mobility of fluoride and aluminum in two different ashfall deposits accumulated during the 2021 eruption of the Tajogaite volcano (La Palma, Canary Islands, Spain), which were exposed to contrasting conditions of ambient humidity and precipitation. We selected one site to the east of the volcanic emission center, located near the top of Cumbre Vieja Ridge and exposed to continuous humidity and rain, and another site to the west of the volcano situated in a lowland and characterized by much drier conditions. The mobility of fluoride and aluminum is markedly different at both sites, with the first sequence suggesting a downwards migration of Al and F, and the second sequence showing no sign of mobility. The migration of aluminum and fluorine results from the dissolution of different fluoride salts (mostly AlF3 and CaF2, as suggested by scanning electron microscopy) followed by vertical transport as ionic complexes (AlF3, AlF2+, AlF4−) during the percolation of rainwater through the ashfall deposits. Geochemical calculations suggest that the mobility of fluorine at neutral to alkaline conditions (pH 7.0–9.0) is likely limited by the solubility of fluorite (CaF2), whereas at slightly acidic conditions (pH < 6.5), the aqueous concentration of aluminum seems to be controlled by the solubility of gibbsite (Al(OH)3). This study demonstrates that aluminum and fluoride can be transported from volcanic ash to the underlying soil or groundwater, which is an environmental concern that should be followed in the future.