Leaching tests reveal fast aluminum fluoride release from ashfall accumulated in La Palma (Canary Islands, Spain) after the 2021 Tajogaite eruption

Leaching tests conducted on fresh ashfall samples taken soon after the onset of the 2021 Tajogaite eruption occurred at the Northwest flank of the Cumbre Vieja Volcanic Ridge in La Palma (Canary Islands) have revealed relevant geochemical trends which have important environmental significance and practical implications. The ashfall deposited on most of the island during the eruptive episode (9/19/2021-12/13/2021) contains, in addition to common constituents in volcanic tephra (e.g. volcanic glass, silicates like olivine, amphibole or pyroxene, magnetite, etc.) a significant amount of very fine-grained (diameter range of 10-500 mu m) crystals of highly soluble salts like AlF3, NaCl, KCl or CaSO4. These salts likely crystallized at high temperature by interaction of acid volatile phases (e.g., HF, HCl, SO2) with volcanic glass and silicate minerals during the ascending flow of the eruptive plume, though some adsorption of marine aerosols on ash surfaces during the subsequent descending flow and transport to more distal areas could have also taken place. The dissolution of these salts in the leaching tests (conducted with ultrapure distilled water during 90 min) reveals a fast release of important amounts of F-, Cl- and SO42- as major anions, and Al3+, Na+, K+, Ca2+, Mg2+ and Si4+ as major cations, in addition to minor amounts of trace metals like Mn2+, Zn2+, Ba2+ or Sr2+. The release of F- and Al3+, both potentially toxic and travelling together as aluminum fluoride species (AlF3, AlF2 +, AlF4- ) is particularly noteworthy and represents a potential threat for the environment (including soils, surface waters or groundwater existing beneath the ashfall layers covering the island) as well as for food safety (e.g. banana plantations) and/or human health (exposure to F- or Al-contaminated water). On the other hand, the tephra does not represent an environmental hazard as regards to other toxic elements traditionally reported for ashfall released in other volcanic eruptions, such as As, Pb, Hg or Se, which were always close to or below the limit of quantification. Besides, the Tajogaite 2021 ashfall shows a significant acid-neutralizing capacity as a result of the partial dissolution of volcanic glass in the F-rich solutions. This study presents a discussion of geochemical and mineralogical controls on element mobility during ashfall/water interaction in the environment which may be worth considering in any future restoration or land management plans for the island.