Post-fire nickel mobility in acid sulfate soils developed in ultramafic wetlands of New Caledonia

Fire is increasingly recognized as a significant driver of trace metal mobility, posing risks to water quality, especially in metal-rich environments such as ultramafic wetlands. In such environments, wildfires can trigger the formation of Acid Sulfate Soils (ASS), further amplifying metal release [1]. This study investigates the geochemical processes controlling nickel (Ni) dynamics in such contexts, focusing on a burned doline and its downstream drinking water catchment on Île des Pins, New Caledonia.Water chemistry analysis and advanced imaging techniques (SEM, TEM, XAS) reveal extreme post-fire dissolved Ni concentrations (up to 368,000 µg/L in the doline and 4,300 µg/L downstream), and transformation of the studied Gleysols into ASS. Those observations are interpreted as resulting from a multi-step reaction sequence: (1) oxidation of Ni-bearing pyrite and millerite due to increased oxygen diffusion, (2) acidification from sulfide oxidation, and (3) acidic dissolution of Ni-bearing chrysotile. This cascade scenario leads to the release of nickel, magnesium, and sulfate, which then precipitate as Ni-hexahydrite [(NixMg1-x)SO4.6H2O)] upon water evaporation.Ni-hexahydrite, which is a highly soluble compound, accounts for 40–50% of the solid nickel in surface soils and drives the nickel dynamics at the soil-water interface and at catchment scale through repeated precipitation/dissolution cycles. These findings show that wildfires in ultramafic wetlands can severely mobilize trace metals via ASS formation, with metal-sulfates playing a crucial role in post-fire geochemical cycling. [1] Thery G., Quantin C., Calmels D., Jeanpert J., Morin G., Montargès-Pelletier E., Bourbon E., Kessie M., Kieffer I., Genthon P., Juillot F., 2025. Nickel dynamics in acid sulfate soils formed after wildfires across ultramafic wetlands of New Caledonia: the key role of Ni-hexahydrite. Journal Soils and Sediments, doi.org/10.1007/s11368-025-04190-9