Purpose <p>Wildfires represent an increasing forcing that can strongly affect water quality by enhancing the mobility of trace metals. This effect is expected to be of particular concern in metal-rich environments, such as ultramafic settings. However, it can also be marked in wetlands through the fire-driven formation of Acid Sulfate Soils (ASS). Beyond concerns regarding post-fire trace metal mobility towards freshwater systems in ultramafic wetlands, these considerations raise the question of the geochemical reactions that drive trace metal dynamics in such a context.</p> Materials and methods <p>The present study contributes to addressing this question by characterizing the water chemistry along a flow path extending from a burned doline to a downstream drinking water catchment (DWC) in the ultramafic context of Ile des Pins (New Caledonia). The crystal-chemistry and molecular-level speciation of nickel in the Gleysols of the burned doline were also investigated using X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), and synchrotron-derived X-ray absorption spectroscopy (XAS).</p> Results and discussion <p>The results obtained revealed exceptionally high post-fire aqueous nickel concentrations at the doline and downstream DWC (i.e., ~ 370&#xa0;mg/L and ~ 4&#xa0;mg/L, respectively, compared with the WHO and EU guidelines of 0.07&#xa0;mg/L and 0.02&#xa0;mg/L). These high concentrations resulted from the wildfire-induced transformation of the studied Gleysols into ASS through a multi-step scenario involving (1) oxidation of Ni-bearing pyrite [(Ni, Fe)S₂) and millerite (NiS) following enhanced O₂ diffusion at depth, (2) water acidification upon sulfide minerals oxidation and (3) acidic dissolution of Ni-bearing chrysotile [(Ni, Mg)₃Si₂O₅(OH)₄)]. This reaction sequence generated large amounts of dissolved nickel, magnesium and sulfate that further reacted to precipitate Ni-hexahydrite [(Ni_xMg_1−x)SO₄.6H₂O)] upon water evaporation. Owing to its high solubility and major contribution to Ni hosting in the surface horizons of the Gleysols (i.e., 40–50% of total solid Ni), this mineral species is considered to have then driven nickel dynamic at the Gleysol/water interface, and by extension at the scale of the whole catchment, through precipitation/dissolution cycles upon successive refilling/drying of the doline.</p> Conclusions <p>This spatial snapshot of post-fire nickel contamination along a doline-DWC hydrological flow path confirms that wildfires across ultramafic wetlands can be a major cause of trace metals mobility from soils/sediments towards freshwater systems through the formation of ASS. It also emphasizes the importance of metal-sulfates as secondary mineral species resulting from post-fire geochemical reactions to control trace metals dynamics during the wildfire-induced transformation of Gleysols into ASS at such ultramafic settings.</p>