Résumé
Purpose 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.
Materials and methods 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).
Results and discussion The results obtained revealed exceptionally high post-fire aqueous nickel concentrations at the doline and downstream DWC (i.e., similar to 370 mg/L and similar to 4 mg/L, respectively, compared with the WHO and EU guidelines of 0.07 mg/L and 0.02 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)S-2) and millerite (NiS) following enhanced O-2 diffusion at depth, (2) water acidification upon sulfide minerals oxidation and (3) acidic dissolution of Ni-bearing chrysotile [(Ni, Mg)(3)Si2O5(OH)(4))]. This reaction sequence generated large amounts of dissolved nickel, magnesium and sulfate that further reacted to precipitate Ni-hexahydrite [(NixMg1-x)SO4.6H(2)O)] 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.
Conclusions 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.