Abstract
More than 30% of continental surface is affected by wildfires every year. These fires arestrongly influenced by natural climatic phenomena such as El Niño, but their frequency and intensity are increasing as a result of current climate change. In addition to their devastating impact on biodiversity, fires represent a real threat to the quality of water resources, as they can lead to contamination by trace metals. In New Caledonia, soils developed on the Peridotite Nappe are naturally rich in trace metals such as nickel and chromium, and around 3% of the archipelago burns each year. The impact of fires on Cr mobility is first explored through laboratory heating experiments in which different soil horizons from New Caledonia are subjected to temperatures mimicking the conditions encountered during a wildfire (200, 400 and 600°C). The Cr(III) initially present in the soil goethite is not fully incorporated into the hematite formed by thermal transformation of the goethite, and a fraction is oxidized to Cr(VI). This mineralogicaltransformation resulting from the action of heating would therefore be a major driving force behind the oxidation of Cr(III) to Cr(VI), thus facilitating the mobility of Cr in surface environments. Secondly, the occurrence and persistence of nickel contamination (up to 4200 μg/L) in a drinking water supply catchment on the Ile des Pins in the south of the archipelago after a fire episode is analysed. The coupling of Ni and S isotopic analysis with a detailed analysis of solid nickel speciation enabled us to propose a scenario explaining the evolution of this contamination. The wildfireepisode during dry season would have led to the oxidation of nickel-bearing sulphides present in the soils of the drained wetland in the watershed, and the acidity thus produced would have favoured the dissolution of nickel-bearing chrysotiles, particularly when the wetland supplying the catchment was re-watered. The persistence of Ni contamination can be explained by the formation of nickel-rich hexahydrite in these wetlands during dry periods. When these areas are re-watered during the first rains, the hexahydrite dissolves immediately, which explains the presence of Ni andS-rich waters in the water supply catchment.