How wildfires can impact nickel concentration and biogeochemistry at ultramafic drinking water catchments: An example study in New Caledonia

In New Caledonia, a significant fraction of soils developed on the Peridotite Nappe are naturally enriched in trace metals, such as nickel and chromium, that can be remobilized upon wildfires (Thery et al., 2022). In this Pacific archipelago, the average annual burnt vegetation surface is estimated to be 30,000 ha, representing 2% of the total land surface (Dumas et al., 2013). However, much larger surfaces can burn during strong El Nino years. This was notably the case in 2016 at Ile des Pins, in the South part of the archipelago, where the burnt surface reached 1000 ha compared to an average annual value of 300 ha. Concomitantly, a dramatic increase in nickel concentrations could be observed in some water supply catchments, with some values reaching up to 4000 µg/L compared to the WHO and European guidelines of 70 µg/L and 20 µg/L, respectively. This situation led the authorities to order some investigations to better understand the link between these increased wildfires and the degradation of freshwater quality.

In this presentation, we will discuss the results of these investigations performed for two years on the dynamics and biogeochemistry of nickel across a drinking water catchment supplied by both surface and groundwater. The surface water originates from a doline, which is a characteristic feature of karstic landscapes frequently observed in the lateritic landscapes on ultramafic rocks from New Caledonia (Jeanpert et al., 2016). Geochemical analyses of the surface water collected in the burnt doline showed very high nickel concentration (i.e. up to 300,000 µg/L) compared to groundwaters (i.e below 30 µg/L). These surface waters were also found enriched in sulfate (i.e. up to 3200 mg/L) compared to groundwaters (i.e. below 8 mg/L). Water isotopes analyses allowed to propose a simple mixing model between these two end-members to reconstitute the water supply at the drinking water catchment. In addition, mineralogical characterization of the doline sediments and XAS-derived analysis of nickel speciation allowed to evidence a mixed Mg/Ni-sulfate and Ni/Fe-sulfides as the two major Ni-bearing mineral species. Although the sulfides are common species in sedimentary settings, the occurrence of a mixed Mg/Ni sulfate was considered to result from the large 2016 wildfires that impacted the nickel biogeochemistry in the sediments. The high solubility of this latter mineral species is probably playing a major control on nickel concentration in the water that is supplied to the downstream drinking water catchment.

This study brings further understanding on how wildfires can impact drinking water catchments quality by modifying the biogeochemical cycling of trace metals across their related watersheds. In the case of New Caledonia where most of drinking water catchments are supplied by surface water (a significant fraction of them being related to ultramafic watersheds), it spreads awareness to local policy-makers about the vulnerability of the water resource relative to wildfires. At a larger scale, it also put some warning on the possible impact of wildfires on drinking water catchments related to ultramafic watersheds worldwide.