Bioavailability of chromium, nickel, iron and manganese in relation to their speciation in coastal sediments downstream of ultramafic catchments: A case study in New Caledonia.

Coastal sediments downstream of ultramafic catchments can show Ni and Cr concentration well above sediment quality guidelines. Despite their potential ecological impact, the bioavailability of these trace metals in such sedimentary settings has been poorly investigated. In this study, we tried to fill this gap by performing kinetic EDTA-extractions across a shore-to-reef gradient in lagoon sediments downstream of an ultramafic catchment in New Caledonia and interpreting the results in regard of synchrotron-derived speciation. Measured bioavailability ranged from very low for Cr (below 1% of total Cr) to medium for Ni (below 5% of total Ni). Both trace metals showed a decreasing shore-to-reef bioavailability gradient reflecting the larger deposition of ultramafic sediments close to the shore. According to synchrotron-derived speciation data, the very low bioavailability of Cr is attributed to its major occurrence as Cr(III)-bearing Fe-(oxyhydr)oxides and phyllosilicates, with no evidence of Cr(VI). Considering the low occurrence of Fe-sulfides, the medium bioavailability of Ni is considered to arise mainly from the reductive dissolution of Ni-bearing Fe-(oxyhydr)oxides during early diagenesis. This reaction also explains the medium bioavailability of Fe (up to 15% of total Fe) and the positive correlation observed with Total Organic Carbon (TOC). In this regard, this latter parameter appears as a major driver of Ni and Fe bioavailability in coastal sediments downstream of ultramafic catchments. On the opposite, in the absence of Mn-oxides, TOC has no influence on Mn bioavailability (up to 30% of total Mn) that appears more likely driven by sediment sources. From an ecological point of view, considering the Australian and New-Zealand High Interim Sediment Quality Guidelines (ANZ-ISQG-H), Cr should not represent a significant risk towards benthic communities in coastal sediments downstream of ultramafic catchments. On the opposite, Ni, Fe and Mn might represent an ecological risk that should be further investigated in such sedimentary settings.