Mobility of Ni, Co, and Mn in ultramafic mining soils of New Caledonia, assessed by kinetic EDTA extractions

The aim of this study was to determine the mobilization capability of Ni, Co, and Mn contained in New Caledonian ultramafic soils. Two series of soils were sampled: bare-surface mining soils in a Ni-mining context (n=10), and forest soils, either in the vicinity of mine-working areas (n=3) or far away from any known mining activity (n=2). We focused on the <100m soil fraction, because of its sensitivity to wind erosion, and its possible dissemination toward urbanized areas. In order to assess maximum potential metal mobility, EDTA kinetic extractions were performed over 24h. Extraction curves were modeled as the sum of two first-order reactions. The first EDTA extracted pool corresponds to quickly released metals, while the second pool corresponds to slowly released metals. The remaining fraction is the EDTA non-extractable pool. Extractable Ni, Co, and Mn were always low in relation to total concentrations (<5% for Ni, and 5-35% for Co and Mn). The extraction rate of the less labile pool was significantly higher for forest soils than for mining soils, whatever the metal. Despite the greater extractability potential in forest surface soils, mining soils represent a bigger environmental risk, because of their high metal content and, above all, because of their predisposition to surface runoff and eolian deflation.