Ultramafic geoecosystems as a natural source of Ni, Cr, and Co to the environment: A review

Ultramafic soils are in equal parts fascinating and dangerous. Developed on rocks derived predominately from the Earth's mantle and metamorphosed at the ocean floors, ultramafic soils form in the places where tectonic forces brought these rocks from mantle depths to the surface. As it is common in nature, both ultramafic rocks and soils are site-specific, and vary in character and composition; however, they have one thing in common, they are enriched in certain elements and three metals in particular form an "ultramafic" triad: Ni, Cr, and Co. These three metals are far from being human-friendly and strict legislative limits are established for maximum allowable concentrations of these metals in soils, but mostly in the case when the metals are of anthropogenic origin. However, ultramafic soils are a natural phenomenon where increased metal content is not the result of pollution, but rather referred as a peculiar geochemical background, therefore there is no reason for their remediation. At the same time, it is not that easy to actually find an ultramafic soil that does not overstep the limits (for the sake of this paper we use median world Regulatory Guidance Values - RGVs). Often, mobile Ni and Co concentrations are above the guidelines when doing tests to estimate the bioavailable fraction (EDTA and DTPA), and high concentrations of Ni are also commonly present in excluder plants (also edible ones). Also waters in ultramafic areas often exceed Ni and Cr(VI) limits. It is therefore expected that the ultramafic metals are present in the food chain and they might constitute a potential health risk. Thus, there is a need for additional research focused on assessment of the potential health consequences of chronic high exposure on naturally occurring Ni, Cr, and Co.