DOES SOIL DEVELOPMENT CONTROL SOURCING OR RELEASE OF TOXIC (CD, PB) AND POTENTIALLY TOXIC METALS (CU, NI) FROM GREY SHALE-DERIVED SOILS?

Shale is an important lithology across the world due to its wide spatial abundance and potentially elevated trace element concentrations under certain depositional environments. Once part of the terrestrial environment, shale-derived soils inherit organic matter and oxides (Fe and Mn) from grey shale, which can promote accumulation and retention of both geogenic and exogeneous trace metals. Identifying and quantifying the sequestration and transport of cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb) are important due to their potential impacts on water resources and aquatic ecosystems. Overall, total concentrations of Cd and Cu in the grey shales and their soils were below concentrations known to be toxic and below concentrations observed in black shales. However, Ni and Pb concentrations were elevated in soils at several sites. Tau values show that shale-derived soils are net accumulators of Cd, Cu, Ni, and Pb for most sites, despite the variation in climate and suggest human pollution sequestration. When considering sorption of the metals, < 5% of the metals were in oxidizable forms (e.g. organic matter bound) but 20 to 60% of Cu and Cd and 5 to 20% of Ni and Pb were within reducible forms (e.g. Fe oxides). This suggests that a combination of inherited and neoformed oxides are retaining an important fraction of toxic and potentially toxic metals. These results imply that soils derived from grey-shale are net accumulators of metals from exogeneous sources.