Trace-Metal Accumulation In Regolith Derived from Black Shale In Humid and Semi-Arid Weathering Regimes, USA

Redistribution of metals during weathering of black shale is influenced in part by climate and composition of parent shale. Devonian black shale beneath large areas of the eastern United States weathers in a humid, temperate climate. In contrast, Cretaceous calcareous black shale exposed in the Midwest and Rocky Mountain Region weathers in a semi-arid climate. Understanding processes that affect the distribution of metals in contrasting climates is needed to anticipate environmental impacts due to land use and climate change on these black shale terrains.

Weathering of Devonian New Albany Shale in eastern Kentucky produces a metal-rich, acidic solution that exceeds the acid-neutralizing capacity of the shale. Precipitation exceeds evapotranspiration, so acid-soluble metals (Co, Mn, Ni, Zn, Fe and As) are transported in shallow ground water to nearby streams where acid neutralization and oxidation result in coprecipitation with Fe, Al, or Mn-oxyhydroxides. Other metals accumulate in refractory phases in the acidic regolith, including Cu (56 mg/kg) bound to organic litter, Mo (96 mg/kg), Cr (124 mg/kg), Se (4 mg/kg), Tl (8 mg/kg), and V (465 mg/kg). Illuviation further concentrates metals at the surface, where they are mechanically removed during erosion.

The Cretaceous Mancos Shale in southwest Colorado is calcareous and contains less metal than New Albany Shale (regolith concentrations: Cu, 22 mg/kg; Mo, 6 mg/kg; Cr, 55 mg/kg; Se, 2 mg/kg; Tl, 1 mg/kg; and V, 150 mg/kg). Nonetheless, these metals are of concern due to their accumulation in readily soluble forms in neutral to alkaline regolith where evapotranspiration exceeds precipitation. Some metals have high concentrations in saturation paste extracts (Se, 16 mg/L; Mo, 1.2 mg/L; and As, 0.11 mg/L) and are known environmental contaminants (Mancos Shale is the major source of Se contamination in the Colorado River), illustrating the importance of understanding the redistribution of these metals during weathering.