THE IMPACT OF BLACK SHALE WEATHERING ON STREAM AND STREAM SEDIMENT CHEMISTRY IN EASTERN KENTUCKY

Black shales contain high concentrations of trace elements in addition to their elevated organic carbon contents. Once exposed at the surface, weathering processes release these elements into the environment, including many (e.g., As, Cd, U, Zn) that could pose threats to human and environmental health. However, few studies have focused on the impact of natural weathering of black shales on local stream water chemistry or the transport and partitioning of these geogenic contaminants in stream environments.

This study examines the major and trace element chemistry of stream waters and sediments in eastern Kentucky. This area, along the eastern flank of the Cincinnati Arch, is a near-ideal setting to study the environmental impacts of black shales as their outcrops lay adjacent to similarly aged gray shales, allowing for optimal control of external factors. The chemistry of 237 stream sediment samples and 633 stream water samples from the National Uranium Resource Evaluation (NURE) database were geostatically evaluated along with an additional 23 water and 11 stream sediment samples collected from northeastern Kentucky where NURE data were not available.

Our results show that the aqueous concentrations of Ba, Cd, Co, F, Mn, Rb, Sb, Se, Sr, U, and Zn are significantly higher (a = 0.05) in headwater streams draining watersheds where black shales outcrop than in streams that do not drain black shale exposures, with concentrations of toxic Cd as high as 10 ug/L. Many of these elements are released via the oxidative dissolution of sulfide phases, which are abundant in the black shales. The concentration of SO₄ was sufficiently elevated in many black shale streams to shift the water type from the Ca-HCO₃ type prevalent in the region to Ca-SO₄ type. Some stream segments draining black shales were also much more acidic (pH<5, and as low as 3.5) than control streams (avg = 7.8+/-0.5). Sediments from stream segments draining black shales were also significantly elevated in As, Co, Cu, Ni, V, and Zn; average sediment concentrations of As and Ni in these stream segments (n = 60) exceeded consensus-based probable effective (toxic) concentrations for freshwater benthonic organisms.