Impact of Black Shale Weathering on Stream Water and Sediment Quality in Eastern Kentucky

Author

Brett Hopt, Portland State UniversityFollow

Sponsor

Portland State University. Department of Geology

First Advisor

Robert B. Perkins

Term of Graduation

Summer 2025

Date of Publication

9-9-2025

Document Type

Thesis

Degree Name

Master of Science (M.S.) in Geology

Department

Geology

Language

English

Subjects

Black Shales, Kentucky, Organism Health, Sequential Extractions, Trace Metals

Physical Description

1 online resource (ix, 137 pages)

Abstract

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

This study examines the impacts that black shale surface exposures have on 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. Here, black shale outcrops lay adjacent to similarly aged gray shales, providing optimal control of external factors that affect weathering rates. A two-pronged approach was used to analyze the effects of these shales. First, sediment and water samples from the National Uranium Resource Evaluation (NURE) database were geostatically analyzed for differences in metal concentrations between watersheds with black shale exposures and surrounding watersheds where black shale are absent. Secondly, an additional 23 water and 11 stream sediment samples were collected from northeastern Kentucky where NURE data was unavailable. These samples allowed a more detailed examination of sediment and water chemistry, with sediment samples analyzed by X-ray diffraction, scanning electron microscopy, and a four-stage sequential extraction analysis. Water samples were analyzed for many metals that were excluded from the NURE data, proving better insight into the aqueous trace element chemistry.

Results show that black shale exposures significantly affect the water and sediment chemistry of small streams in Kentucky. The oxidative dissolution of sulfide phases has sufficiently elevated the concentration of sulfate in many streams whose watersheds include black shale exposures to shift the water type from the Ca-HCO₃ type prevalent in the region to Ca-SO₄ type. Aqueous concentrations of Ba, Cd, Co, F, Mn, Rb, Sb, Se, Sr, U, and Zn were found to be significantly higher (a = 0.05) in lower (1^st - 4^th) order 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. Those "black shale streams" with pH < 6 had Cd and Ni concentrations which were higher than Kentucky fresh-water aquatic standards.

Measured concentrations of As, Ba, Cd, Co, Cu, Mn, Ni, Pb, Mo, V, U, and Zn in sediments from stream segments draining black shales were significantly elevated over control stream sediments. Sequential extraction results show that Ba, Cd, and Mn in black shale stream sediments have the highest potential mobility and bioavailability of all the metals, as the exchangeable and reducible fractions hosted over 80% of their total recoverable fractions. Cr has the lowest bioavailability with an average of 75% of the total amount recovered in the residual fraction. Other elements, including As, Co, Cu, Ni, Pb, U, V, and Zn were more evenly partitioned across various fractions. The sediment concentrations of As, Ni, Mo, and Cd exceed consensus-based probable effective (toxic) concentrations for fresh water benthic organisms at many sample locations, with Ni and Cd being the most problematic of these metals due to their relative higher bioavailability. Arsenic concentrations in the black shale stream sediments were found to be sufficiently elevated to pose an increased carcinogenic risk for both adults and children.

The findings from this study may assist in guiding future studies focused on the biological effects produced by the weathering of black shales, bring awareness to the public about the potential human health hazards due to black shales, and add to our understanding of the effects that black shales have on environmental metal loading.

Rights

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Persistent Identifier

https://archives.pdx.edu/ds/psu/44106

Recommended Citation

Hopt, Brett, "Impact of Black Shale Weathering on Stream Water and Sediment Quality in Eastern Kentucky" (2025). Dissertations and Theses. Paper 6923.