Isotopes of selenium as redox tracers of geochemical processes influencing the export and behavior of selenium contamination in coal mine watersheds

Selenium is a redox-active element that is necessary for life in small quantities but causes toxic effects for wildlife above 5 μg/L in water and for humans above 50 μg/L in drinking water. Se contamination is released by oxidative weathering of Se-bearing shales, and this contamination is often associated with mining activity. Selenium is most mobile and toxic in its oxidized Se(VI) form, less mobile as Se(IV), and immobile in the Se(0) or Se(-II) forms. Se isotope ratios have been developed to measure the presence and extent of geochemical processes affecting the mobility and environmental impact of Se. Se isotope systematics are well defined through laboratory experiments but have seldom been applied to real environments. The leftover material from coal mines within the Appalachian region is constructed into valley fills along mountain slopes, which produce Se-laden effluents exceeding EPA regulations. The redox conditions within these watersheds are relatively unknown, and the behavior of Se is not well studied. Water samples collected from 3 valley fills and their respective watersheds were analyzed for Se concentration, Se speciation, and 82Se/76Se isotope ratios. A leaching experiment on shale to determine the Se(VI) isotopic composition before the Se(VI) undergoes redox reactions yielded a δ82/76Se = -0.46‰ for Se(VI). The δ82/76Se values of surface water samples observed at each site for Se(VI) were; Mary’s Fork (2.56 – 4.54‰), 4-Mile Site (3.38 – 3.86‰), and the T&M Site (3.53 – 4.35‰). The isotopically heavy values of Se(VI) suggest that reduction of Se(VI) occurs within the fills. A trend of increasingly heavy Se(VI) values further downstream is hypothesized to occur due to Se(VI) reduction in the hyporheic zone. The groundwater and isotope data suggest that the effluents leaving the toe are a mix of runoff from oxic and suboxic/anoxic portions of the fill, highlighting the heterogeneous structure and varying redox conditions within valley fills.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Geology

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Stiegman, Matthew S

Contributors dc:contributor

• Johnson, Thomas M

• Isotope Geochemistry
• Selenium

Statement dc:rights

• Copyright 2024 Matthew Stiegman

Language dc:language

eng, en

Handle dc:identifier

https://hdl.handle.net/2142/124565

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/124565