Masters Theses

Date of Award

8-1991

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Don W. Byerly

Committee Members

Michael L. McKinney, Claudia I. Mora, R. Bruce Robinson

Abstract

Road construction on the Tellico Plains Robbinsville Scenic Highway encountered sulfidic lithologies in rocks of the Precambrian Ocoee Supergroup which upon exposure and weathering can produce acid drainage. Encapsulating fills designed to restrict oxygen and water from contacting the sulfidic material are being utilized to mitigate acid drainage. The encapsulated design utilizes a compacted surface soil layer to restrict reactants for sulfide oxidation, a basal limestone aggregate layer to neutralize any acidic leachate, and a drainage network to divert water around and under sulfidic materials.

The present study proposed to monitor gas and temperature variations within the encapsulated fills and water quality from fill underdrains. The information was used to assess the design integrity in mitigating acid drainage, determine factors which influence fill atmosphere composition, determine gas and water transport pathways, and assess the activity of iron-oxidizing bacteria.

Oxygen, carbon dioxide, and temperature were measured in acid- producing highway fills over a 17 month period to provide in situ evaluation of mitigation designs. All parameters measured showed distinct seasonal trends at all depths with carbon dioxide concentration and temperature highest and oxygen concentration lowest in the summer months. Oxygen concentrations generally increased with depth due to a compacted surface soil layer restricting diffusion from the surface and a well oxygenated basal limestone aggregate layer used for drainage diversion and neutralization of any acidic leachate from the fill. Changes in oxygen and carbon dioxide were correlated throughout the study area. The changes in concentrations could not be related to any single controlling mechanism, rather, as indicated by carbon isotope analyses, a combination of organic and inorganic sources. Oxygen is apparently consumed from both biotic respiration and oxidation reactions and lesser amounts of carbon dioxide result from a combination of neutralization reactions and biotic respiration. Correlations of gas composition with temperature were weak due to the influence of additional variables such as barometric pressure, precipitation, and fill porosity. Temperatures remained conducive to the growth of iron oxidizing bacteria throughout most of the year.

Isotopic data from aqueous carbonate species from saturated upslope soils and fill drainage indicate 2 dominant water transport pathways. In fills constructed on gentle slopes, water is transported as throughflow in sub-fill soils and eventually is discharged into the limestone aggregate layer. Aqueous carbonate species from these fills had δ13C values ranging from -15% to -25% (PDB) indicating that the water chemistry is strongly influenced by organic sources of carbon dioxide (δ13C = -21% to -26%%) with only a slight contribution from limestone dissolution (δ13C-2%). In contrast, water transport pathways in fills constructed on steep, rocky slopes occur through fractured bedrock, eventually discharging into the limestone aggregate layer.

Isotopic data (δ13C = -5% to -20%%) and high concentrations of sulfate in discharge waters indicate a greater interaction of water with sulfidic fill material and subsequent development and neutralization of acid drainage. Isotopic data from dissolved sulfate showed that water derived oxygen contributes approximately 75 percent of the oxygen in sulfate. The results indicate that bacteria-catalyzed reactions are the primary mechanism for sulfide oxidation within the encapsulated fills. This conclusion was also supported by confirmatory tests on fill leachate for the presence of the iron and sulfur-oxidizing bacterium Thiobacillus ferrooxidans.

The encapsulating design functions to restrict the transport of water into the encapsulated sulfidic material, limits transport of oxidation products from the fill, and neutralizes acidic drainage prior to release to the environment. Water quality data, however, indicate greater interaction of water with sulfidic material in fills constructed on steep, rocky slopes. Recommendations from this study include increasing the thickness of the basal limestone aggregate, especially on steep, rocky slopes, to provide more efficient drainage and prevent the development of acid drainage. In addition, a short-term feasibility study of the effectiveness of oxygen traps installed within the drainage network is recommended.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS