Masters Theses

Date of Award

5-1989

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Don W. Byerly

Committee Members

Richard Arnseth, Otto C. Kopp

Abstract

Highway construction in the southern Blue Ridge province of Tennessee and North Carolina encountered sulfide-bearing, Precambrian strata that produced acid drainage (AD) upon exposure and weathering. The situation prompted the development of preconstruction evaluation techniques and encapsulation measures for these materials.

The present study proposed to: examine effluent quality over time in a series of laboratory leaching tests incorporating sulfidic material from a constructed highway segment; aid in the development of a more economical encapsulation method using fieldscale leaching tests; review current Federal Highway Administration predictive and mitigative measures using the leach test results and surface water data; examine reaction pathways, rates and mechanisms of sulfide oxidation; and examine mineralogical changes in leached or encapsulated material over time.

Seven bench-scale leaching tests were constructed using 1 kg of material from various intervals of the highway corridor which demonstrated acid-base accounting values ranging from +50 to -50 tons CaCO3 per 1,000 tons material. Eight field-scale variations of the present encapsulation method were constructed with similar source material in 30 gal (100 liter) polyethylene tanks. Baseline water quality comparisons were done using effluents from an encapsulation site underdrain and several surface water points draining from untreated roadcut exposures. Comparisons of pH, conductivity, sulfate, acidity, Al, Fe, Mn, Cu, and Zn were made among both laboratory and field models, as well as to actual fill leachates, in order to meet the study objectives. A mineralogical characterization using XRD and visual methods and sulfur phase distribution studies were performed on leachates and rock material to examine species redistribution patterns in both the leaching studies and the roadfills.

Major conclusions of the study include:

1. Sulfur bound in sulfide minerals is redistributed to sulfate forms or flushed from a system during oxidation and leaching. Sulfur present as organic phases seems to remain relatively constant. The duration of this process and hence, the significant acid-production life of Ocoee rocks, is about two years in laboratory leaching studies such as those used in this study.

2. Accelerated weathering in the laboratory leaching tests indicates that sulfidic Ocoee rocks appear to be capable of significant acid production for several decades in the field.

3. The oxidation of sulfides, along with leaching and hydration of other primary minerals such as albite, muscovite and chlorite, in acidic water, provides a source for species which result in abundant secondary mineral formation. Mineral phases resulting from the weathering of Ocoee rocks include jarosite, copiapite, ferrihydrite, aluminum oxides and hydroxides, and kaolinite.

4. Current predictive methods generally appear to have good correlation with lab leaching tests using similar source materials.

5. Field-scale models of encapsulation sites show that separate reductions in the thickness of limestone underblankets and topsoil cover used in current roadfills by 60% and 50% respectively, do not significantly affect water quality.

6. Current encapsulation site design seems to be an effective long-term method to mitigate AD in highway construction settings. A more economical variation, however, might include reductions in the amount of limestone underblanket and/or topsoil cover.

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