Date of Award:

5-2016

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Civil and Environmental Engineering

Advisor/Chair:

William J. Doucette

Abstract

Trichloroethylene (TCE) is a carcinogenic, chlorinated volatile organic compound that was commonly used as a degreasing solvent for aircraft maintenance at many US Air Force bases. Past improper disposal of TCE has resulted in contaminated groundwater at many of these facilities. Phytoremediation, defined as the use of plants and their associated microorganisms to stabilize or remove contamination, has been implemented as part of a TCE groundwater cleanup at Travis Air Force base near Sacramento, CA and is being considered as a remediation option at other bases. Volatilization of TCE from leaves and the surface of the soil near the trees were shown to be the most important removal mechanisms at the Travis site. Past studies conducted on indigenous trees growing above TCE contaminated groundwater at several Hill Air Force Base (HAFB) locations have also shown that TCE is taken up and volatilized by the trees. However, phytoremediation has not been implemented, in part because of the difficulty in predicting the potential effectiveness of TCE removal over time.

Flow through or recirculating chambers were used to quantify the amount of TCE removed by volatilization through leaf, trunk, and soil surfaces. Tenax™ sorbent tubes, used to collect TCE from the chambers, were analyzed by thermal desorption gas chromatography/mass spectrometry. Tree cores were collected using an incremental borer and analyzed by headspace GC/MS to quantify the TCE mass contained in the trees.

Field measured transpiration stream concentrations (TSC) and groundwater data were used to calculate transpiration stream concentration factors (TSCF) for TCE. Comparing current and historical data, it was found that trees reach a steady state TSCF value of 0.26 after about 15 years.

Using this information, it was predicted that a phytoremediation plot containing 40 poplar trees located in a seep area within HAFB OU2 would remove 4.82 kg of TCE annually. A larger plot covering the entire hillside above this seep (160 trees) could remove up to 19.28 kg of TCE annually, once trees reach a steady state TSCF.

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