Date of Award:

5-2016

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Civil and Environmental Engineering

Committee Chair(s)

William J. Doucette

Committee

William J. Doucette

Committee

Bruce Bugbee

Committee

Ryan Dupont

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 plumes surrounding 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 effort 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 as the trees mature.

To help address this shortcoming and evaluate the potential removal effectiveness of phytoremediation, a demonstration plot was initiated within a TCE contaminated seep area at HAFB OU2 in the spring of 2013. Poplar poles were planted in holes dug to near the depth of the groundwater to eliminate the need for an irrigation system, but consecutive hot, dry springs resulted low survival rates and only 25% of the planted poplar poles survived after the end of the second summer. In order for a test plot to have a higher success rate, an irrigation system should be installed.

Samples were taken in order to quantify the amount of TCE removed by volatilization through leaf, trunk, and soil surfaces. Volatilization samples were collected on Tenax™ sorbent tubes and analyzed by thermal desorption gas chromatography/mass spectrometry. Tree cores were sampled using an incremental borer and analyzed by headspace GC/MS to quantify the TCE mass contained in the trees.

A Thiessen polygon approach was used to scale the measured TCE removal from the sampled trees to the entire site. A polygon was generated for each sampling location and assuming that the single measured value was representative of the entire area. The TCE leaving each polygon was calculated and the mass removed from each polygon was summed to estimate the total TCE removal from the site. Groundwater plume maps were used along with the sample locations to define polygon boundaries.

The mass of TCE contained in the groundwater plume was estimated from historical groundwater data using a polygon approach. The polygon boundary used was the same as the boundary used to scale flux sampling at the site. The volume of the plume was estimated from the average plume depth for each polygon. This was calculated by subtracting the plume depth elevation from the maximum groundwater elevation for the sampled wells, assuming each polygon had a uniform depth. By multiplying the plume depth by the average of the groundwater polygon area, the volume was estimated. Once plume volume was found, the mass of TCE contained in the plume was estimated. There is currently an estimated 916 kg of TCE contained in the groundwater plume. HAFB estimated 1280 kg TCE contained in the plume in 2006 using a similar approach.

Based on a 260 day growing season and the existing trees at the site, it was estimated that 0.278 kg TCE is removed annually at HAFB Operable Unit 2. A prediction of future removal was also estimated for a phytoremediation demonstration plot containing 40 poplar trees located over the seep area and a large scale plot containing 160 trees over the entire hillside above the contamination plume.

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.

It was predicted that a phytoremediation demonstration plot containing 40 poplar trees at HAFB OU2 would remove 4.82 kg of TCE annually. A larger plot covering the entire hillside at HAFB OU2 (160 trees) could remove up to 19.28 kg of TCE annually, once trees become mature

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