Date of Award:

5-2011

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Civil and Environmental Engineering

Advisor/Chair:

William Doucette

Abstract

Past use of trichloroethylene (TCE) as a degreasing solvent for aircraft maintenance has resulted in widespread groundwater contamination at Air Force Bases around the world. Travis AFB in California and Fairchild AFB in Washington are evaluating phytoremediation as a treatment option, since trees have been reported to take up dissolved TCE from shallow groundwater and volatilize it to the atmosphere while enhancing the volatilization of TCE from surrounding soil. Previous studies generally focused on the identification of removal mechanisms. The emphasis of this research was to quantify total TCE removal from phytoremediation demonstration plots at Travis and Fairchild AFBs.

Tree cores, collected using an increment borer and analyzed using headspace GC/MS, were used to determine the relative TCE concentrations within the plume beneath the trees and to estimate the mass of TCE in each tree. To estimate the volatilization of TCE from leaves, a small section of tree branch was placed inside a flow-through glass chamber. Continuous air flow through the chamber maintained normal transpiration and temperature. Air exiting the chamber was sampled for TCE using Tenax® tubes. Humidity probes placed at the chamber entry and exit were used to estimate transpiration. Volatilization of TCE from tree trunk and soil surfaces was measured by enclosing a section of trunk or ground surface within a small stainless steel chamber. Fans in the chamber mixed the air that was recirculated through Tenax® tubes to continuously remove TCE. After a measured time interval, the Tenax® tubes were analyzed for TCE by thermal desorption GC/MS.

By using a Thiessen polygon method, the removal of TCE was estimated to be 839 g/yr at Travis and 18 g/yr at Fairchild with the majority from leaf and soil volatilization. Soil surface volatilization of TCE was greater inside the planted areas than outside the planted areas, indicating that the trees enhance this removal by this mechanism. Based on these estimates phytoremediation removed 5 and 50% of the mass of TCE in the groundwater at Fairchild and Travis Air Force sites, respectively.

Comments

This work made publicly available electronically on February 14, 2011.