Volatilization of Trichloroethylene from Shallow Subsurface Environments: Trees and Soil
Abstract (Summary)Results from two previous studies conducted at Operable Unit 2 (OU2) of Hill Air Force Base, Utah indicate that the phytovolatilization (volatilization from leaves and trunk) of TCE by indigenous trees as well as soil surface flux may play a significant role in the removal of TCE from shallow groundwater plumes around the base. Previous studies investigated late summer and early autumn TCE leaf volatilization but no attempt was made to examine potential TCE volatilization seasonal variability and the volatilization of TCE directly from tree trunks. Whole tree transpiration rates were also not directly measured. To address those limitations and improve removal estimates, TCE removal via volatilization from leaves and tree trunks at OU2 was measured monthly during a growing season. Sap flow sensors were installed in several representative trees to directly measure transpiration rates. Transpiration rates were estimated between 15 and 160 L/day by sap flow meter data collected in 2007 and 2008. With an average growing season of 150 days, estimated TCE loss to the atmosphere through leaf volatilization was 107 to 211 mg/tree/year. An additional 4.1 mg/tree/year was estimated to volatilize directly from tree trunks. No definite seasonal trends in phytovolatilization were observed. Soil surface flux over 12,200 m2 equated to an overall loss of 390 g/year (180 days per year), with combined losses from all volatilization pathways of a maximum of 424 g/year, assuming an estimated 30 trees. This was one-sixth the removal of the interceptor trench installed in 1997, which is significant considering there was no additional cost for natural attenuation removal. Tree cores, branches, groundwater, precipitation, and nearby canal samples were collected to analyze for stable isotopes of hydrogen and oxygen. Stable isotope results, low summer precipitation, and TCE core sample concentrations suggest that the trees are using shallow groundwater as their primary source of water. There was no indication of any significant yearly or seasonal variability in TCE leaf and trunk volatilization, groundwater concentrations, and groundwater use by trees.
School:Utah State University
School Location:USA - Utah
Source Type:Master's Thesis
Date of Publication:12/01/2008