Assessment of Subsurface Chlorinated Solvent Contamination Using Tree Cores At the Front Street Site and a Former Dry Cleaning Facility At the Riverfront Superfund Site, New Haven, Missouri, 1999-2003.


Tree-core sampling has been a reliable and inexpensive tool to quickly assess the presence of shallow (less than about 30 feet deep) tetrachloroethene (PCE) and trichloroethene (TCE) contamination in soils and ground water at the Riverfront Superfund Site. This report presents the results of tree-core sampling that was successfully used to determine the presence and extent of chlorinated solvent contamination at two sites, the Front Street site (operable unit OU1) and the former dry clean-ing facility, that are part of the overall Riverfront Superfund Site. Traditional soil and ground-water sampling at these two sites later confirmed the results from the tree-core sampling. Results obtained from the tree-core sampling were used to design and focus subsequent soil and ground-water investiga-tions, resulting in substantial savings in time and site assessment costs.The Front Street site is a small (less than 1-acre) site located on the Missouri River alluvium in downtown New Haven, Missouri, about 500 feet from the south bank of the Missouri River. Tree-core sampling detected the presence of sub-surface PCE contamination at the Front Street site and beneath residential property downgradient from the site. Core samples from trees at the site contained PCE concentrations as large as 3,850 μg-h/kg (micrograms in headspace per kilogram of wet core) and TCE concentrations as large as 249 μg-h/kg. Soils at the Front Street site contained PCE concentrations as large as 6,200,000 μg/kg (micrograms per kilogram) and ground-water samples contained PCE concentrations as large as 11,000 μg/L (micrograms per liter). The former dry cleaning facility is located at the base of the upland that forms the south bank of the Missouri River alluvial valley. Tree-core sampling did not indi-cate the presence of PCE or TCE contamination at the former dry cleaning facility, a finding that was later confirmed by the analyses of soil samples collected from the site. The lateral extent of PCE contamination in trees was in close agreement with the extent of subsurface PCE contamina-tion determined using traditional soil and ground-water sampling methods. Trees growing in soils containing PCE concentrations of 60 to 5,700 μg/kg or larger or overlying ground water containing PCE concentrations from 5 to 11,000 μg/L generally contained detectable concentrations of PCE. The depth to contaminated ground water was about 20 to 25 feet below the land surface. Significant quantitative relations [probability (p) values of less than 0.05 and correlation coefficient (r2) values of 0.88 to 0.90] were found between PCE concentrations in trees and subsurface soils between 4 and 16 feet deep. The relation between PCE concentrations in trees and underlying ground water was less apparent (r2 value of 0.17) and the poor relation is thought to be the result of equilibrium with PCE concentrations in soil and vapor in the unsaturated zone. Based on PCE concentrations detected in trees at the Front Street site and trees growing along contaminated tributaries in other operable units, and from field hydroponic experiments using hybrid poplar cut-tings, analysis of tree-core samples appears to be able to detect subsurface PCE contamination in soils at levels of several hundred micrograms per liter or less and PCE concentrations in the range of 8 to 30 μg/L in ground water in direct contact with the roots.Loss of PCE from tree trunks by diffusion resulted in an exponential decrease in PCE concentrations with increasing height above the land surface in most trees. The rate of loss also appeared to be a function of the size and growth characteristics of the tree as some trees exhibited a linear loss with increasing height. Diffusional loss of PCE in small (0.5-inch diameter) trees was observed to occur at a rate more than 10 times larger than in trees 6.5 inches in diameter. Concentrations of PCE also exhibited directional variability around the tree trunks and con-centration differences as large as five-fold were observed around the trunks of several trees. The directional differences were attributable to spatial differences in PCE concentrations in soils around the trees and to natural “twisting” of the tree trunks. The directional differences also may be caused by diffusion of PCE vapors in the unsaturated zone into the tree roots. Comparison of PCE concentrations in core and sap samples confirms laboratory sorption studies and indicates that the vast majority (greater than 95 percent) of the PCE and TCE reside in the wood phase and not the transpiration stream.


Civil, Architectural and Environmental Engineering

Keywords and Phrases

Chlorinated and Halogenated Hydrocarbons; Environmental Analysis; Organic Compounds; Tetrachloroethylene; Trichloroethylene

Document Type

Article - Journal

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