USGS Washington Water Science Center
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WA448 - Analyzing Tree Cores to Locate the Extent of TCE Contamination in Ground Water at a Location on McChord Air Force Base, Washington - Completed FY2002
Problem - In 1983, trichloroethene (TCE) was detected in ground water beneath the McChord Air Force Base at a site referred to as SS-34N (fig. 1) during an investigation of a JP-4 bulk fuel storage area. Monitoring wells CW-25 and CW-26 (fig. 2) were installed in 1994, and TCE was detected above the cleanup level of 5 micrograms per liter in both wells. In May 1995, a soil gas survey was performed to determine the areal extent of the contamination, but it was unsuccessful. Five additional monitoring wells were installed along the western boundary of the base in November 1999, and TCE was detected in all wells, including one that was expected to represent background conditions. In March 2000, six monitoring wells were installed in the residential area to the west of the base (fig. 2), and TCE was detected at concentrations above the cleanup level in samples from three of the wells. The source of the contamination is unknown, but spills and improper disposal of solvents that may have been used in building 1104 (fig. 2) are suspected. McChord Air Force Base is starting a remedial investigation at site SS-34N to determine the extent of TCE contamination in ground water. Because the soil gas survey was not successful, the Air Force is faced with the expense of installing and sampling monitoring wells unless another method can be used to determine the extent of contamination.
Objectives - The objective of this study is to determine if concentrations of TCE in samples of tree cores can be used to determine the extent of TCE contamination in ground water at and adjacent to site SS-34N on McChord Air Force Base.
Relevance and Benefits - This proposed work is an extension of ongoing studies conducted by the USGS Washington Water Science Center on Ft. Lewis. While sampling on Ft. Lewis has demonstrated the utility of sampling tree cores to determine the extent of TCE contamination, this project provides an opportunity to test the technique under more challenging conditions (greater depths to ground water). Should it prove out, it will provide a substantial cost savings to the remedial investigation to be conducted by the Air Force at site SS-34N. If no correlation between TCE concentrations in tree cores and ground water is found, then the project provides information about the limitations of this technique.
Approach - Core samples from twenty trees, located within, upgradient, and downgradient of the known areal extent of TCE contamination, will be collected twice and analyzed for TCE. Samples will be collected from the twenty trees during March or April 2001, and the same trees will be sampled again during the peak period of soil-moisture deficit in late July or early August 2001. When possible, the sampled trees will be located in close proximity to monitoring wells so that concentrations of TCE in tree cores can be compared with concentrations in ground water. If TCE is detected in the tree-core material, the correlation or lack of correlation between concentrations in tree cores and ground-water samples may be used to evaluate the effectiveness of this technique at site SS-34N.