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Scientific Investigations Report 2005-5035

Hydrogeology and Trichloroethene Contamination in the Sea-Level Aquifer Beneath the Logistics Center, Fort Lewis, Washington

Prepared in cooperation with
Fort Lewis Public Works

By R.S. Dinicola


The U.S. Army disposed of waste trichloroethene (TCE) and other materials in the East Gate Disposal Yard near the Logistics Center on Fort Lewis, Washington, from the 1940s to the early 1970s. As a result, ground water contaminated with primarily TCE extends more than 3 miles downgradient from the East Gate Disposal Yard. The site is underlain by a complex and heterogeneous sequence of glacial and non-glacial deposits that have been broadly categorized into an upper and a lower aquifer (the latter referred to as the sea-level aquifer). TCE contamination was detected in both aquifers. This report describes an investigation by the U.S. Geological Survey (USGS) of the source, migration, and attenuation of TCE in the sea-level aquifer.

A refined conceptual model for ground-water flow and contaminant migration into and through the sea-level aquifer was developed in large part from interpretation of environmental tracer data. The tracers used included stable isotopes of oxygen (18O), hydrogen (2H), and carbon (13C); the radioactive hydrogen isotope tritium (3H); common ions and redox-related analytes; chlorofluorocarbons; and sulfur hexafluoride. Tracer and TCE concentrations were determined for samples collected by the USGS from 37 wells and two surface-water sites in American Lake during 1999-2000. Ground-water levels were measured by the USGS in more than 40 wells during 2000-01, and were combined with measurements by the U.S. Army and others to create potentiometric-surface maps.

Localized ground-water flow features were identified that are of particular relevance to the migration of TCE in the study area. A ridge of ground water beneath American Lake diverts the flow of TCE-contaminated ground water in the sea-level aquifer to the west around the southern end of the lake. Tracer data provided clear evidence that American Lake is a significant source of recharge to the sea-level aquifer that has created that ridge of ground water. High ground-water altitudes at locations north and northeast of the Logistics Center combined with the ridge beneath American Lake prevent TCE contaminated water beneath the Logistics Center from migrating toward municipal water-supply wells northeast of the site.

The 1999-2000 TCE concentrations measured by the USGS at older wells screened in the sea-level aquifer were similar to those measured since 1995, but the known downgradient extent of the TCE contamination expanded nearly 2 miles after the Army installed and sampled new wells during 2003-04. Concentrations of TCE in the sea-level aquifer were consistently highest in the upper part of the aquifer throughout the plume, although TCE has spread throughout much of the thickness of the aquifer in the downgradient portions of the plume.

Environmental tracer data indicated that the primary pathway for contaminant migration into the sea-level aquifer is through the previously identified confining unit window, an area where the predominately fine-grained confining unit is relatively coarse grained and more permeable. Other less substantial pathways for contaminant migration also were identified near the East Gate Disposal Yard and the I-5 pump-and-treat facilities. Those areas are near active pumping wells and ground-water reintroduction facilities, but there is no evidence that the contaminant migration was caused or enhanced by those activities.

Within the sea-level aquifer, TCE concentrations continue to migrate westward in the flow field strongly influenced by ground-water recharge from American Lake. Historical data are not available to definitively determine if the 5-µg/L leading edge of the current TCE plume is stable or if it is still moving downgradient. However, an evaluation of the available data combined with TCE traveltime estimates indicates that the peak TCE concentrations in the sea-level aquifer may have not yet reached the wells near the currently defined leading edge of the plume. Hypothetically, the 5-µg/L leading edge of the current TCE plume will move farther downgradient in the years ahead.

Attenuation of TCE concentrations in the sea-level aquifer is most rapid near the confining unit window, but slows substantially farther downgradient in the current contaminant plume. Biodegradation of TCE is not an important attenuation mechanism in the sea-level aquifer. As the plume advances downgradient into previously uncontaminated areas, attenuation due to sorption likely will continue, but the longevity of sorption-related attenuation is uncertain. Conditions become more favorable for attenuation due to lateral and vertical dispersion and mixing in areas west of American Lake.

TCE concentrations already have been detected in two Army water-supply wells in the sea-level aquifer that are within the footprint of the existing contaminant plume, and there is a potential for TCE migration toward additional water-supply wells west of American Lake. The risk is low for TCE migration to a Fort Lewis backup supply well west of the lake, but the TCE plume possibly will migrate to a more distant public-water supply in the years ahead. However, the likelihood for detecting TCE at concentrations greater than the 5-µg/L drinking-water standard in that well is low.


Hydrogeologic System
Study Methods
Hydrogeology and Contamination of the Sea-Level Aquifer
Implications for Remediation Plans
References Cited
Appendix A. Duplicate and Field-Blank Sample Results for USGS Data
Appendix B. Chlorofluorocarbon Data

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Send questions or comments about this report to the author, R.S. Dinicola, (253) 428-3600 ext. 2603.

For more information about USGS activities in Washington, visit the USGS Washington District home page.

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