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The Hidden Valley Landfill site is listed by the U.S. Environmental Protection Agency (EPA) on the National Priorities List (NPL). The site occupies 52 acres of a larger, 72-acre site. Pierce County began landfilling operations in the mid 1960s, and sold the site to a private operator (Land Recovery, Inc.) in 1977. The landfill is not lined and is situated over very porous soils. Due to complaints of odors in the past, a methane collection and burn system has been installed on a portion of the site.

The site is not fenced but has a gate across the entrance road to the site. It is active and open to any commercial operation or private citizen who wishes to dispose of solid waste.

The following documents were provided to ATSDR for review: Hazard Ranking System Package, March 7, 1985; Thun Field Preliminary Assessment, May 14, 1985; Thun Field Consent Order, August 27, 1987; and Thun Field Landfill current Situation Report, October 9, 1987. These documents form the basis of this preliminary health assessment.


Groundwater on-site has been found to be contaminated with benzene (8 ppb), tetrachloroethylene (12 ppb), trichloroethylene (9 ppb), methylene chloride (23 ppb), pentachlorophenol (14 ppb), carbon tetrachloride (4 ppb), manganese (10 ppb), and dibenzofurans (5 ppb). In addition, chromium (10,000 ppb) has been found in liquid waste on-site. A monitoring well located immediately off-site has found benzene (6 ppb), vinyl chloride (2 ppb), napthalene (10 ppb), acenaphthene (8 ppb), and manganese (10 ppm) in the groundwater.

There are no physical hazards reported on this site.


The potential environmental pathway is contaminated groundwater and chromium-contaminated liquid waste on-site. Since odor complaints have been documented relative to this site, volatile organic chemicals in air may be an environmental pathway.

The potential human exposure pathways are ingestion and other uses of contaminated groundwater, inhalation of VOCs, and dermal contact and absorption of chromium compounds; although we understand that bulk liquid waste is no longer accepted and previous bulk liquid waste containing chromium should be buried, thus possibly limiting the potential impact of the dermal contact pathway.


Approximately 1,725 people reside about one mile from the site. Adjacent to the site are the Thun Field air strip, an active gravel pit, and a gun club. There are 311 private wells, 11 municipal wells, 4 irrigation wells, and 3 industrial wells within a 3-mile radius of the site.

Within the next five years, a residential community of about 10,000 is planned for an area approximately 0.5 mile from the site.


There are numerous wells within a 3-mile radius of the site. The County has applied for sole-source aquifer status for the Clover-Chambers Creek Aquifer; to date the government has not declared it as such. Further, the landmass between the landfill and the aquifer presumably is porous and permeable to percolation and infiltration to the groundwater by site contaminants. Off-site contamination of the groundwater has already been documented. The extent of groundwater contamination in the area should be evaluated. Information on the direction and velocity of groundwater flow (if contaminated) is important relative to the plans to construct a residential community nearby. Contact with on-site wastes should be a remote possibility, and is not apt to be of public health concern as long as that potential contact is only incident to short-term presence on-site for the purpose of dumping. Inhalation of VOCs by persons on-site may need to be evaluated if those persons spend more than several minutes on-site. If the concentrations reported in groundwater thus far are representative of true levels, contaminant concentrations in air off-site are not apt to be a significant pulic health problem.

ATSDR has prepared, or will prepare, Toxicological Profiles on the site contaminants (with the exception of manganese, acenaphthene, and dibenzofuran) noted above.


Based on the available information, this site is considered to be of potential public health concern because of the risk to human health caused by the possibility of exposure to hazardous substances via groundwater, and of secondary importance, inhalation of VOCs.

Further environmental characterization and sampling of the site and impacted off-site areas during the Remedial Investigation and Feasibility Study (RI/FS) should be designed to address the environmental and human exposure pathways discussed above. When additional information and data become available, e.g., the completed RI/FS, such material will form the basis for further assessment by ATSDR at a later date.

Table of Contents The U.S. Government's Official Web PortalDepartment of Health and Human Services
Agency for Toxic Substances and Disease Registry, 4770 Buford Hwy NE, Atlanta, GA 30341
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