Impermeable Isolation Cap at the Thea Foss Waterway Superfund Site, Tacoma, Washington

Site Name:

Thea Foss Waterway Superfund Site



Period of

November 13, 2003 - Impermeable cap installed



--Impermeable cap (3/8-inch high-density polyethylene [HDPE] plastic) comprised of 10 foot (ft) by 5 ft panels with a total area of 5,250 square feet (75 ft by 70 ft).
--After placement of the HDPE cap, additional 4 to 6 ft of sand was placed over it as part of the Waterway's continuous thick sand cap.

Cleanup Authority:


EPA Contact
Kira Lynch
U.S. EPA Region 10
1200 Sixth Avenue
Seattle, WA 98101
Phone: 206-553-2144

Utilities Contact
Jackie Wetzsteon
PacifiCorp Environmental Remediation Company
825 NE Multnomah, 906 LCT
Portland, OR 97232

Gary Braun
Tetra Tech EC, Inc.
19803 N. Creek Parkway
Bothell, WA 98011

City of Tacoma Contact
Mary Henley
City of Tacoma Public Works Department
Environmental Services Science and Engineering
2201 Portland Avenue
Tacoma, WA 98421-2711
Phone: 253-502-2113

Polycyclic aromatic hydrocarbons (PAHs)

Waste Source:
Historical operations and practices associated with the Standard Chemical Company from 1915 to 1922. Two 96-inch stormwater outfalls that discharge to the Head of the Waterway also were identified as contamination sources.

Type/Quantity of Media Treated:
Sediment (quantity not documented)

Purpose/Significance of Application:
The cap was designed to (1) halt dense non aqueous phase liquid (DNAPL) seepage and reduce groundwater flow through the preferred pathways and (2) lengthen the groundwater flow paths through fine grained sediment before discharge into the overlying sand capping material.

Regulatory Requirements/Cleanup Goals:
The cleanup goals for the contaminants found at the site were Sediment Quality Objectives (SQO). SQOs for contaminants that exceeded or equaled the SQO concentrations in the latest round of monitoring data (2008) are listed below.

Contaminant Sediment Quality Objective (SQO) in µg/kg
Zinc 410,000
Phenanthrene 1,500
Fluoranthrene 2,500
Pyrene 3,300
Benzo(a)anthracene 1,600
Benzo(b+k)fluoranthenes 3,600
Benzo(a)pyrene 1,600
Indeno(1,2,3-cd)pyrene 690
Dibenz(a,h)anthracene 230
Benzo(g,h,i)perylene 720
Total HPAH 17,000
Bis(2-ethylhexyl)phthalate 1,300
Mercury 590
Notes: µg/kg - Microgram per kilogram HPAH - high molecular weight PAHs

Baseline monitoring was conducted in April 2004, and monitoring has continued annually. Year 4 monitoring (2008) shows the following:

-- All early-warning samples (0 to 2 centimeters [cm]) had Bis(2-ethylhexyl)phthalate (BEHP) concentrations above the SQO, with a maximum of 14,000 µg/kg at SC-03. Early-warning samples were collected to provide warning for possible "top-down" contamination from sources, such as stormwater.
-- Compliance samples (0 to 10 cm) concentrations of BEHP and HPAH south of the State Route (SR) 509 Bridge appear to have generally increased between 2007 and 2008, even though they had generally decreased between 2006 and 2007. The average concentration of BEHP increased from approximately 3,100 to 4,500 µg/kg. The average concentration of HPAH increased from approximately 8,700 to 10,800 µg/kg.
-- Concentrations in core samples did not exceed SQOs indicating no evidence of bottom-up recontamination and that the cap is performing as designed.

Year 4 monitoring activities also included visual inspection, bathymetric survey, and Sediment Profile Imagery (SPI) survey. Observations during the visual inspection conducted in May 2008 noted the scour protection apron was functioning as intended, no obvious signs of erosion, and side slops with no visible evidence of erosion. The bathymetric survey was conducted in August 2008 and results indicated the sediment cap to be relatively stable. Results of the SPI survey indicate conditions are similar to the previous study conducted in Year 2 (2006), and mostly positive changes have been observed.

Cost Factors:
The final cost for the installation of the impermeable cap was $11.5 million. The final cost included a capital cost of $10.5 million and $1 million for monitoring. The capital cost included:

- $726,000 for site investigation
- $2.2 million for design of the impermeable cap
- $7.2 million for the construction of the impermeable cap
- $11,500 for community relations, and
- $333,000 for insurance premiums.

Monitoring costs are projected costs for monitoring the performance of the impermeable cap from April 2004 to October 2014.

The Thea Foss Waterway (formerly known as the City Waterway) extends approximately one-and-a-half miles through the downtown area of the City of Tacoma, Washington. It is one of seven inlets connecting to Commencement Bay at the southern end of the main basin of Puget Sound.

A localized, intermittent, non aqueous phase liquid (NAPL) seep was discovered underneath the SR 509 Bridge located at the Head of the Waterway shortly after its construction in the mid-1990s. NAPL droplets were observed on the water surface in this seep area during periods of low tide. Site investigations lead to the discovery of a buried coal tar-derived deposit or NAPL deposit. Based on past studies and reviews of historical information, it was determined that the most likely source of this NAPL contamination in the Waterway sediments was due to operations and practices associated with the Standard Chemical Company from 1915 to 1922. It is believed that the NAPL began to seep from the deposit during the construction of the SR 509 Bridge in the mid-1990s. NAPL in deeper sediments of the Waterway and in the SR 509 seep area has been described as containing primarily PAHs. Another source of contamination is the presence of two 96-inch stormwater outfalls (known as the Twin 96 outfalls) that discharge to the Head of the Waterway.

The remedial action for this portion of the Waterway included installation of a sheet pile wall, dredging, and placement of a HDPE cap, a sand cap over the HDPE cap, and a slope cap and armor material on Waterway slopes. This case study focuses on the HDPE cap, which was installed on November 13, 2003. Baseline monitoring was conducted in April 2004, and monitoring has continued annually. Year 4 monitoring (2008) concluded that the sediment cap remains stable and is effective at minimizing migration of contamination upward. However, "top-down" recontamination is occurring, most likely from nearby stormwater outfalls.