Facility Site ID: 84531356 Cleanup Site ID: 3618

Site Status

Información en español.  

We held a public comment period from June 15 to July 17, 2023, for review of several documents: Amendment 1 to Agreed Order DE 11099, Revised Cleanup Action Plan (RCAP), and State Environmental Policy Act Determination of Non-Significance. We received one comment. We considered the comment, and finalized the documents without modifying them after the comment period.

The comment and our response is available in our Responsiveness Summary.

Amendment 1 to Agreed Order DE 11099 is a legal agreement between Ecology and USG Interiors (USG) to implement the RCAP and modify the schedule. USG is the potentially liable person responsible for cleanup of the site. The RCAP describes the plan for cleaning up the site. 

Site Description

Approximate property boundary. The site is between Pacific Hwy 99 (left of the site) and Interstate 5 (right of the site).
Approximate property boundary. The site is between Pacific Hwy 99 (left of the site) and Interstate 5 (right of the site).
The USG Interiors Inc 99 site is generally located at 7110 Pacific Highway East (aka Highway 99) in Milton. The site is on a narrow strip between Pacific Hwy and Interstate 5. Hylebos Creek (shown in blue in Figure 1 below) runs along the south and east side of the site, where it flows downstream to the Hylebos Waterway and into Puget Sound.

Before 1985, USG disposed of waste at the site from their rock wool manufacturing plant in Tacoma. Rock wool is an insulating and fire-proofing material made by melting rock and blowing it or spinning it into thin fibers. 

Contamination

USG's Tacoma plant used slag (a waste) from the former Tacoma Asarco copper smelter to make rock wool. The slag contained a high concentration of arsenic—a toxic metal.

USG buried waste and by-products of rock wool manufacturing at the site. The buried waste was used as fill to raise the height of the site to the level of Highway 99. The waste is the source material for the arsenic contamination at the site.

Cleanup

From 1985 to 1986, USG dug up and removed 20,000 to 30,000 cubic yards of arsenic-contaminated waste from the site and disposed of it at a hazardous waste landfill. We required USG to monitor the groundwater after the waste removal. 

In 2007, Ecology received a report from USG that showed arsenic contamination of soil and groundwater. Eight of nine soil samples and all nine groundwater samples exceeded the state cleanup levels for soil (20 mg/kg) and for groundwater (5 µg/l). The arsenic contamination required cleanup under the state’s toxics cleanup law, the Model Toxics Control Act (MTCA).

After a public comment period, AO DE 11099 was signed and the original cleanup plan was finalized in 2016. However, pilot testing of the cleanup method described in the plan showed it would not reduce the arsenic contamination to the levels desired.  

The Revised Cleanup Action Plan (RCAP) was developed to address the arsenic contamination in a reasonable time frame, less than 20 years. Amendment 1 to the AO was negotiated to implement the RCAP and modify the cleanup schedule.

Revised Cleanup Action Plan

Figure 1. Area of cleanup work described in the RCAP. The approximate area of sediment removal is shown in black. Other areas are described in the text.
Figure 1. Area of cleanup work described in the RCAP. The approximate area of sediment removal is shown in black. Other areas are described in the text.

The RCAP is designed to reduce arsenic in groundwater.

Soil:

  • Core remediation area: In-situ solidification will be used to treat all soil above and below the water table containing arsenic greater that 250 mg/kg.  Solidification stops groundwater from contacting arsenic in the soil. Treatment of the core remediation area targets where the highest concentrations of arsenic-contaminated soil is located. Afterwards asphalt pavement will cover the site to reduce contact with soil.
  • P429 Plus area: This area is the P429 parcel combined with the right-of-way next to Pacific Hwy. All contaminated soil in the P429 Plus area with greater than 20 mg/kg of arsenic will be excavated and disposed of off-site. 

Groundwater: 

  • Natural attenuation: Groundwater will flow around the solidified soil, which reduces groundwater exposure to contamination. 
  • Contingency plan: Condition of groundwater will be monitored regularly. If the reduction of arsenic in groundwater is too slow, then groundwater will be treated by injecting iron into groundwater to bind the arsenic to the soil.  

The cleanup of Hylebos Creek sediment, the long term monitoring plan, and institution controls described in the RCAP are the same as described in the original cleanup action plan. 

Contaminated sediment and some streambank soil will be removed in the area shown in black in Figure 1. Low permeability barriers will be installed, the excavation backfilled, and a new creek bed constructed. Monitoring wells will be installed and the creekside buffer restored.

Groundwater will be monitored to make sure the cleanup is successful and an environmental covenant will be filed for the site. The covenant will protect the integrity of the cleanup, restrict land use, and prohibit water removal from the site for any use. While the covenant is in place, we will review conditions at the site every five years.

Descripción Del Sitio

El sitio está ubicado entre la Interestatal 5 y la Pacific Highway 99 en Milton. 

Antes de 1985, USG eliminaba los desechos en el Sitio de su planta de fabricación de aislamiento de lana de roca en Tacoma. USG usó escoria de la fundición de cobre ASARCO en Tacoma como materia prima para la producción de lana de roca. La escoria contenía una concentración alta de arsénico—un metal tóxico. En el Sitio, USG enterró desechos y subproductos de la fabricación de lana de roca. Los desechos enterrados se usaron como relleno para elevar la altura del Sitio al nivel de la Carretera 99. Los desechos son  el material de origen de la contaminación por arsénico en el Sitio.

En 1984 y 1985, se excavaron y retiraron 20,000 a 30,000 yardas cúbicas de desechos contaminados con arsénico del Sitio. El muestreo en 2006 reveló que la contaminación por arsénico del suelo y las aguas subterráneas estaba por encima de los niveles de limpieza, lo que requería limpieza según la ley estatal de limpieza de tóxicos, la Ley Modelo para el Control de Sustancias Tóxicas (MTCA, por sus siglas en inglés).

Después de un período de comentarios para revisar el AO 11099 y el plan de acción de limpieza original, se firmó el AO, y se finalizó el plan en 2016. Sin embargo, la prueba piloto del método de limpieza descrito en el plan mostró que no reduciría la contaminación del arsénico en un tiempo razonable (menos de 20 años).

El RCAP está diseñado para reducir el arsénico en las aguas subterráneas en un periodo de tiempo razonable (menos de 20 años).

Una lista de datos brevemente resumiendo información sobre el sitio y el plan de limpieza (el plan de remedio).

Legal 7
Document Title Document Date Document Type
First Amendment to Agreed Order USG HWY 99 -ExhibitA&B with Revised CAP 7/31/2023 Agreed Order Amendment
Final USG Hwy 99 Agreed Order 6/24/2016 Agreed Order
PLP Determination Letter - Eric Thompson 3/30/2015 Final Potentially Liable Person Status Letter
PLP Determination Letter - Linda Plein 3/27/2015 Final Potentially Liable Person Status Letter
Preliminary PLP Status Letter - Linwood Custom Homes 12/23/2014 Final Potentially Liable Person Status Letter
Preliminary PLP Status Letter- General Trailer Co 12/23/2014 Final Potentially Liable Person Status Letter
USG Highway 99 Site Agreed Order Signed 10/19/2009 Agreed Order
Public Information 8
State Environmental Policy Act 2
Document Title Document Date Document Type
US HWY 99 SEPA Checklist & DNS for REVISED Cleanup Action Plan 6/1/2023 SEPA Documents
USG Hwy 99 SEPA Checklist and DNS 7/8/2015 SEPA Documents
Technical Reports 14
Document Title Document Date Document Type
USG Interiors, Inc 99 - Notice of Proj Coordinator Designation 4/9/2024 Site Specific Administrative Document - other (Administrative correspondence)
Final Engineering Design Report 11/22/2023 Engineering Design Report
USG Interiors HWY 99 - Revised Cleanup Action Plan 7/31/2023 Cleanup Action Plan
Geotechnical Field Investigation and Bench Scale Treatability Study 8/11/2021 Site Specific Technical Document - other
Field Pilot Study Evaluation Report 6/12/2020 Site Specific Technical Document - other
Final hWY 99 Conceptual Design Report 4/16/2020 Site Specific Technical Document - other
Notification of Project Coodinator Designation 10/4/2016 Site Specific Technical Document - other
Inadvertent Discovery Plan for the USG Interiors Highway 99 Site Project, Pierce County, Washington 7/22/2016 Site Specific Technical Document - other
Feasibility Study 6/23/2016 Feasibility Study
Remedial Investigation Report 6/23/2016 Remedial Investigation Report
US Highway 99 Draft Cleanup Action Plan 4/24/2014 Cleanup Action Plan
US Highway 99 Draft Feasibility Study 9/13/2013 Feasibility Study
US Highway 99 Draft Remedial Investigation Report Addendum 2/15/2013 Remedial Action Report
US Highway 99 Draft Remedial Investigation Report 7/11/2012 Remedial Investigation Report
There may be more documents related to this site. To obtain documents not available electronically, you will need to make a public records request.

Places to see print documents

  • Southwest Regional Office
    300 Desmond Dr SE
    Lacey, 98503-1274
    Please schedule an appointment to view print documents at this location.

Contaminants 1

Contaminant Type
Soil
Groundwater
Surface Water
Air
Sediment
Bedrock
Metals - Metals Priority Pollutants R C
S
Suspected
C
Confirmed Above Cleanup Levels
B
Below Cleanup Levels
RA
Remediated-Above
RB
Remediated-Below
R
Remediated
This contaminant list was based on our best information at the time it was entered. It may not reflect current conditions at the site.