USG Interiors Inc 99 Site

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. 

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. 

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.

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.

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.

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).