Reserve Silica Corporation
26000 Black Diamond & Ravensdale Rd, Ravensdale, WA 98051



The Site is generally located at 28130 Black Diamond–Ravensdale Road near Ravensdale, Washington.

Earlier investigations suggest that the Site may be contained on King County Tax Parcel Numbers 3522069018, 3622069138, 0121069011, and 3522069046. Under this Order, the results of the RI will determine the final boundaries of the Site.

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The Reserve Silica Site was used for coal and sand mining until 2007. Now the site is in the reclamation phase - the mine pits are being backfilled with materials from construction sites in the Puget Sound area.

In the 1970s and 1980s, cement kiln dust (CKD) from a local cement plant was placed as backfill in two of the mine pits on the property.

  • The Lower Disposal Area (LDA) was an open pit sand mine reclaimed by placing CKD and other materials into the excavation between 1979 and 1982.
  • The Dale Strip Pit (DSP), constructed as an open pit coal mine, was reclaimed in the 1970s and 1980s by filling with CKD and other materials. 
The LDA and DSP are permitted as closed solid waste landfills by Public Health Seattle King County (Public Health).


Groundwater moving through the CKD in the Lower Disposal Area discharges to the surface in seeps. Groundwater and surface water contaminants include:

  • High pH (corrosive) water.
  • Metals, including arsenic and lead.

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King County reports document drainage seeping through the waste from the Lower Disposal Area as early as 1986. Trenches were installed around that time to control the drainage but were not effective. Since 2002, Holcim (US) Inc. has conducted activities at the property to reduce and control the seepage.

  • 2002: Developed a monitoring well system and reported data quarterly to Ecology and Public Health.
  • 2006: Installed monitoring wells in bedrock and reported data quarterly. Ecology and Public Health reviewed and approved the work plans.
  • 2007: Upgraded the soil cover on the Lower Drainage Area to a 2-foot minimum thickness to reduce infiltration. Regraded the cover to improve surface water runoff. Constructed a ditch to divert surface water runoff on the uphill side of the cover.
  • 2008: Constructed and monitored a test trench system to collect high pH seepage from the Lower Disposal Area. Evaluated the feasibility of installing a full-scale trench system.
  • Late 2010 to early 2011: Upgraded the Dale Strip Pit soil cover to ensure 2-foot minimum thickness, positive drainage, and surface vegetation.
  • February 2013: Installed a ditch and concrete catch basin to collect surface water runoff. Captured leachate seeps from the Lower Disposal Area and directed them from the catch basin to the infiltration ponds.
  • September 2013: Installed a gravel-filled trench to prevent groundwater from flowing into the Lower Disposal Area.
  • 2013-2014: Conducted hydrological investigations and groundwater and surface water statistical evaluations of the Lower Disposal Area.

These measures did not eliminate the seepage.

In June 2016, Ecology’s Water Quality Program issued a Notice of Violation for high pH water draining into the infiltration ponds. In 2017-2018, Holcim (US) Inc. installed a treatment system to lower the pH and remove arsenic from the seepage before it enters the infiltration ponds. They extended the existing collection trench to the north to capture additional seepage. They also installed fencing around the seeps, drainage ditches, and infiltration ponds. Ecology and Public Health reviewed and approved the system design before construction. The treatment system began operating in 2018.


Map showing site location as King County, WA SITE INFORMATION


View Electronic Documents

Cleanup Site Details Report

Facility Site ID: # 2041

Cleanup Site ID: 4728

Ravensdale, King County

Status: Cleanup Started   Get definitions of Status terminology

Tim O'Connor
Site Manager
(425) 649-7051

Alan Noell
Site Manager
(425) 649-7015

Nancy Lui
Public Involvement Coordinator
(425) 649-7117

Document Repositories:

Northwest Regional Office
3190 160th Ave SE
Bellevue, 98008

Maple Valley Public Library
21844 SE 248th Street
Maple Valley, 98038