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Spokane, WA 99260
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Solid Waste - Mica Landfill

Mica Landfill, a municipal solid waste facility is situated approximately one mile north of the town of Mica, Washington. The landfill operated from 1972 to late 1992 and received residential and municipal solid waste, sewage treatment plant sludge, and industrial wastes, including secondary aluminum slag (black dross) and solvents. In 1981, the U.S. Environmental Protection Agency (EPA) conducted a preliminary investigation at the site that was followed up by a more intensive investigation by Spokane County.

On the basis of data from their preliminary 1981 investigation, the EPA entered the site on the national priorities list (NPL) for cleanup and designated the Washington State Department of Ecology (Ecology) as the lead agency. In 1988, Spokane County and Ecology entered into a consent decree that required the county to conduct a Remedial Investigation/Feasibility study (RI/FS) at the site, followed by appropriate removal or remedial actions, as necessary. A work plan for the RI/FS was completed in mid-1988 and work was begun on the RI , which was completed in September 1992. As a result of the findings of the RI Ecology concluded that a dangerous waste cover (two impermeable layers) was necessary at Mica.

In 1993, CH2M-Hill was contracted to design the cover system. Delhur Industries was awarded the construction contract to install the cover, a combination high density polyethylene geomembrane / geosynthetic clay liner. Construction began in early 1994 and was completed in July 1995. Sixty acres of mostly steep hillside were covered. The closure project included construction of a landfill gas collection system consisting of 49 gas wells and 29 trench risers. Landfill gas is collected at the wells and risers and directed through a manifold to passive flares for combustion. Another piping system was installed under the cover to collect and direct leachate to one of the four leachate storage ponds on site.

Spokane County is currently in the third year of a five-year Interim Action Monitoring Program. The program includes monitoring landfill gas, surface water, leachate, and groundwater. Landfill gas is monitored at various points on the gas collection system as well as 11 gas probes outside the boundaries of the cover. Surface water is sampled annually at three sites. Leachate is samples twice a year. There are 31 groundwater monitoring wells and three privately owned wells that are sampled on a quarterly, semiannual, and / or annual basis. The overall effectiveness of the cover will be reviewed at the end of the five-year monitoring program. At that time the site may be subjected to additional remedial activities and /or undergo modifications to the monitoring program.