Asotin County Landfill

The Asotin County Regional Landfill is an operating facility that accepts municipal solid waste (MSW). The landfill has two MSW landfill areas: a closed unit and a new unit. It also has a moderate risk waste facility.

The closed landfill covers 46 acres, is unlined, and received waste from 1975 until it closed in 1993. From 1975 to 1985, treated sewage, septic sludge, and industrial wastes were disposed at the landfill, which was allowed then. The closed landfill only received MSW after the law changed in 1985. The closed unit contains an estimated 500,000 tons of waste by weight, or 1,000,000 cubic yards of waste by volume. 

Municipal solid waste has been placed in the new landfill since 1992, and it is scheduled to remain open until 2037. When full, the new landfill will contain roughly 2,200,000 tons of waste by weight, or a volume of 4,000,000 cubic yards of waste. 

Asotin County Regional Landfill now has a multi-unit designation, so both the closed unit and newer, lined unit are regulated under current laws.

The closed landfill was filled with waste from west to east. Routine groundwater sampling in 1989 showed low levels of volatile organic compounds (VOCs) beneath the western side. Groundwater results around the lined landfill units to the east have not shown VOCs.

Asotin County routinely submits all groundwater results to us in their monitoring reports. Groundwater sample results from 1989 to the present show VOC levels at levels requiring treatment along the western side of the landfill, next to the closed landfill. The primary VOCs of concern are tetrachloroethylene and trichloroethylene. Nitrate is a non-volatile inorganic chemical that is also contaminating groundwater at levels requiring cleanup.

• To remove potential threats to human health. 
• To protect the surrounding areas from environmental impacts. 
• To safeguard ground and surface waters. 
• To ensure that contamination does not migrate to the Snake and Clearwater rivers.

1993: Covered the closed landfill with low permeable soil to reduce surface water percolation.

1996: Installed a landfill gas removal and treatment system. With the soil cover and gas removal system in place, groundwater VOC levels are much lower but still high enough to require further action to protect human health and the environment. 

2009: Investigated the source and extent of VOCs. Soil samples were dry, showing that leachate may not be the cause. Rather VOCs in groundwater may have come from landfill gas. VOCs have not been detected in samples taken outside the landfill property.

2023: Prepared a draft Cleanup Action Plan that proposes to continue operating a vapor extraction (VE) system at the closed landfill. The VE system extracts landfill gas containing VOCs (including tetrachlorethylene and trichloroethylene) from the soil above the groundwater, then routes the gas to a permitted flaring system for thermal destruction. Asotin County proposes to refine or optimize the VE system, including installing additional VE wells.

Landfills emit both gases and liquids over time. Leachate is liquid in the waste or flowing through it that leaks from the bottom of a landfill. More leachate is produced when rain or snow percolates through the waste. Bacteria in the waste produce the landfill gas in the moist environment. If more water enters the waste, the bacteria produce more gas. Both landfill gas and leachate may dissolve into and contaminate groundwater.

Prior to 1985, landfills were not required to use plastic liners, and liquid disposal was not regulated. Older, unlined landfills often contaminate the underlying soil and groundwater.