Leachate – formed when rain water filters through wastes placed in a landfill. When this liquid comes in contact with buried wastes, it leaches, or draws out, chemicals or constituents from those wastes.

Two of the major issues when prepping a landfill site for building are dealing with gases that emanate from the site and subsidence, or sinking and settling of the land as waste decomposes. As the waste sinks and settles over time, not only do depressions develop on the surface of the site, but leachate, or liquid, passes through the waste carrying contaminants as it goes, possibly spoiling ground water and the surrounding soil.

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In addition, gas or vapors from the decomposing material works its way into surrounding soil and then up and out of the ground. If there are no structures on top of the site, the gas releases into the air. If it’s trapped by a building’s foundation, the result can be a dangerous accumulation of gas.

To mitigate the risk, contractors can install a permeable layer of gravel, which provides a pathway of escape for gas, and then a membrane underneath the slab of a building, Amarandos said. The gas can then be piped out from underneath the building, up the sides and then released at rooftop level.

• Older landfills (pre 1960’s) have largely settled and the landfill gas generation has reached its peak whilst newer landfills represent a greater landfill gas risk due to a higher organic content in the waste. However, no matter what age or class a landfill may be, all require the same assessment approach. The Langley City Dump and Burning pit opened in 1947. It accepted household, yard, demolition, construction, appliances, ammunition from firing range, vehicles (as example) as waste from the 98260 area. The Dept of Ecology mandated the closing of the burn pits in 1971. Dumping continued into the 1990’s.

There is no record of remediation concerning the Langley City Dump.

• Remediation of landfill sites can be a costly and time consuming exercise and requires a greater depth of assessment, particularly in relation to understanding structural stability, landfill gas protection measures and separation of the development from the waste, as well as well developed conceptual site models combined with detailed human health risk assessment.

• The redevelopment of a landfill will receive more scrutiny then other sites and it is unlikely that the site will ever achieve an unrestricted use classification under the Contaminated Sites Act 2003.

The 1986 Island County Groundwater Quality Assessment and Monitoring Program. Download below. It characterizes the water quality in the vicinity of nine county waste disposal sites and design groundwater monitoring programs for the health department. Starting on page 50 (page 126 of pdf), with several diagrams, the Langley Dump site is described in detail. It explains the size of the dump, that dumping continued after 1986, low water table in valley, steps to monitor.