Hidden Underground

By Gregg George   •  

Taking the Underground Tour in Pioneer Square should be on every visitor to Seattle's list of things to do. It is fascinating to see how the city has evolved and grown up, and over, it's original beginnings. Pioneer Square isn't the only place with secrets hidden below. The rest of Seattle, and many other developed areas, are built up over previous uses. When starting a new construction project we get to discover what those uses were and in this article we will look at 3 of the most common.

Gas Station

The corner gas station is an American icon dating back to the golden age of the automobile. But as times change and land uses change, new buildings are built where gas stations once stood. The biggest risk in these locations is of course the underground gasoline and diesel storage tanks. Removing an intact tank from underground is a relatively straightforward process. Unfortunately if the tank has been leaking, which is somewhat common, the soil around it is contaminated. This requires either the removal of contaminated soils, or treatment of the soil and groundwater left in place through wells. Testing wells are left in place with a cap poking through the building's garage slab. Testing continues through these wells even after construction is complete.


The South Lake Union neighborhood of Seattle was once the laundry district and home to many large laundries from the 1920's onward. Today a few of these historic structures survive, including the Supply Laundry, Troy Laundry, and the New Richmond Laundry, and are finding new uses as office space, restaurants, and retail. Laundry operations of the past incorporated heavy usage of PCE (perchloroethylene), detergent, and bleach, with poor handling and disposal practices generating significant chemical releases into the soil below these facilities. Many technologies can be employed to determine the level of contamination including color-tec screening, monitor wells, membrane interface probes, and passive diffusion bags to name a few. Remediation can include excavation of the contaminated soil, soil vapor extraction (wells with negative pressure to pull contaminated vapors to the surface for treatment), injection of treatment materials for reductive dechlorination (biological treatment of contamination with ethane as the final product), and chemical oxidation injection (chemically breaks down contaminates and coverts to harmless byproducts).


And finally, many new projects are built on the sites of previous manufacturing facilities. The process of manufacturing includes the use of many chemical elements, including chromium, a hard metal that resists tarnishing and has a high melting point. Many manufacturing processes also employ the use of TCE (trichloroethylene) as a cleaning solvent. If these chemicals find their way into the ground below a project, we must first mitigate them before building. In the case of TCE, one strategy is to inject permanganate into the soil. This is an oxidizer that neutralizes the compound. The allowable level of TCE is zero so over-excavation may be required to make sure all of it is gone.

Project sites in urban areas are kinda like a box of chocolates; you never know what you are going to get. But by looking into the history of use on the site and taking sample borings, you can get a pretty good idea of what to expect and then are able to prepare for it and abate it.

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