Pacific Highway South and South 250th Street
(classified as level 3 by Sound Transit for hazmat levels).
During 1980, this site was cleaned up for PCB contamination
The possible source could have been when Sea Tac Raceway occupied the property across the street (where Fred Meyer stands today) from 1949-1964.
When the property was torn down in 1964 to construct Valu-Mart (Fred Meyer) the oil, car parts, and waste from the former racetrack may been simply dumped there since it wasn't developed at the time (or may have been disposed of on the site when the track was in operation) since there was little or no regulation at the time to prevent it from happening.
When Harvey's was constructed in the 1970's, an area, in front of the parking lot was used as a bus stop to pickup students going to Totem Junior High/Middle School and Woodmont Elementary.
This site is close to another part of another branch of McSorley Creek that empties into Saltwater State Park as well.
While there are a few black and white photos of this track in existence available on Facebook (no permission has been obtained to use them at this time) but Willamette Speedway in Lebanon Oregon, is a good example of the same style raceway, size, and racing car classification that exists today.
Willamette Speedway (Lebanon, OR)-(Source: Tim Babcock)