Southgate Oil

Ecology received Post-Remediation Compliance Monitoring report on October 30, 2024.  Based on a review of the remedial action documented in previous submitted reports, and the additional data provided in this report, Ecology determined that No Further Action is necessary to clean up contamination at the Site.  Ecology issued a No Further Action Letter on December 9, 2024. 

The majority of the Site footprint is located on one 0.64-acre King County parcel 2500600480 in Kent, Washington (Property). Based on data, the Site also includes the adjacent property to the north (King County Parcel 2500600465), and adjacent property to the southeast (King County parcel 2500600481).

A former bulk fuel distribution facility operated on the Property for approximately 75 years, most recently under the business name of Southgate Oil.  The facility stopped distributing fuel in or around 1999.  The facility included at least 11 underground storage tanks (USTs), multiple fuel dispensers, and an auto repair garage.  The USTs were installed between the 1950s and 1990s, and removed in 2000 and 2002.

Sound Transit acquired the Property and demolished all remaining building structures by 2020. The north portion of the Property has been developed with a new right-of-way, South 234^th Street. The south portion is currently used as an equipment staging area for light rail facilities construction.

Environmental assessments and remedial actions were conducted at the Site since 2000.  In 2000 and 2002, 11 USTs and associated fuel dispensers were removed from the Property.  Petroleum contaminated soil were discovered in the UST excavations and under the dispensers.  Approximately 748 tons of contaminated soil were removed from the Property in 2002.  

In 2017 to 2020, multiple environmental site assessment confirmed that petroleum contaminated soil were present throughout the Property to at least 21 feet deep. The deepest soil contamination is discovered on the central portion of the Property. The Petroleum contamination has migrated to soil beneath the north- and southeast-adjacent properties to approximately 15 feet deep.

In addition, shallow soil contaminated with chlorinated solvent, heavy metals, and polycylic aromatic hydrocarbons (cPAHs) were discovered near the former service garage on the northwestern portion of the Property.

In 2019 to 2020, soil excavations were completed throughout the Site (on all three properties) to remove contaminated soil. Soil excavations were completed to up to 20 feet deep on the western portion of the Property, 30 feet deep on the central portion of the Property, 12 feet deep on the eastern portion of the Property, and 15 feet deep on the adjacent properties to the north and southeast. A total of approximately 6,570 tons of contaminated soil has been removed from the Site. All known soil contamination has been sufficiently removed.

In 2024, a post-remediation groundwater exploration boring was advanced to 65 feet deep. No groundwater was observed in the boring. Because groundwater is at least 35 feet deeper than the deepest contaminated soil, the groundwater at the Site is not impacted by the contaminated soil.

Total petroleum hydrocarbons as gasoline, diesel, and heavy oil (TPHg, TPHd, and TPHo); benzene, methylene chloride, tetrachloroethene (PCE), naphthalenes, carcinogenic polycyclic aromatic hydrocarbons (cPAHs), cadmium, lead, and mercury into the Soil