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Background

The site is a former service station and bulk petroleum plant in Sunnyside, Washington, where petroleum hydrocarbons were discovered leaking from underground storage tanks (USTs) in the late 1990s. West Central Environmental Consultants (WCEC), an environmental consulting company highly experienced in addressing petroleum-contaminated sites, investigated the site and observed light non-aqueous phase liquids (LNAPL or free product). In response, WCEC completed interim remedial measures, including excavating grossly impacted soils and installing a free product recovery system. WCEC completed further remedial investigation over the next several years. In Situ remediation activities began in the early 2000s with the injection of oxygen enhancing amendments to stimulate biologically-mediated aerobic oxidation of petroleum hydrocarbons. Investigation and oxidation injections were expanded to address other impacted site areas in phases. Although the oxygen enhancement injections reduced the hydrocarbon mass, soil and groundwater contamination remained, including MTBE (methyl-tert-butyl-ether) concentrations exceeding the Washington Department of Ecology’s action level.