Active Service Station, 1301 North State Street, Ukiah

A sudden release of gasoline to the subsurface occurred at this service station in 1986, resulting in a plume of gasoline contaminants (including MTBE) migrating over an area of approximately 600 feet in length by 200 feet in width. In 2001, Stratus was retained to provide complete characterization and assessment of the contaminants in the ground and to them complete cleanup. Plume migration was towards the Russian River (approximately ¾ mile away) under a relatively steep groundwater gradient (over 20 feet of groundwater elevation difference over the length of the plume) and regulatory pressure to address the concern was extremely high.

Stratus completed lateral and vertical assessment, installed monitoring wells (CPT and hollow stem auger rigs were used). Prior to beginning remediation, MTBE concentrations were detected as high as 270,000 µg/L onsite, and as high as 27,000 µg/L approximately 300 feet downgradient of the UST/fuel dispenser leak source. Stratus evaluated, pilot tested, selected, and then implemented a combination of remediation technologies -- DPE followed by in-situ chemical oxidation, using ozone injection to mitigate groundwater impact at the site. In addition, limited over-excavation of contaminated soils was completed near the fuel dispenser/former UST area at the time of service station renovation.

Implementation of corrective action included obtaining permits, preparing design drawings, obtaining approval from a neighboring property owner to allow cleanup to proceed beneath their property, and providing a public notification to the neighborhood surrounding the site. Groundwater and soil vapor were extracted and treated using a 400 cfm Solleco thermal oxidizer and three 1000-lb granular activated carbon vessels. A total of 24 specialized ozone injection wells were installed using the assistance of a geophysical survey, across much of the area with previously documented MTBE impact to groundwater. Two high-output ozone injection systems were used to supply a combination of ozone and air to the 24 wells.

Combination of the technologies used resulted in a rapid reduction in dissolved fuel contaminant concentrations across a widespread area of the subsurface. By January 2008, MTBE concentrations in the monitoring well network across the site had been reduced to levels ranging from 1.3 g/L to 28 µg/L. Following successful remediation of the site, Stratus worked with the RWQCB to develop a post-remediation groundwater monitoring plan and a path to close the environmental case at this site.


MTBE Impact to Groundwater, March 2004


MTBE Impact to Groundwater, January 2008