Formatted Title
Progressing a High Mass Petroleum Site Using In Situ Desorption/Sorption
Background/Objectives
A popular service station site and key stop off point with access to the lakes north of the Twin Cities in Minnesota is the location of an ongoing remediation project resulting from a leaking underground unleaded gasoline storage tank beneath a fuel dispenser. An unknown, substantial volume of unleaded gasoline was released to the soil and groundwater beneath the service station with the risk of migrating offsite and potentially to drinking water sources. A vulnerable drainage ditch linked to the city storm sewer system just down gradient of impacts heightened the risk from the release. Over more than a decade, several investigative actions and free petroleum product recovery have been completed, but only more recently has the combination of in situ desorption followed by in situ sorption progressed the site to meet regulatory expectations.
Approach/Activities
While no specific regulatory goal was established for the site, the regulators expected a demonstration of progress toward improving conditions. Initially, direct push injection of a low-risk in situ chemical oxidation (ISCO) amendment coupled with high vacuum extraction from newly installed recovery wells was attempted at both pilot and full scale to reduce high gasoline mass in the subsurface and mange potential migration to sensitive receptors. Despite three rounds of full-scale injection and a dozen extraction events, subsurface contamination still was not reduced to a reasonable level. Addressing the gasoline mass inside the tank pit where the original leaking underground storage tank resided would be the key step in reducing risk from the contamination. A modified injection using a different remedial fluid, PetroFix® (carbon and nutrients), to flood the tank basin and to create a barrier along the eastern boundary of the basin was designed. This approach trapped the ongoing release of gasoline mass and allowed it to be slowly biodegraded. The process had never been used in Minnesota and had only been used at two other locations around the country. The approach used direct push injection of the PetroFix® into the subsurface on the west side of the tank basin coupled with a high vacuum extraction on the east side wells drawing the remedial fluid into the tank basin and sealing the gasoline mass in place where it could be biodegraded on the surface of the carbon amendment.
Results/Lessons Learned
Approaching two years later, significant reductions in contaminant mass of about 80% at well locations downgradient from the tank basin were achieved following an August 2022 injection into the tank basin. Specific well locations even showed decreases as great as 95%. This presentation will show the progress across the site but will also look at the design approach and lessons learned from challenges working onsite.