Formatted Title
Active Service Station: A Better Understanding of the Hydrogeology Facilitated Successful Cleanup and Cost Settlement
Background/Objectives
The North Shore Lee Site (Site) has been an active gas station since 1986 in Federal Way, Washington. Results from a 1991 subsurface investigation determined that a gasoline and diesel fuel release occurred between 1987 and 1991. An air sparge (AS)/soil vapor extraction (SVE) system was installed and operated between 1996 through 2010. The system was shut down in 2010 when a 12,000-gallon gasoline underground storage tank (UST) failed, releasing product to the subsurface. An emergency interim action was conducted that consisted of installing several 4-inch recovery wells immediately adjacent to the UST basin to pump and recover light non-aqueous phase liquid (LNAPL). A large volume of product was recovered but did not eliminate LNAPL accumulating in wells and elevated dissolved-phase total petroleum hydrocarbon (TPH) concentrations in groundwater remained. In 2012, the previous consultant did a pilot test to expand the AS/SVE system, but the system was never expanded. As a part of cost settlement efforts, Floyd|Snider was retained in 2014 to define the extents of the pre-1991 and 2010 plumes and to determine the hydrocarbon mass remaining in the subsurface associated with each release. A review of the data and ineffectiveness of the AS/SVE system indicated that the geology beneath the Site was not completely understood nor was the Site adequately characterized. It was determined that a re-evaluation of the hydrogeology, conceptual site model (CSM), and approach to remediation was necessary. The Site was entered into the Washington State Pollution Liability Insurance Agency’s (PLIA’s) Technical Assistance Program and additional characterization was conducted to determine the most efficient remedial solution while also assisting with cost recovery efforts.
Approach/Activities
Using sonic drilling for additional subsurface characterization, a perched water-bearing zone consisting of gravelly sand was identified and present at depths between 22 and 33 feet at the base of very dense glacial drift material. The gravelly sand in the water-bearing zone overlies a continuous clayey silt/silty clay aquitard approximately 5 feet thick. The overall subsurface topography of the aquitard slopes downward to the northeast. Additionally, the area where the aquitard surface was encountered at the deepest elevations coincides with where the perched water-bearing zone is the thickest. The perched water-bearing zone thins outward in all directions from where it is the thickest. The water-bearing unit is not present to the south or southeast, nor present further north on the adjacent property. These observations explained why there is a preferential pathway to the northeast while the groundwater flow direction is to the east-southeast. These findings helped to determine why the previous AS/SVE system was not effective after hundreds of thousands of dollars had been spent. A remedial alternative using surfactant injections and extraction and targeted in situ chemical oxidation (ISCO) injections within the water-bearing zone was selected and approved by PLIA in 2019. In 2020, a pilot study was conducted within an area containing LNAPL and the greatest concentrations of dissolved-phase hydrocarbons. Post-pilot study results indicated that wells within the radius of influence of injection points had concentration reductions between 40 and 80%, with the greatest reductions more than 80%. Additionally, LNAPL was no longer detected at a recordable thickness. A Site-wide remedial implementation was conducted the following year.
Results/Lessons Learned
Following Site-wide remediation activities in 2021 and 2022, subsequent groundwater monitoring results showed significant reductions in groundwater contaminant concentrations across the Site, with one well that previously contained LNAPL showing TPH concentrations less than cleanup levels. The remedial activities were successful because of the understanding of the localized geology and hydrogeology. Importantly, the results are supported by PLIA as the agency providing cleanup oversight and reduce cost uncertainty for the past and current facility operators, thereby facilitating cost settlement efforts and Site finality. Although successful, the Site-wide ISCO injections took considerably longer to inject when compared with the pilot study because injections occurred during high groundwater levels when the gravelly sand water-bearing unit was at capacity. Therefore, a very slow injection rate was required; if future injections are deemed necessary, conducting the work during the dry season will likely be just as effective.