Contour performed a Limited Phase II Site Investigation to investigate on-site and off-site environmental concerns. The Limited Phase II findings indicted the presence of petroleum constituents in the soil and groundwater from direct push borings advanced on-site. The source of the petroleum constituents was attributed to the former underground storage tank (UST) system. Contour installed four (4) permanent monitoring wells as part of a Corrective Action Plan – Part A (CAP-A) investigation.
The CAP-A report was submitted with recommendations for additional assessment, a chemical injection pilot study, and preparation of a CAP-Part-B (CAP-B). Concurrent with CAP-A preparation, a supplemental source assessment was performed in February 2005 to further assess source areas for remediation prior to redevelopment of the property. The findings of the assessment suggested that past releases of leaded and unleaded gasoline had occurred from the USTs and associated dispensers.
Contour excavated, removed, transported, and disposed of approximately 1,800 tons of petroleum contaminated soil from the southwestern portion of the site. The soils were transported to a Subtitle D landfill for disposal. The excavation area included the former UST basins and a portion of the adjacent petroleum dispenser area. This work was performed prior to re-development of the property, which included installation of an underground detention gallery for storm water drainage.
Prior to backfilling, a horizontal injection gallery was installed at a depth of approximately 20-22 below ground surface. The injection gallery consisted of three 1-inch diameter horizontal injection wells ranging from 20-30 feet in total length with 15-20 foot slotted screened sections. The horizontal wells were installed into a bed of large, cobble-sized gravel used to backfill the excavation. The wells were used for chemical oxidation treatment of the groundwater.
Contour removed four (4) abandoned USTs located in and adjacent to the DOT right-of-way for Highway 20 and 78. These USTs were discovered during the soil over-excavation work described above. The USTs reportedly ranged in size from 650 to 2,000 gallons.
An initial in-situ chemical oxidation (ISCO) pilot test was performed by Contour. The injection was performed using a 17-18% solution of hydrogen peroxide. A total of 1,100 gallons of hydrogen peroxide was injected during this event.
Contour performed a second injection using a different chemical oxidant blend, and contracted with EXOTECH, Inc. to perform an ISCO pilot injection using a mixture of hydrogen peroxide (12-13%) and sodium persulfate (5-6%). In this application, sodium persulfate acted as the primary oxidizer when “activated” with a dilute solution of hydrogen peroxide. The combination of these oxidants releases both sulfate and hydroxyl radicals as part of a dual oxidant process. The redox potential of these two oxidants combined is estimated at 2.6-2.7 electron volts, the same/similar as a standard Fenton reaction using hydrogen peroxide.
Based on the corrective action performed and the absence of nearby groundwater receptors, a “Monitoring Only” program was recommended for this site. The vast majority of impacted soils and source materials were removed during the over-excavation and subsequent UST closure. The ISCO pilot injections successfully reduced dissolved benzene concentrations to below 10,000 μg/l in the vicinity of MW-2, located outside of the footprint of the excavated area.