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Seneca Environmental Services removed three underground storage tanks (USTs) in December 2012. Contamination was present below the tanks. The soil waste was profiled and approximately 42 tons of soil was removed for disposal at a landfill.
A Tier 1 Investigation took place from September 2013 through November 2013. A daycare facility with a basement was located adjacent north of the site. A Tier 2 Investigation included a subsurface structure vapor intrusion assessment via sub-slab sampling in the daycare and subsurface soil vapor sampling and analysis with an on-site laboratory to map out the distribution of petroleum vapors in the subsurface. The subsurface structure survey and sub-slab sampling revealed no evidence of vapor intrusion, however the soil gas investigation mapped elevated vapors above site-specific target levels (SSTLs) to be migrating toward the foundation of the daycare.
Nebraska Department of Environmental Quality (NDEQ) and Seneca determined the need for vapor intrusion prevention via soil vapor extraction (SVE) remediation. A SVE pilot test was conducted in December 2014 by installing a 4-inch vapor recovery well in the heart of the mapped vapor plume and inducing vacuum on the recovery well with a vacuum truck and control manifold to monitor radius of influence, flow rates and vapor recovery. This enabled Seneca Environmental and Remediation to design a full-scale system.
The SVE system was approved by NDEQ and was installed in April 2015. The full-scale system included the installation of four additional 4-inch vapor recovery wells, subsurface trenching and piping, installation of temporary power and mobilization of a trailer mounted SVE system.
The SVE system was started on May 18, 2015, running full throttle 24/7. Monitoring of system performance and collection of emissions samples to gauge remedial progress was subsequently conducted 12 hours following startup, weekly thereafter for three weeks, and monthly after that.
Based on laboratory analytical results of the emission samples, benzene concentrations were reduced by over 99 percent within the first two weeks of operation. Following one month of operation, the SVE system was programmed to “pulse” six hours on and six hours off to alter preferential air pathways in the subsurface. Emissions plateaued at low levels after six months.
The SVE system was shut down December 2, 2015. After allowing two months for the subsurface to stabilize, a post remedial subsurface soil vapor investigation took place on February 9, 2016 to confirm the SVE system had reduced benzene concentrations below SSTL’s.
The following day, a post remedial sub-slab confirmation sampling event was completed in the daycare basement, which all resulted below indoor air standards. Per request of NDEQ, two additional quarterly sampling events were required on May 11, 2016 and August 11, 2016 to exhibit that benzene concentrations continued to remain below indoor air standards. Concentrations remained at acceptable levels and NDEQ granted no further remedial action in a letter dated December 22, 2016. The trailer was demobilized, the system decommissioned and all monitoring and remediation wells were plugged and abandoned in January and February 2017. The key component to the success of the project was the communication and cooperation between Seneca’s Environmental Services, Remediation and Waste Solutions divisions, providing the customer with “The Complete Solution.”