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<br /> SEC ® R
<br /> 7-Eleven Store#19976
<br /> Quarterly Groundwater Monitoring and Remediation Progress Report(1Q2004)
<br /> June 3, 2004
<br /> Page 3
<br /> Sod boring SB-1 and nested wells MW-414A and MW-515A were installed in May 1999, as requested
<br /> by the SJCPHS, to provide additional lateral and vertical plume delineation Soil boring SB-1 was
<br /> advanced to 66 feet bgs near the UST area, to evaluate the vertical extent of the plume The nested
<br /> wells were installed to access the saturated zone at depths of 10-30 feet bgs (MW-4 and MW-5), and
<br /> 45-50 feet bgs (MW-4A and MW-5A) Soil sample data from boring SB-1 indicated that the base of
<br /> the benzene, TPHg, and MtBE/oxygenate plume was at between 40 and 55 feet bgs, as evidenced
<br /> by non-detectable concentrations of constituents Samples from well MW-5 contained low
<br /> concentrations of gasoline constituents from 30-35 feet bgs, with no detectable concentrations at
<br /> deeper depths (Shaw, 1999)
<br /> Well MW-6 was installed in December 1999, to a depth of 30 feet bgs (Figure 2) Benzene and
<br /> MtBE were not detected above laboratory reporting limits in sod samples The maximum TPHg
<br /> concentration in sod was reported at 20 feet bgs (15 4 mg/kg) Delineation of hydrocarbons in soil
<br /> was considered complete at this time (Shaw, 2000) x
<br /> On September 18, 2000, as requested by the SJCEHD, eleven soil-gas survey points were
<br /> advanced to 3 feet bgs Significantly elevated MtBE concentrations (2,200 pg/L) were detected in
<br /> only one of the points (SG-9), located along the western edge of the UST pit about 20 feet south of
<br /> MW-2
<br /> On November 1, 2000, monitoring well MW-7 was installed to a depth of 30 feet bgs and soil boring
<br /> SB-2 was drilled to a depth of 45 feet bgs (Figure 2) Sod samples analyzed from both well MW-7
<br /> and soil boring SB-2 did not contain benzene, toluene, ethylbenzene, xylene (BTEX), TPHg, and
<br /> MtBE above laboratory reporting limits A grab groundwater sample was collected from boring SB-2
<br /> at a depth of 40 feet bgs The water sample contained total xylenes at a concentration of 1 5 NgIL
<br /> Soil vapor extraction (SVE) testing conducted in 2001 using the two horizontal vent wells installed in
<br /> the former UST tank pit (IT Corporation, September 2001) indicated extracted vapor concentrations ;
<br /> of over 10,000 parts per million by volume (ppmv) at applied vacuums ranging between 9 and 32
<br /> inches of water Induced vacuum responses of more than 0 2 inches of water were observed in
<br /> wells MW-1, MW-2, and MW-3 during extraction from the horizontal wells Vacuum response in
<br /> these wells was likely facilitated by relatively higher-permeability conditions in the tank cavity backfill ,
<br /> as opposed to native soils Testing was also conducted on the groundwater monitoring wells, lower � 1
<br /> permeability and limited screen above the water table likely accounted for low extraction rates/low t �
<br /> induced vacuum responses observed during extraction from the wells Extracted vapor
<br /> concentrations ranged up to 0 91 ppmv benzene, 220 ppmv MtBE, and 880 ppmv TPHg The data �
<br /> indicated that vapor extraction was a viable remediation method at the site, particularly when the
<br /> horizontal wells were used F'
<br /> x�=
<br /> Double-nested sparge well SP-1 was installed on October 29-30, 2001 to a depth of 50 feet bgs and
<br /> screened from 30-35 and 45-50 feet On February 6, 2002, an air sparge test was conducted using r ;
<br /> well MW-2 for vapor extraction, and double-nested sparge well SP-1 for air injection Pre-sparging ,
<br /> extracted vapor levels ranged up 1,000 ppmv, with dissolved oxygen (DO) levels of up to 0 72 mg/L
<br /> Air was then injected into the deeper section of well SP-1 using an oil-less air compressor Vapor "
<br /> levels in well MW-2 increased to 1,600 ppmv, and DO levels to 8 mg/L, after 30 minutes of sparging
<br /> The data indicated the area of sparging influence in SP-1 included the region of MW-2, and that
<br /> treatment using well SP-1 should be successful at reducing dissolved hydrocarbonsloxygenates in
<br /> well MW-2 and the surrounding area Air sparge testing was repeated on May 23, 2002, using well
<br /> MW-1A as the injection point, and the horizontal wells as the SVE point Results indicated no
<br /> breakthrough of sparged air into the vadose zone accessible by the horizontal wells
<br /> 19976 04-1Q Report doc SECOR International Incorporated
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