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Fourth Quarter 2002 Monitoring,Sampling, and Remediation Progress Report 3
<br /> 7-Eleven, [no,Store#19976, 1399 North Main Street, Manteca,CA December 24,2002
<br /> ' Upon receipt of these data, the SJCPHS requested further lateral delineation Well MW-6 was Installed in
<br /> December 1999, to a depth of 30 feet bg (Figure 2) Soil samples collected during well Installation
<br /> contained no detectable benzene or MTBE, and up to 15 4 mg/kg TPH-G, with the highest levels detected
<br /> ' at 20 feet bg (capillary fringe soils) Dissolved concentrations in the well when first sampled ranged up to
<br /> 1 1 pg/L benzene, 1,010 pg/L TPH-G, and no detectable MTBE
<br /> ' On September 18, 2000, as requested by the SJCPHS, eleven soil-gas survey points were pushed to
<br /> 3 feet bg Significantly elevated MTBE concentrations(2,200 pg/L)were detected in only one of the points
<br /> ' (SG-9), located along the western edge of the UST pit about 20 feet south of MW-2
<br /> On November 1, 2000, as requested by the SJCPHS, monitoring well MW-7 was Installed to a depth of
<br /> ' 30 feet bg and soil boring SB-2 was drilled to a depth of 50 feet bg (Figure 2) All soil samples analyzed
<br /> from both well MW-7 and soil boring SB-2 were non-detectable for BTEX, TPH-G, and MTBE A grab
<br /> groundwater sample collected from boring SB-2 at a depth of 40 feet bg indicated non-detectable levels of
<br /> all analyzed constituents except total xylenes at a concentration of1 L A4.9• A fs/.rsGs < 45- (/A,,
<br /> SVE testing conducted In 2001 using the two horizontal SVE wells installed In the former UST tank pit(IT
<br /> Corporation, September 2001) indicated extracted vapor concentrations of over 10,000 parts per million
<br /> by volume (ppm[v]) at applied vacuums ranging between 9 and 32 Inches of water (IW) Induced vacuum
<br /> responses of more than 0 2 IW were observed In wells MW-1, MW-2, and MW-3 during extraction from the
<br /> horizontal wells Vacuum response In these wells is likely facilitated by relatively higher-permeability
<br /> conditions In the tank cavity backfill as opposed to native soils Testing was also conducted on the
<br /> ' groundwater monitoring wells, lower permeability and limited screen above the water table likely account
<br /> for low extraction rates/low Induced vacuum responses observed during extraction from the wells
<br /> Extracted vapor concentrations ranged up to 0 91 ppm(v) benzene, 220 ppm(v) MTBE, and 880 ppm(v)
<br /> ' TPH-G The data Indicated that vapor extraction Is a viable remediation method at the site, particularly
<br /> when the horizontal wells are used
<br /> ' Double-nested air-sparge (AS)/SVE well SP-1 was Installed In October 2001 to a depth of 50 feet bg, and
<br /> is perforated at 30-35 feet bg and 45-50 feet bg On February 6, 2002, an AS test was conducted using
<br /> well MW-2 for vapor extraction, and well SP-1 for air Injection Pre-sparging extracted vapor levels ranged
<br /> up 1,000 ppm[v], with dissolved oxygen (DO) levels in groundwater of up to 0 72 mg/L Air was then
<br /> injected into the deeper section of well SP-1 using an oil-less air compressor After 30 minutes of AS,
<br /> extracted vapor levels in well MW-2 increased to 1,600 ppm(v), and DO levels increased to 8 mg/L The
<br /> data indicated that the area of AS Influence in SP-1 includes the region of MW-2, and that treatment using
<br /> well SPA should be successful at reducing dissolved hydrocarbons/oxygenates In the well and the
<br /> ' surrounding area AS testing was repeated on May 23, 2002, using well MWAA for injection and the
<br /> horizontal wells for SVE Results Indicated no breakthrough of sparged air into the vadose zone
<br /> ' accessible by the horizontal wells
<br /> 19976024 DOC
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