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6425 PACIFIC,STOCKTON -2- 17 July 2400
<br /> was found at 50 feet where the top of a fin�gramed sand unit is present across the site Eight
<br /> monitoring wells have been installed on-site, and groundwater has been monitored from December 1988
<br /> to January 1999 Free product gasoline and high concentrations of TPHg and BTEX were identified in
<br /> groundwater samples collected from on-site monitoring wells MW-1, -3, and—10. Maximum
<br /> groundwater monitoring results for on-site wells sampled on 8 January 1949 show TPHg at 1700 pg/l,
<br /> benzene at<4 5 pgll, ethyl benzene at 12 µg/1, and xylenes at 1.20 pgll.
<br /> Regal operated an on-site SVE system from October 1991 to December 1998, and an air sparging system
<br /> from December 1997 to December 1998. Monitoring wells show a decrease in TPHg from 290,000 YO
<br /> to 1700 µg/l (MW-10), and benzene from 68,000 pg/1 to<0 5 pg/1 (MW-10). Seven confirmation soil
<br /> borings were completed in June 1997, and Regal estimated that 62,430 pounds of TPHg and 872 pounds
<br /> of benzene were removed from soil and groundwater
<br /> Off-site Investigation
<br /> Five monitoring wells have been installed off-site. Groundwater samples from three of these monitoring
<br /> wells Zsff-5, -7, and—11 have been consistently below detection limits. Samples from monitoring wells
<br /> 1V1N'V-8 (150 feet off-site) and MW-12 (300 feet off-site)have reported high concentrations of TPHg, and
<br /> minor concentrations of benzene and other gasoline constituents Maximum groundwater monitoring
<br /> results for off-site wells sampled on 8 January 1999 show TPHg at 5,400 pg/l,benzene at 7 pg/l, toluene
<br /> at 55 pg/i, ethyl benzene at 170 ppll, xylenes at 1,100 pgll, and MtBE at 16 pg/1. MtBE was first
<br /> reported in September 1997, and the maximum groundwater concentration by EPA Method 8250 is
<br /> 33 pg/1 at monitoring well MW-12. Other fuel oxygenates and additives have not been analyzed
<br /> The TPHg plume in Figiire 1 shows that the TPHg increases downgradient toward monitoring wells
<br /> MW-8 and MW-12, and concentrations in these wells show instable or shghtly increasing trends from
<br /> 1994 to 1999 (see Figures 2 and 3 from WEGE's January 1999 quarterly monitoring report). The extent ,
<br /> of contamination downgradient of MW-12 has not been defined, and the isoconcentration line shown on �f
<br /> Figure 1 is inferred No off-site remediation has been conducted
<br /> Sensitive Receptor Survey !s Av-
<br /> Blakely's Closure report contained a Well Receptor Survey, and three wells were idenufl thin
<br /> 2,000 feet of the site. The closest well is a 120-foot deep domestic well app{oximately 950 feet
<br /> southeast of the site and cross gradient. City water is supplied in the area, and it is unknown if the v7 {
<br /> domestic well is active No samples have been collected from this well The other two wells are
<br /> upgradient of the site greater than. 1,000 feet.
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<br /> Conclusion
<br /> Staff gnizes the substantial effort Regal has made However, based on the high concentrations of
<br /> H d the lack of lateral and vertical defnition off-site and downgradient of the site,we cannot
<br /> concur with San Joaquin County's closure recommendation at this time. For us to consider concurrence,
<br /> Regal must submit a report which defines the lateral and vertical extent of groundwater contamination at
<br /> their site,present an analysis of groundwater samples for all five fuel oxygenates, 1,2-dichloroethylene, -` 9
<br /> /-ethylene dibromide, ethanol, and methanol by EPA Method 82E0B, detemvne the status of the closest
<br /> domestic water supply well, and complete a mass balance calcuIadon of groundwater contamination d vF
<br /> using all monitoring well data �! 1�V '.
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