SITE INFORMATION AND CORRESPONDENCE

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M - _ 17 July 2000 6425 PACIFIC,STOCKTON was found at 50 feet where the top of a fine-grained sand`unit is present across the.site. Eight, + monitoring wells have been installed on-site, and groundwater has been monitored from December 1988 to January 1999. Free product gasoline and high concentrations of TPHg and BTEX were identified in _ groundwater samples collected from on-site monitoring wells MW-1, -3, and-10. Maximum groundwater monitoring results for on-site wells sampled on 8 January 1999 show TPHg at 1700 gg/l, benzene at<0.5 gg/l,' ethyl benzene at 12 µg/1, and xylenes at 120 µg/l. Regal operated an on-site SVE system from October 1991 to December 1998, and an air sparging system from December 1997 to December 1998. Monitoring wells show a decrease in TPHg from 290,000 µg/1 to 1700 gg/l (MW-10), and benzene from 68,000 µg/1 to <0.5 µg/l (MW-10). Seven confirmation soil borings were completed in June 1997, and Regal estimated that 62,430 pounds of TPHg and 872 pounds of benzene were_removed from-soil and groundwater- Off-site roundwaterOff-site Investigation Five monitoring wells have been installed off-site. `Groundwater samples from three of these monitoring , wells MW-5, -7, and—11 have been consistently belowtdetection limits. Samples from monitoring wells MW-8 (150 feet off-site) and MW-12 (300 feet off-site)have reported high concentrations of TPHg, and minor concentrations of benzene and other gasoline constituents. Maximum groundwater monitoring results for off-site wells sampled on 8 January 1999 show TPHg at 5,400 µg/1, benzene at 7 µg/1, toluene ` ` at 55 1, ethyl benzene at 170 1 xylenes at 1 100 1, and MtBE at-'16 1.. MtBE was first µg/ Y gg/l, Y , µ� µg/ reported in September 1997, and the maximum groundwater concentration by EPA Method 8260 is +' 33 gg/l at monitoring well MW-12. Other fuel oxygenates and additives have not been analyzed. The TPHg plume in Figure 1 shows that the TPHg increases downgradient toward monitoring wells MW-8 and MW-12, and concentrations in these wells show unstable or slightly increasing trends from 1994 to 1999 (see Figures 2 and 3 from WEGE's January 1999 quarterly monitoring report). The extent of contamination downgradient of MW-12 has not been defined, and the isoconcentration line shown on Figure 1 is inferred. No off--site remediation has been conducted. Sensitive Receptor Survey µ. Blakely;s_Closure report contained a,Well Receptor Survey; and three wells were identified within'. 2,000 feet of the site. The closest well is a 120-foot deep domestic well approximately 950 feet southeast of the site and cross gradient. City water is supplied in the area, and it is unknown if the domestic well is active. No samples have been collected from this well. The other two wells are ., upgradient of the site greater than 1,000 feet. Conclusions r Staff recognizes the substantial effort Regal has made. However, based on the high concentrations of TPHg and the lack of lateral and vertical definition off-site and downgradient of the site, we cannot concur with San Joaquin County's closure recommendation at this time. For us to consider concurrence, Regal must submit a report'•which defines the lateral and vertical extent of groundwater contamination at . . their site, present an analysis of groundwater samples for all five fuel oxygenates, 1,2-dichloroethylerie, ethylene dibromide, ethanol, and methanol by EPA Method 8260B, determine the status of the closest domestic water,supply well, and complete a mass balance calculation of groundwater contamination using all monitoring well data. + t