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r Ms. Lori Duncan Page 3 of 5 San Joaquin County Environmental Health Department November 23, 2005 reported in September 2005 was in VW-3,at the southern end of the UST complex(6,800 micrograms per liter [t.g/L]). Non-detectable concentrations of GRO at MW-5 suggest that significant downgradient migration of impacted groundwater has not occurred.Isoconcentration maps and cross-sections of GRO in groundwater are presented in Figures 3,4, 10, and 11. Benzene-impacted groundwater is confined to the UST complex and extends downgradient(Figures 5 and ,6). The maximum benzene concentration (110µg/L)reported in groundwater for September 2005 was in �{ VW-1.Isoconcentration maps and cross-sections of benzene in groundwater are presented in Figures 5,6, 12, and 13. Groundwater impacted by MtBE is confined to the site in the area of the former UST complex; it extends r' east and northeast from the UST complex(Figures 7 and 8).The maximum reported concentration of MtBE reported during September 2005 was 120 A.g/L in both AS-1 and VW-1.MtBE isoconcentration maps and rl . cross-sections are presented in Figures 7, 8, 14, and 15. Figures 3 through 8 and 10 through 15 show that petroleum-impacted groundwater is located primarily ' around the former UST complex and downgradient from the complex for approximately 50 feet. The northward extension of the plumes depicted in Figures 3,5,and 7 was caused by the release of product into soil during the drilling of boring B-18 which punctured a product line in 1993. This release resulted in groundwater contamination around VW-4,which has now become commingled with the larger plume around the former_UST.complex-"""'� - _ -- - •- --�� C C N ' Based on the minimal amount of plume migration since groundwater monitoring and S VE/AS mitigation of the plume began,which is shown by the decreasing concentrations over time,groundwater impacts at the site do not present a threat to the municipal supply wells identified in the sensitive receptor survey. HYDROCARBON MASS ESTIMATES Mass Estimates The mass of petroleum hydrocarbons beneath the site remains similar to that calculated for the August 2004 r� SCM report. "jv ,t."f 4ucce:4;t w'J-e)aAj;,A, ? �! CONCLUSIONS ' The results from additional investigative work conducted at the site support the initial SCM conclusions including: ,+ • The remaining hydrocarbon impacts identified in groundwater are in the vicinity of the vapor extraction wells near the UST complex, particularly in wells VW-3, VW-1,and AS-1. ' • Because of the relatively low permeability soils (predominantly silts) and the relatively low hydraulic gradient(0.002 ft/ft to 0.03 ft/ft),the remaining hydrocarbon ted groundwater is unlikely to migrate vertically or laterally to a significant extent. Esi-.:iM7r� scop&jf rjf ? • The extent of the plume has been defined in both the vertical and horizontal directions. KAWprocessU5 BP ARCO\0054812005 SCM Update\SCM Update Text.doc