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` Ms.Lori Duncan Page 7 of 9 I ? San Joaquin County Environmental Health Department August 25,2004 Mass Estimates Values, calculations, and calculated results for hydrocarbon mass estimates are provided in Table r 7;the results follow. • Approximately 12.6 lbs of GRO remain in the subsurface below the site. • Approximately 0.2 lb of Benzene remains in the subsurface below the site. ' + Approximately 0.4 lb of MtBE remains in the subsurface below the site. er"y CONCLUSIONS sjAV14 Information was requested from the CSPWD to determine the locations of municipal production wells relative to the site. Current groundwater and historical soil analytical data were compiled and are presented in tabular and graphical formats. Following are the conclusions of the investigative work. • Subsurface conditions at the site have benefited from the historical use of SVE and air injection. SVE has contributed to the remediation of soil and groundwater by source removal. Air injection has contributed to the remediation of groundwater through volatization and the introduction of oxygen into the subsurface to encourage the growth of naturally occurring bacteria and associated aerobic biodegradation. • To date, approximately 2,100 pounds of petroleum hydrocarbons have been removed from the site. 0 \One active groundwater production well is present within a 2,000-foot radius of the site. --._ dThe well is screened at various depths, ranging from 170 to 240 feet bgs. Because the area of impacted groundwater is relatively stable and shallow, and the municipal well is 950 feet from the site,.URS concludes that the impacted groundwater at the site does not pose �a threat to either of these municipal wells. • 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 and VW-1. • Because of the relatively low permeability soils (predominantly silts) and the relatively law hydraulic gradient (0.002 ft/ft to 0.03 ft/ft), the remaining hydrocarbon-impacted groundwater is unlikely to migrate vertically or laterally to a significant extent. • Lateral definition in the downgradient direction has not been defined at the groundwater interface. Lateral definition cross-gradient and upgradient is defined by the current monitoring well and vapor well network. • Vertical definition of impacted groundwater has not been defined. KAWprocessV5 BP ARCO\0054812004 SCMXSite Conceptual Model V2.doc