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rrrry S E C ® R Conceptual Site Model March 24, 2004 Page 10 Groundwater analytical data presented in, Chevron Groundwater Monitoring Report (402003), was used to estimate the approximate mass remaining in groundwater The equation used for estimating the mass of MtBE remaining in groundwater is presented in Appendix H Groundwater mass balance calculations indicate that approximately 631 7 lbs, 16 5 lbs, 68 7 lbs, and 81 1 lbs of TPHg, TPHd, benzene, and MtBE, respectively, are present in groundwater beneath the Site kk 5 2 Chemical Concentrations versus Time Concentrations of TPHg and MtBE have generally declined in wells MW-1 through MW-3 since S monitoring and sampling began An uncharacteristic increase in TPHg and MtBE concentrations was detected in groundwater samples collected from MW-2 on November 3, 2003 Plots of chemical concentration versus time are presented on Figures 10 through 12 5 3 Groundwater Elevations versus Time yt Groundwater elevations have risen approximately 10 feet since monitoring and sampling began In March of 1995 -wholtMW-1 monitoring well installation groundwater was encountered at approximately 35 feet bgs Based on field observations, during monitoring well installation, MW-1 through MW-3 were screened from approximately 30 feet bgs to 50 feet bgs ' Historical depth to y water data indicates that MW-1 through MW-3 have been monitored with submerged screens from approximately first quarter 1996 Depth to water versus time for MW-1 through MW-3 are presented as Figures 10 through 12 M 6.0 SOURCES OF IMPACTS a Primary sources of petroleum hydrocarbon impacts to soil and groundwater were identified by a database search conducted by EDR and a review of historical reports EDR reviewed databases maintained by federal and state environmental agencies The purpose of the review was to identify f reported listings for the subject site and properties in the vicinity The reviewed databases include ; federal and state lists of known or suspected contaminated sites, known handlers or generators of r hazardous wastes, known waste disposal facilities, and permitted USTs The databases which were researched are presented within the Database Radius Report(Appendix A) a Based on a review of the database search conducted by EDR and historic reports for the site vicinity, twenty four facilities were identified (excluding the site) within a 1/a- mile radius of the site Eleven of y the twenty four sites are listed to have had a potential release Six leaking underground storage tank (LUST) sites were either adjacent to or inclose proximity to the Site These sites, together with the { subject Site, are • Chevron Service Station 9-2033, located at 508 West Charter Way, Stockton, California • Former Texaco Service Station, located at 440 West Charter Way, Stockton, California • Former Arco Service Station 2168, located at 441 West Charter Way, Stockton, California r • The Transmission Shop, located at 515 West Charter Way, Stockton, California I IChc,vron1920331REPORTSICSMICSM 92033(REV 8) doc