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E"'41-lted Mass of Petiolewot lfrdrocurbons Constituents Rewuning in Groundwater CoIbLt goat R orks,401 North Stockton Street,Stockton,California Page 2 Based on Mr Wong's response letter included as Appendix A, Apex submitted a workplan entitled Workplan foi Additional Subsurface Investigation and Site Characterization dated June 9, 1999 On September 29, 1999, Apex personnel Implemented this workplan and installed five geoprobe locations on the subject site in order to establish the extent of petroleum impacted groundwater east of monitoring well MW-3 and to incorporate this data with a new mass balance calculation All field work was conducted according to Apex's Standard Operating Procedures (SOP) Included as Appendix B The boring logs for each hydropunch are included as Appendix C and the laboratory analytical reports and chain-of-custody forms are Included as Appendix D ESTIMATION OF REMAINING PETROLEUM HYDROCARBON CONSTITUENTS Since laboratory analytical results for soil samples collected from the most recent subsurface investigation do not show significant concentrations of petroleum hydrocarbon constituents, the mass remaining in soil has not been recalculated Table 1 summarizes the laboratory results for soil from the September 1999 subsurface investigation The 20-foot sample from soil boring SB- 20 contained minor amounts of toluene and total xylenes Also, the 20-foot sample collected from soil boring SB-21 contained MTBE slightly above the detection limit of 0 025 milligrams per kilogram Static groundwater beneath the site was encountered at an approximate depth of 15 feet below grade surface (bgs) indicating that both soil samples containing minor amounts of petroleum hydrocarbons were actually collected in the saturated zone and do not represent a vadose zone source In order to estimate the mass of remaining petroleum hydrocarbon constituents present in groundwater, it is assumed that petroleum hydrocarbon impacts will remain within the upper 10 feet of the shallow aquifer Using analytical data from the most recent groundwater sampling event conducted on October 21, 1999, including the groundwater elevation data, and the subsurface investigation conducted on September 29, 1999, iso-concentration contours were created using the Golden Software, Inc Surfer for Windows Version 6 (refer to Tables 2 and 3 for groundwater elevation data and groundwater analytical data, respectively) The distribution maps for groundwater elevations, TPHg, benzene, and MTBE remaining in groundwater are shown in Figures 3, 4, 5, and 6, respectively The mass of TPHg, benzene, and MTBE present in groundwater beneath the site was estimated by finding the area of each iso-contour, calculating the volume of groundwater in each Iso-contour, and, in order to present a conservative estimate, using the maximum concentration of each constituent inside each Iso-contour CALCULATION OF ESTIMATED MASS The first step to calculating the mass of petroleum hydrocarbons remaining in groundwater is to identify the volume of affected groundwater inside each contour Assuming the petroleum hydrocarbons are contained within the upper ten feet of the shallow aquifer and the effective porosity is 0 35, the thickness of the aquifer is multiplied by the effective porosity and the area inside the contour to determine the volume of affected groundwater For a sample calculation, the volume inside the 0 micrograms per liter (µg/L) and 100 �1g/L contour for groundwater is as follows Volunie,,, , = 1,541 ftz k 10 ft "0 31 W 5,394ft3 In order to present a conservative estimate of the mass of TPHg, benzene, and MTBE remaining in groundwater, the maximum concentration for a specified contour interval was used for each