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V DISCUSSION Results of this groundwater monitoring event indicate that levels of dissolved gasoline hydrocarbons remained generally consistent in wells MW-1 through MW-7 from the previous sampling events, however, methyl tertiary butyl ether was detected in well MW-7 at a concentration of 4,400 µg/L The groundwater flow direction (northeast to southeast) and gradient are consistent with previous sampling events Groundwater elevation increased 2 62 feet from the previous sampling event (November 1997) The remediation system was started up on January 24, 1997 The remediation system consists of a King, Buck and Associates MMC-5A Catalytic Oxidizer rated at 100 cubic feet per minute The remediation system is connected to vapor extraction wells VW-4 through VW-13 The system was operational for 42 percent of the time and extracted a total of 328 pounds of TPHG during the first quarter 1998 The low uptime was due to pulsing the system for three weeks to evaluate system effectiveness and due to high water levels in the condensate knock-out A cumulative amount of 9,024 pounds of TPHG has been removed since commencing SVE activities (see Table 5) Figure 5 presents cumulative mass of TPHG removed versus time and TPHG extraction rate versus time Vapor extraction wells VW-10, VW-11 (adjacent to the former underground storage tank complex), and VW-12 (adjacent to pump islands) are still contributing to influent soil vapor petroleum hydrocarbon concentrations during the first quarter 1998 (see Table 6) As documented in Pinnacle's letter, dated April 8, 1998, to the SJCPHS, and as shown in Figures 4 and 5, influent TPHG and benzene concentrations continue to decline with system operation and did not rebound back after pulsing the system for a 3-week period Monitoring data (see Table) indicate that benzene has declined to non-detectable levels As a result, Pinnacle will suspend vadose zone remediation ATTACHMENTS • Table 1 Groundwater Monitoring Data • Table 2 Groundwater Analytical Results for Oxygenates • Table 3 Operational Uptime Information of Soil Vapor Extraction System • Table 4 Flow Rates and Analytical Results of Air Samples • Table 5 Extraction and Emission Rates • Table 6 Vapor Extraction Well Status • Figure 1 Site Location Map • Figure 2 Generalized Site Plan • Figure 3 Groundwater Contour Map (February 9, 1998) • Figure 4 TPHG Concentration Vs Time • Figure 5 Extraction Rate and Cumulative Mass of TPHG Removed Vs Time • Appendix A Field Procedures • Appendix B Laboratory Analysis Reports for Groundwater Samples and Chain-of-Custody Records • Appendix C Groundwater Monitoring Field Data Sheets • Appendix D Laboratory Analysis Reports for Air Samples and Cham-of-Custody Records • Appendix E Operation and Maintenance Field Data Sheets cc Elizabeth Thayer, California Regional Water Quality Control Board Kai Chen, San Joaquin Valley Unified Air Pollution Control Board i SAC'N\P7fp805\08051461 1AS-98ynusmer 1 P"MaC& Rev 0 7/16/98