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,• DISCUSSION: On October 30, 1997, EMCON installed groundwater monitoring well MW-4 at the subject site. Appendix A presents the groundwater monitoring well installation details. Groundwater samples were analyzed for total petroleum hydrocarbons as gasoline (TPHG), and for benzene, toluene, ethylbenzene, and total xylenes (BTEX). The sample collected from dual-completion vapor extraction well VW-3 came from the deep screen interval located between approximately 42 and 61 feet below ground surface (bgs). Results of this monitoring event indicated that levels of dissolved gasoline hydrocarbons are generally consistent with levels detected during previous sampling events for wells MW-1 through MW-3. BTEX was detected in well VW-3 at concentrations of 4.6, 4.4, 2.6, and 6.7 micrograms per liter (pg/L), respectively. In addition, the groundwater samples were analyzed for methyl tertiary butyl ether (MTBE) using U.S. Environmental Protection Agency (USEPA) Method 8020. MTBE was below laboratory detection limits for wells MW-1 through MW-3 and VW-3 using USEPA Method 8020. Well MW-4 contained detectable concentrations of MTBE at a concentration of 11 µ.g/L. The groundwater gradient and direction of groundwater flow (east) were generally consistent with the previous sampling events. Groundwater elevation increased approximately 0.67 foot from the previous sampling event (August 1997). Per ARCO's request, groundwater samples were analyzed for the oxygenates tertiary butanol, MTBE, di- isopropyl ether, ethyl tertiary butyl ether, and tertiary amyl methyl ether using USEPA Method 8260 for the two monitoring wells with the highest historical concentrations of TPHG, BTEX, and MTBE. EMCON evaluated third quarter data and selected wells MW-1 and VW-3 to be analyzed for these oxygenates. In addition, the newly installed monitoring well MW-4 was also analyzed for the oxygenates listed above. Well MW-1 contained 7.8 µg/L of MTBE, and well MW-4 contained 190 and 13 pg/L of tertiary butanol and MTBE, respectively. No other oxygenates were detected in the samples from wells MW-1, MW-4, and VW-3 using USEPA Method 8260. During the first quarter 1998, a groundwater sample from well MW-4 will again be analyzed for the oxygenates to confirm detection of tertiary butanol. ATTACHMENTS: Table 1 Groundwater Monitoring Data Table 2 Additional Groundwater Analytical Data • Table 3 Quarterly Groundwater Flow Directions Figure 1 Site Location Map • Figure 2 Generalized Site Plan • Figure 3 Groundwater Contour Map(November 20, 1997) • Appendix A Groundwater Monitoring Well Installation Details • Appendix B Sampling and Analysis Procedures • Appendix C Laboratory Analysis Reports and Chain-of-Custody Records Appendix D Field Data Sheets cc: Elizabeth Thayer, California Regional Water Quality Control Board I SAGS-\PJ"80SU8051470.3DS-98rzusmer.2