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C DISCUSSION: Groundwater samples were analyzed for TPHG and 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. Results of this monitoring event indicated that levels of dissolved gasoline hydrocarbons were generally consistent with levels detected during previous sampling events for wells MW- 1 through MW-3 and VW-3. In addition, the groundwater samples were analyzed for MTBE using USEPA Method 8020. MTBE was below laboratory detection limits for wells MWA through MW-3 and VW-3 using USEPA Method 8020. Well MW-4 contained detectable concentrations of MTBE at a concentration of 29 micrograms per liter (µg/L). The groundwater gradient and direction of groundwater flow (east to southeast) were generally consistent with the previous sampling events. Groundwater elevation increased 4.76 feet from the previous sampling event (November 1997). Per ARCO's request, the groundwater sample from well MW-4 was analyzed for the oxygenates tertiary butanol, MTBE, di-isopropyl ether, ethyl tertiary butyl ether, and tertiary amyl methyl ether using USEPA Method 8260. The sample from well MW-4 contained 180 and 25 �tg/L of tertiary butanol and MTBE, respectively. No other oxygenates were detected in the sample from well MW-4 using USEPA Method 8260. ATTACHMENTS: • Table 1 Groundwater Monitoring Data • Table 2 Groundwater Analytical Results for Oxygenates • Table 3 Historical Groundwater Flow Directions • Figure 1 Site Location Map • Figure 2 Generalized Site Plan • Figure 3 Groundwater Contour Map (February 17, 1998) • Appendix A Sampling and Analysis Procedures • Appendix B Laboratory Analysis Reports and Chain-of-Custody Records • Appendix C Field Data Sheets cc: Elizabeth Thayer, California Regional Water Quality Control Board SAC\Ni\PJ0\0805108051470.5AS-9$\???:I