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i MW-2 (2.7 ppb), MW-4 (17.5 ppb), MW-5(4.9 ppb), and MW-6 (0.6 ppb); however, it was not detected in any of the other samples. Table 2 in the attached report summarizes groundwater analytical data and laboratory reports are included as Appendix B. The highest concentrations of hydrocarbons appear to remain onsite in the vicinity of the pump islands (MW- 1 and MW-3). When compared to the July 1999 sampling event, hydrocarbon concentrations increased in MW-3 while they decreased in MW-2, MW-4, and MW-5. Hydrocarbon concentrations remained relatively unchanged in each of the wells. Changes in hydrocarbon!. are most likely a result of groundwater elevation fluctuations. Additional monitoring data will be collected to further evaluate the dissolved hydrocarbon plume. Remediation Activities-Fourth Quarter 1999 - In August and September 1999, SECOR installed a vapor extraction/air sparging system at the subject site to remediate hydrocarbon-impacted soils and groundwater. System operation began December 13, 1999. The vapor extraction system consists of ten wells (W-1 through W-8, MW-1 and MW-3) manifolded within the remediation compound. Vapors are extracted with a blower and destroyed with a catalytic oxidizer with a destruction efficiency of at least 95%. The air sparging system consists of 4 wells (AS-1 through AS-1). An air compressor is used to introduce air into the sparging wells,'and vapors are be recovered and destroyed i with the SVE system. System operation and performance will be discussed in the next quarterly groundwater monitoring report. If you have any questions regarding these matters, please do not hesitate to contact me at(206)286-4495. f Sincerely, Time Oil Co. r Anastasia E. Duarte-Wilkinson ti Environmental Toxicologist w 1 I } cc: Ms. Elizabeth Thayer' California Regional Water Quality Control Board _ y letterslcalrf1404W4049146.doc i 2 i