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can arm Ms. Mary Meays December 20, 1995 (9347-3011) Ground Water Monitoring Wells: Monitoring wells were first installed on the site in September 1990. The average depth to stabilized ground water at the site has decreased from approximately 36 feet below surface (bs) in August 1992 to approximately 29 feet bs in May 1995. The vadose zone soils, soils above the i saturated zone and capillary fringe, are primarily composed of clay with some silt and sand. However, the soil in and beneath the area of the former UST cluster appear to be primarily sand with silt and clay which is conducive to remediation using a VES (RESNA, 1991b). The airflow to the VESs has averaged in excess of 128 standard cubic feet per minute. Ground Water Monitoring and Sampling: The ground water gradient is flat, averaging about 0.01 foot per foot in a easterly-northeasterly flow direction, away from the Stockton Deep Water Channel. The downgradient extent of the ` dissolved-phase petroleum hydrocarbons in ground water has been defined. No benzene has been detected above the reported laboratory detection limits in MW-4, the farthest downgradient well, since } February 1994. Results of the November 1995 sampling event indicated: (1)benzene was detected at up to 0.8 parts per billion (ppb) only in the water samples collected from wells MW-3 and MW-5; (2) TPH-G was not detected above reported detection limits in the water samples collected from wells MW-1 through MW-5, RW-1, and RW-2; and (3) toluene, ethylbenzene, and total xylenes were detected at 1, 1.2, and 2.3 ppb only in the water samples collected from well MW-3. Ground water monitoring and sampling results are summarized in the attached Fourth Quarter 1995 Ground Water Monitoring Report prepared by Fugro on December 18, 1995. Soil and Ground Water Remediation Systems: Shell has been operating the various vapor extraction systems (VESs) to extract petroleum hydrocarbon vapors from soil and ground water at the site since January 1992. The VESs have been augmented by a ground water oxygenation (GWO) system since July 1994. In addition, a ground water recovery system has been in operation at the site since March 1993 to enhance the petroleum hydrocarbon recovery of the VES system and control the migration of the petroleum hydrocarbon in ground water. The remedial options selected and installed at the site soil the best available remedial technology and were approved by the SJCPHS/EHD. The following is a summary of remedial activities conducted at the site to date: Vapor extraction was initiated at the site in late January 1992 using venting wells VEW-1 through VEW4 and an internal combustion (IC) unit. An estimated 70 pounds of TPH-G was removed from the site by the IC engine. (RESNA, refer to 04/02/92 letter to APCD). A thermal oxidation unit replaced the IC unit in March 1992. The thermal oxidation unit was then replaced by an activated vapor-phase carbon unit in July 1992 (RESNA, 1992b). An estimated 1,545 pounds of TPH-G and benzene were removed from the site by the thermal oxidation and carbon units between i - April 2, 1992 and September 24, 1992(RESNA, 1992c). 93473011.BRP -3-