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I Mr Ron Rowe Page 3 Groundwater Sampling Summary MTBE was detected in wells MW4 and MW9 for the second consecutive quarter In addition to the high MTBE levels in MW4, the hydrocarbon concentrations detected in well MW4 have drastically increased sine:July 1997 Since the groundwater flow direction has been consistently ' toward the east-southeast, these concentrations present in well MW4 do not appear to be related to onsite releases rAfter the second quarter of oxygenate analysis, the only apparent trends are the presence of MTBE in wells MW4 and MW9, the presence of DIPS in well MW 11, and the presence of TBA in well MW12 However, the presence of DIPE and TBA in groundwater at this site seems unusual based on the letter from the Regional Water Quality Control Board, Central Valley Region dated August 1997 The letter suggests that in San Joaquin County DIPE has only recently been detected in groundwater and that TBA has only been detected at one location VAPQR EXTRACTION SYSTEM ACTIVITIES i I VES Operation The soil vapor extraction system operated on vapor wells VW2, VW3, VW5, and VW6 for a total of 908 hours between January 1 and March 30, 1998 Vapor well VW3 was brought on line January 5 The system automatically shut down several times during January but was successfully restarted after adjusting gas regulator pressure and dilution air However, attempts to restart the system and isolate the problem following an auto shut down on January 29 were unsuccessful The system controls display indicated a malfunctioning air flow sensor switch (one of three) The sensor malfunction apparently was a result of moisture and the system was restarted and placed back in full time operation on March 10 following the cessation of torrential rains The system was shut down on March 30 to remove and replace a broken pitot tube and was restarted March 31 E F During the quarter, the VES was monitored on a weekly basis The systems monitoring data ` sheets are included in Attachment C On January 12, March 16 and March 27 influent and effluent vapor samples were collected and submitted to Sequoia Analytical for analysis. The samples were analyzed for total purgeable petroleum hydrocarbons (TPPH) and BTEX Vapor Sample Analytical Results Laboratory reports are included in Attachment D and are summarized in Table 4 Based on the TPPH concentrations, the destruction efficiency of the thermal oxidizer during the January and March 26 sampling was approximately 96% However, the samples collected March 16 indicated a destruction efficiency of 75% These results were thought to be erroneous (possibly • roeklgmrk1gm1998 s..