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Mr Ron Rowe Page 3 MW4 at concentrations of 19 and 85 parts per billion (ppb), respectively DIPE was detected in the samples from wells MW3 (1 1 ppb), MW11 (Ilppb), and MW12 (1 9ppb) TBA was detected in all the samples, except from MW13, at concentrations ranging from 8 2 to 49 ppb ETBE and TAME were not detected in any samples The oxygenate concentrations are summarized in Table 3 Groundwater Sampling Summary MTBE was detected in well MW4 for the third consecutive quarter However, concentrations have shown a decreasing trend during that period MTBE was also detected in well MW3, located downgradient of MW4, for the first time Dissolved hydrocarbon concentrations in samples from well MW4 have decreased to near historical levels after two consecutive quarters of increased concentrations. Contaminant concentrations present in well MW4 do not appear to be related to onsite releases due to the lack of MTBE in other site wells and the consistent east-southeast groundwater flow direction In addition, the appearance of MTBE in MW3 and decrease of MTBE in MW4 suggest that an off site plume may be migrating southeastward across the site After the third quarter of oxygenate analysis, the only apparent trends are the presence of MTBE in well MW4, the presence of DIPE in well JAW11, and the presence of TBA in well MW12 However, the presence of DIPS 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 VAPOR EXTRACTION SYSTEM ACTIVITIES VES Operation The soil vapor extraction system operated on vapor wells VW2, VW3, VW5, and VW6 for a total of 1,512 hours between April 1 and June 30, 1998 The VES shut down on Apnl 21, 1998 as a result of a tripped thermal overload switch The system was restarted and placed back in full time operation on May 11, 1998 The system also shut down on June 15 and 20, and was restarted after each occasion The system apparently shut down as a result of temperature fluctuations During the quarter, the VES was monitored on a weekly basis The systems monitoring data sheets are included in Attachment C On April 13, May 18, and June 15 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 • rc:klaznrl2crni S98 F