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i 1 Mr Ron Rowe Page 3 Groundwater Sampling Summary MTBE was detected in well MW4 for the fifth consecutive quarter, and concentrations increased significantly from the previous quarter MTBE was also detected in well MW3, located downgradient of MW4, for the third consecutive quarter (at the highest concentration to date) Contaminant concentrations present in wells MW4 and MW3 do not appear to be related entirely to onsite releases due to the presence of MTBE, the period of onsite UST use/removal and the easterly groundwater flow The sudden appearance of MTBE in these wells in conjunction with ' increased gasoline concentrations suggests an offsite contributing source It is suspected that the Top Filling Station at 101 S Wilson Way, the ARCO Station at 130 S Wilson Way, or both may be the source of this encroaching contamination tAfter the fifth quarter of oxygenate analysis, the only apparent trends are the presence of MTBE in wells MW3 and MW4, the presence of DIPE in wells MW 11, and MW 12, and the presence ' of TBA in wells MW1, MW11, and MW12. The presence or absence of TBA in wells MW3, MW4 and MW 13 during this sampling round is uncertain due to elevated detection limits resulting from dilution of these samples ETBE and TAME have not been detected in any ' samples. VAPOR EXTRACTION SYSTEM ACTIVITIES VES Operation The soil vapor extraction system operated on vapor wells VW2, VW5, and VW6 for a total of ' 1777 hours between September 14 and December 31, 1998 Following the completion of repairs in October, the system operated continuously for the balance of the quarter System failure caused a shutdown on September 14 The nature of the failure was not readily apparent A manufacturers representative determined that the internal logic system was corrupted The system was reprogrammed on October 8 However, memory could not be ' retained by the controller The digital recorder/controller was removed and shipped to the manufacturer for repair On October 14 the recorder/controller was reinstalled and the logic system reprogrammed Startup was successful The system subsequently was down from ' October 15 through 19 due to a plastic bag clogging the supplemental air flow blower The system was restarted on October 19 and operated continuously through December 31 ' During the fourth quarter 1998, the VES was monitored on a weekly basis The systems monitoring data sheets are included in Attachment C On October 26, November 23, and ' December 21, 1998 influent and effluent vapor samples were collected Additional influent samples were collected on December 7, 1998 and submitted to Sequoia Analytical for analysis in order to confirm the significant decrease in influent concentrations reported for the November ' 23 sampling event The samples were analyzed for TPPH and BTEX i I