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I 1 ' Mr Ron Rowe Page 3 located east of MW4, for the second consecutive quarter Dissolved hydrocarbon concentrations in samples from well MW4 have increased to the middle of the recent historic range after one ' quarter of decreased concentrations. Contaminant concentrations present in well MW4 (and perhaps MW3) do not appear to be related entirely to onsite releases due to the presence of MTBE in these wells, the period of UST use/removal on the property and the easterly groundwater flow direction The recent appearance of MTBE in MW3 and increase of MTBE in MW4, corresponding to an increase in gasoline hydrocarbon concentrations in MW3 and 1 1 MW4, suggests that a plume originating offsite may be migrating eastward across the site 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 ' After the fourth quarter of oxygenate analysis, the only apparent consistencies are the presence of MTBE in wells MW3 and MW4, the presence of DIPE in wells MW11, and MW12 and the ' presence of TBA in wells MW 1, MW3, MW4, MW 11 and MW 12 ETBE and TAME were not detected in any samples ' Hydrocarbon concentrations in several wells have exhibited an overall trend of decrease since 1995196. Some of these wells, however, have had their screens submerged for a year or more Importantly, wells MW2 and MW 13 are constructed such that the tops of their screens remain above the water table. These wells are generally downgradient of the onsite contaminant source and both have shown a trend of decreasing concentrations, particularly MW2 The operation of the soil vapor extraction system, along with natural attenuation, may be asserting a ' measurably positive influence on dissolved contaminant concentrations VAPOR EXTRACTION SYSTEM ACTIVITIES ' VES Operation The soil vapor extraction system operated for a total of 612 hours between June 28 and ' September 14, 1998 During this period an estimated 1,600 pounds of hydrocarbons were removed from the subsurface. Vapor wells were VW2, VW3, VW5 and VW6 were all online until August 25 when VW3 was taken offline due to high groundwater levels which had 1 submerged the screen System operation was very erratic during the quarter with a tendency to operate for a matter of ' days or hours and then automatically shut down "Air Flow Error" was generally indicated on the combustion control system Additionally, the thermal overload circuit would often be ' tripped The manufacturer's representative suggested several possible faults causing this problem malfunctioning digital flow transmitter, malfunctioning air pressure switch transmitter, current overload to the blower motor and high ambient air temperature exacerbating the ' foregoing New components were installed and a current overload situation apparently rectified raeklgmr13gm1998 dao I I I