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Ms. Margaret Lagorio Page 7 of 8 The socks will be periodically replaced as necessary. One month prior to each quarterly sampling event the socks will be removed in order to allow the collection of samples representative of the formation water. Manufacturer's information on ORC is included in Attachment D. ENCROACHMENT OF OFFSITE PLUME PHS/EHD has indicated that they do not believe there is"conclusive information" as to the source of groundwater contamination in monitoring wells MW3, MW4 and MW14. We believe the information is as conclusive as can be until the Endich Brothers' Top Filling Station installs shallow monitoring well(s) to the east of their site. As described above, the mix of contaminants in Knowles wells MW3, MW4 and MW14 ("Top Filling Plume") is distinct from the Knowles Plume and the ARCO Plume. The configuration of the plume and the time of discovery also indicate a source distinct from either ARCO or Knowles. Recently installed monitoring well MW14 has verified the existence of this plume at the upgradient edge of the 102 Wilson Way property. PHS/EHD apparently agrees in correspondence to others (letter to ARCO, January 24, 2001) that the contamination in these wells does not originate at 102 S. Wilson Way - "Contaminant levels in the water samples from monitoring wells MW3 and MW4 at 102 S. Wilson Way have increased since November 1998. It is suspected that the source of the increase in contaminant levels is from your site, especially since the levels of MTBE in the water samples from these wells are high and the underground tank system was removed from 102 S. Wilson Way in 1988" [sic, tanks actually were removed in 1978]. ARCO's consultant, Delta Environmental, does not believe that the contaminants in their well MW4 (located near Knowles' MW4) originate from their site (Well Installation Report, October 4, 2000). We agree. The source is almost certainly the Top Filling Station at 101 S. Wilson Way. At the Top Filling Station, flooded vapor well VW3B, located on the south side of the building, has contained extremely high concentrations of TPHg, BTEX and MTBE. This well is screened from 19 to 39 feet and intercepts the water table surface. Flooded vapor well VW3A, at the same location, is screened from 43-63 feet and also contains very high concentrations of these hydrocarbons. Monitoring well MW2 is located to the southeast of these contaminated vapor wells (generally downgradient) but this well rarely contains more than a trace of contaminants. This is quite likely a result of the screened interval (60 to 80 feet) the top of which is currently approximately 22 feet below the water table surface. Top Filling's consultants, Advanced GeoEnvironmental acknowledge (Quarterly Report — First Quarter 2001) that this well "...may not yield samples fully representative of shallowest groundwater conditions". Figure 8 is an east-west cross section from the Top Filling Station across Wilson Way to the Knowles site. This figure depicts the lithology in Top Filling wells VW3 and MW2 and in Knowles wells MW3, MW4 and MW14. Top Filling well MW2 is screened beneath a low permeability unit underlying the shallow aquifer zone most affected by contamination. MW2 does not provide a definition of the lateral extent of shallow groundwater contamination related to the Top Filling Station release. Top Filling's consultants collected discrete groundwater samples from boring CPT1A east of the source area. However, groundwater samples were not collected from depths above 75 feet. Although significant groundwater contamination was identified deep roek\closure\workpla \respo e,doc