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fol PHS/EHD •) October 3, 2002 Page 3 of 5 Attachment B • SJC/EHD expressed concerns regarding the lack of explanation for calculations of contaminant mass remaining in soil and groundwater at the site Response: Site sketches showing the estimated extent of contamination, which included the �3 "zones" used in the calculating residual soil contamination were inadvertently omitted from the closure request package These sketches, along with the contaminant mass calculations sheets, are included in Attachment C The average concentrations for each zone were calculated using standard convention of averaging all values within the contours, including the contour values themselves For zones that have no specific values, the average value within the zone was taken to be 1/2 the upper contour value Any non-detect results used in the calculations were entered as the detection limit This methodology is considered a conservative approach • SJC/EHD expressed confusion regarding the orientation of the cross section and which wells were"projected" onto the line Response: The cross section is pretty straightforward, drawn to illustrate the relatively uniform soil conditions beneath the site, and including all soil borings and monitoring wells The line of section was oriented from the north end of the monitoring well field (MW2) to the south end of the monitoring well field (MW4) All other wells/bonngs in between were projected onto the line The mayor technical points to be taken from the cross section are 1) shallow site stratigraphy is relatively uniform, with sand comprising the upper 12 to 13 feet of the soil horizon and generally fine-grained material comprising the interval between 12/13 feet to at Sri least 26 feet, and 2) no soil contamination was detected within the fine-grained material A C copy of the cross section, as well as a site plan showing the line of section, are included in r' Attachment D • SJC/EHD states A well receptor survey completed for this site lists six wells as being located within a 2000 foot radius of the site, but a map accompanying the survey shows many more residences located within this area The water sources)for these residences, as well as for the subject site, were not identified Response: The well receptor survey prepared by Ground Zero, which was included in the Workplan for Groundwater Monitoring Well Installation dated AugusR t 11, 2000—included a table and figure with information on specific wells for which Tata was available from the California Department of Water Resources (DWR) and/or PHS/EHD The report specifically states The figure also shows the approximate locations of additional residences, which would presumably have associated domestic wells, located within 2,000 feet of the site based upon a field reconnaissance conducted by Ground Zero personnel No public or private agency provides water utility services to residences or businesses in the vicinity of the site Therefore, the sole source of drinking water is either extracted from on-site wells or bottled water delivered to the site Even so, the nearest potential water supply well downgradient (west or northwest) G 1Data1GROLINDZE\TuffBoy\Corresp10902gpsrevd doe