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i CAMBRIA April 17, 2000 PHSIEHD Letter Response Plume Definition PHSIEHD states in Paragraph 1 of their letter that "the plume is undefined" at this site As previously discussed in our August 26, 1999 work plan the plume is fairly well defined, "The extent of hydrocarbons in groundwater is defined by wells MW-7, MW-13, MW-14, MW-16 and MW-17 " Wells MW-6 and MW-8 also help delineate lateral the extent of the plume All seven of these wells have consistently shown low to non- detectable levels of hydrocarbons, which shows plume definition west, southwest, south, southeast, and east of the site As we stated in our work plan "The primary area where the distribution of hydrocarbons in groundwater has not been completed is in the direction of well MW-15 The hydrocarbon plume appears to be stable and, if the hydrocarbon concentration decreases are due to natural attenuation, the plume is shrinking " If the hydrocarbon plume is stable or shrinking, then additional delineation of the hydrocarbon is not warranted Quarterly chemical analytical data from wells at this site show declining concentrations which suggest the plume is shrinking We agree that the vertical extent of hydrocarbons and gasoline oxygenates in groundwater has not been delineated at this site This is why our original work plan proposed a deep soil boring be drilled in the primary source area to log the soil and to collect soil and groundwater samples for chemical analyses We typically propose one soil boring of this type located in the source area to determine if gasoline constituents have migrated vertically beneath the source area This addresses one of two general scenarios for vertical migration of contaminants The other general vertical migration scenario is where the plume migrates vertically while it migrates horizontally downgradient (a plunging plume) A soil boring drilled in the source area may not be able to detect a plunging plume A second deep soil boring should be drilled downgradient of the source area to determine if the plume is migrating vertically We propose such a boring for this site later in this document if chemical analytical data from these borings indicate vertical migration in groundwater has not occurred, then investigation efforts can be focussed on the shallow groundwater plume If data show vertical migration in groundwater has occurred then it may become necessary to install deeper groundwater monitoring wells to monitor and delineate the extent of the deeper plume Work Plan Directive PHS/EHD's April 17, 2000 letter states in Paragraph 2 "On May 26, 1999 PHS/EHD issued a written directive including the following site-specific requirements " This statement is not consistent with the May 26, 1999 letter The May 26, 1999 PHSIEHD letter states in Paragraph 2 "The responsible party is now directed to submit a workplan to PHS/EHD considering the following comments " Cambria/Equiva complied with 1399 2