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15. Retrieval of three groundwater samples from Well No. 1 is <br /> proposed, with depths corresponding to the mid-points of the <br /> "first three sections of screen. " Since the contaminants of <br /> concern are TCE and PCE, it would seem logical to sample the <br /> bottom of the water column in the well. <br /> • Explain in the text the rationale for not sampling the <br /> lower perforated section. <br /> 16. Because the constituents of concern in this study (TCE and <br /> PCE) are heavier-than-water and commonly known as "sinkers, " <br /> it would seem imperative to sample the bottom screen and to <br /> sample the first three screens within the lower portions of <br /> the screens in both Wells No. 1 and 2 , rather than the mid- <br /> points, as proposed. Unless there is a packer or some <br /> device between the well screens, and considering the primary <br /> contaminants are sinkers, there is really no need to sample <br /> the upper screened intervals at all, since all the water in <br /> the wellbore is mixed anyway. <br /> • DLA should address these concerns in the text of the <br /> work plan. <br /> 17 . For Wells No. 4 and 7, sample collection points are proposed <br /> for relatively high in the water column. Due to the <br /> heavier-than-water character of the constituents of concern, <br /> sample collections points in the lower portion of the wells <br /> would develop data more representative of the groundwater <br /> quality. <br /> • DLA needs to explain the rationale for sampling high in <br /> the water column or revise the approach to address <br /> EPA's concern. <br /> 18 . Well No. 7 is located at the extreme southern part of the <br /> base and is unlikely to be impacted by the TCE and PCE <br /> contamination originating at the DDRW-Tracy site. Because <br /> of its location, this well could provide data on possible <br /> contaminant migration onto DDRW-Tracy. <br /> • EPA recommends that several samples at various depths, <br /> should be obtained in order to effectively screen the <br /> groundwater for a variety of contaminants and due to <br /> the lack of information about screened intervals for <br /> this well. <br /> Page 3-4 , Section 3 . 3 . 3 <br /> 19. It is stated that no sampling is proposed for LM-60 because <br /> the well "has been sampled previously and is known to have <br /> contained detectable PCE and TCE. " Yet on page 4-6 in <br /> Section 4 . 3 . 1. 10, DLA states that the decision not to <br /> 6 of 12 <br />