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• Data obtained from an area known or thought to be contaminated with metals cannot be used to determine background for metals (i.e., the IWPs, the IWPL, the old IWP, the burn pits, possibly underground storage tanks); • Data obtained from an area known or thought to be contaminated with acids, which are likely to mobilize metal constituents, should not be used to determine background for metals (i.e., sumps in the IWPL, the old and present IWPs); likewise, an area contaminated with chlorinated solvents which could mobilize metals might not constitute a suitable background location for metals (i.e., the IWPs, the IWPL, and various other locations); 8. Page 3-9, Sections 3.9.2.1 and 3.9.2.2 The criteria discussed above (Section 3.9.1.3) should be considered when determining background locations for metals and for pesticides. (Background values for VOCs are, presumably, zero concentrations.) 9. Pages 3-9 and 3-10, Section 3.9.2.3 The text states "Much of the existing groundwater data for DDRB'-Tracy is not considered representative of dissolved metals and pesticides concentrations because of sample turbidity." However, EPA does not understand the cause/effect relationship which is asserted in this sentence. • Please explain in the text of the document how sample turbidity would impact the representativeness of dissolved metals and pesticides or delete this sentence. Further, it is unclear if the intent of this paragraph is to invalidate all WCC's groundwater data taken from monitoring wells LM] through LM43 and analyzed for filtered (dissolved) metals as a part of the well monitoring program at DDRW-Tracy. • Please explain in the text whether DDRW-Tracy intends to invalidate some portion of the existing database. [Note that total metals data from more than one quarterly sampling round may be necessary in order to develop the baseline risk assessment (BRA).] Note also that the range of organic carbon partitioning coefficient (Koc) varies greatly depending upon the chemical structure of the pesticide. Jury and Ghodrati (1) demonstrate experimental Koc values ranging from 20 ml/g (or m$/kg) for 2,4- dichlorophenoxyacetic acid (2,4-D) to 240,000 ms/kg for DDT; the Koc value for dieldrin falls between these extremes at 12,000 ms/kg. DDRW-Tracy should treat each of these pesticides in the RI/FS according to its relative mobility. ' Referring to the last sentence on page 3-10, the quarterly groundwater monitoring by j both WCC and JMM, do not suggest the presence of any pesticides that are ubiquitous throughrut the region of DDRW-Tracy. USEPA/R9 12 Apr 93 4119