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characteristics as the median: of concern at the site . . . identifying background location requires knowing which direction is upgradient/upwind/upstream." • This Amendment should provide the plan to achieve DDRW-Tracy-specific background values. Be specific as to the methodology to be used. ti- Page 3-8, Section 3.9.1.3, Second Bullet The statement "there is, as yet, no reason to believe that a significant metals problem exists throughout most of the depot" is misleading. Further, this paragraph appears to erroneously suggest that arsenic background conditions at DDRW-Tracy are similar to conditions at DDRW-Sharp. Whereas EPA believes that the data do not support such an assertion. Further, monitoring well LM3 appears to be intercepting hazardous waste constituents of arsenic (and sometimes chromium and lead) as well as other constituents, probably emanating from either the industrial waste ponds (IWPs, SWMU No. 3) or the industrial waste pipeline (IWPL), referred to as SWMU No. 33 by WCC, or both. Table 2 (attached) shows concentrations of all major waste constituents detected in monitoring well LM3 for September 1991 through February 1992 by WCC and for September 1992 by JMM. Unfiltered (total) arsenic concentrations were 5.8 ug/I at LM3 in JMM's September 1992 quarterly monitoring round, as compared to a (potential background) value of less than 2.3 µg/I. Filtered (dissolved) concentrations of arsenic for well LM3 averaged 7.0 ug/I from August 1991 through September 1992. Furthermore, other downgradient wells -- LM15, LM28, LM49, LM50, LM62, LMI01, LM103, and LM104 -- all had total arsenic concentrations elevated above potential background for that September 1992 sampling round. Even these relatively elevated lavels of arsenic have a maximum reliable concentration, 8.4 µg/l (dissolved) sampled and analyzed by WCC in September 1991, that is much lower than DDRW-Sharpe appears to show as a background concentration. Small groundwater plumes of arsenic (above background) and dieldrin appear to be emanating from the IWPs and/or the IWPL and traveling slowly to the north-northwest and off-site with the groundwater, and that DDRW- Tracy's background concentrations for arsenic are very low. • Please clarify and/or amend these statements regarding background values for arsenic. 7 Page 3-8, Section 3.9.1.3, Third Bullet Background values may be obtained from on-site data with some obvious conditions: • Data obtained from an area known or thought to be contaminated with pesticides cannot be used to determine background for pesticides (i.e., the pesticide burial trenches, the IWPs, the IWPL, the old IWP, burn pits); IISEPA/R9 12 Apr 93 3/19