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1993, the parties to the FFA have agreed to clarify the objectives of the original work plan in this Amendment. Therefore: • Please include a new section in the Work Plan Addendum which restates the sampling objectives of the various phases. In particular, with regard to Phase I sampling, insert text which states "It is not the intent of Phase I sampling to 'deselect'potential source areas from further investigation. Instead, conclusions to cease further investigation, out a source by source basis, will be made via consensus of the parties to the Federal Facility Agreement (FFA)." Sections 3.1.2.3 and 3.1.2.4 of the Comprehensive Work Plan assert an intent to apply statistical tests to all data used as Phase I data.1 5. Background concentrations for arsenic at DDRW-Sharpe (10 to 15 miles from DDRW-Tracy) are discussed in these amendments as if these same background concentrations for arsenic were appropriate for DDRW-Tracy. However, no rationale is presented to support this conclusion. Neither the U.S. Geological Survcy article (Dubrovsky et al., 1991), nor the 1991-1992 well monitoring data (filtered or dissolved arsenic) from both Woodward-Clyde Consultants (WCC) and James M. Montgomery (JMM) at DDRW-Tracy (see the attached Table 1) suggest that there exists high background concentrations for arsenic. (JMM's non-turbid total metals data from September 1992 gives total arsenic concentrations only slightly higher than corresponding dissolved concentrations of arsenic.) Risk Assessment Guidance for Stuperfund (Part A; RAGS, 1989) states in part in Section 4.4.2 on page No. 4-8 that "background samples are collected at or near the hazardous waste site in areas not influenced by site contamination . . . the locations of background samples must be areas that could not have received contamination from the site, but that do have the same basic 1 If DDRW-tract' were to desire to eliminate sites from further consideration, EPA would likely require statistical tests to assure adequate sampling. These statistical tests arc designed to determine whether or not further sampling is required and if additional samples are required per site. Under one hypothetical scenario (developed by WCC), no further samples would be required at some particular sites; the reason for this is that the statistical tests (after Phase I sampling) would indicate that enough samples were already taken, and that no appreciable risks exist at those sites to human health and/or the environment. This is a possibility at some sites. At most sites, however, it is expected that more samples will be required to state with statistical confidence that there exists or does not exist, risk. For locations where only one boring/monitoring well are to be analyzed in Phase I (SWMU Nos. 20, 21, 23, 24, and 27) data may be insufficient to determine "no risk." For three other sites (SWMU Nos. 6, 14, 15) in which the number has been reduced from 3 to 2 samples; it is uncertain how the statistical test will be successfully applied. Other sites which possibly may have insufficient samples to allow "deselection" are SWMU Nos. 3 (IWPs), 33 (IWPL), and burn pit SWMUs Nos. 7, 8, and 16. These latter SWMUs are heterogenous and known to contain, or expected to contain, wide ranges of hazardous waste constituents. USEPA/R9 12 Apr 93 2/19