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• Background data from Phase I of this RI and from the ongoing groundwater <br /> monitoring program at DDRW-Tracy will determine what are background <br /> concentrations at this site. Please clarify these issues in the text. <br /> 10. Page 3-12, Section 3.9.3.7 <br /> It is recommended that the statistical test which will be utilized to distinguish <br /> between background and aquifer horizons be more fully discussed. For example, if <br /> the testing will be based on null hypothesis testing, how many samples will be used <br /> in order for the tests to be statistically valid? This section should be expanded to <br /> more completely explain the tests that will be conducted and the data which will be <br /> used in making the tests. <br /> Ll. Figure 3.9-1 <br /> For completeness, the map should include the locations of SWMU No. 13 and No. 64 <br /> (UST No. 8 believed to have contained waste oils near Building 201.) <br /> • Please include these locations on the update. <br /> BASELINE RISK ASSESSMENT <br /> 12. This Comprehensive RI/FS Amendment to the Baseline Risk Assessment Work Plan <br /> does not provide adequate information for developing the site-wide baseline risk <br /> assessment (BRA). The present version of the document evaluates analytical methods <br /> and detection limits to ensure adequacy of the RI data for the BRA, but does not <br /> provide information on the methodology that will be used in the BRA. For example, <br /> a conceptual site model that will'serve as the iterative foundation for the growth <br /> and development of the final BRA has not been clearly and precisely presented. <br /> Without this conceptual site model attempts to discuss exposure pathways and <br /> estimation of chemical intakes are not meaningful. Also, the ecological assessment <br /> methodology, specifically how ecological exposure pathways will be evaluated, is <br /> lacking from this document. <br /> It is important to the RI/FS process that all BRA methodology be reviewed prior to <br /> the submittal of the RI report. The intent of requesting early explanation of BRA <br /> methodology is to facilitate the BRA process, encourage discussion, and clarify the <br /> reasoning affecting the BRA and ultimately the risk management. This approach <br /> will help to preclude delays in EPA's acceptance of the report and avoid the <br /> inefficient use of time and money by DDRW-Tracy. This Work Plan should provide: <br /> • environmental/ecological exposure pathways, <br /> • a conceptual site model, and <br /> • a BRA methodilogy. <br /> USEPA/R9 12 Apr 93 5/19 <br />