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'N/Ir. Ohland 3 19 December 1994 4.DETERMINATION OF BACKGROUND FOR FUTURE SOLID WASTE RENIEDIAL WORK AT THE SOUTHW3';ST HIDE SITE: Your closure request letter discusses a method of establishing background concentrations by determining a "Threshold Limit Value" (TLV), based on methodology in the USEPA Technical Enforcement Guidance Document (USEPA, 1986). In deriving the TLV for the Southwest Hide site soils, you considered analytical results spanning a very wide range of TDS concentrations. from under 10 ppm to values in six figures, without consideration for the non-uniform distribution of the results. We believe that this approach is inappropriate for application to the soil remediation problem at Southwest Hide, because the clustered distribution of high concentrations can be clearly related to the known distribution of historical activities and structures having a high potential to act as sources of salt pollutants. This point is illustrated by comparison of Figure 2, a location map of historical site activities, with Figure 4, soil sample locations, and the analytical results summarized in Table 3 (all included with your closure request). Presence of low TDS and chloride values in some samples shows that some areas of the site have not been affected, or have been affected less severely. Plotting the sample results on a histogram demonstrate that the values are clustered, and reference to the maps shows that the clusters are non-randomly distributed on the site. A statistical treatment which attempts to flatten or normalize the extreme values ignores the realities of the site. Background should reflect conditions on portions of the site which are least likely to have been impacted by historical activities, or should be obtained from off-site measurements if no unimpacted areas can be found on-site. Further, since future solid waste remediation will involve, at minimum, removal of"hot spots" from the soils, it is not reasonable to include the worst cases in the background determinations. 5. CLOSURE REQUIREMENTS UNDER CHAPTER 15: Page 6 of your April 6 1994 letter states that "...Southwest Hide has complied with the requirements of Chapter 15, Section 2582.b.I to close" the impoundments. Based on our determination, as discussed in Item 3 above, that contaminated subliner soils remain in the impoundments, we disagree with this statement. Section 2582.b.1 is a clean closure. We cannot consider the closure to be clean if salts remain in the subliner soils at levels exceeding background. The Final Closure Plan dated November 20, 1992, submitted by RESNA on your behalf, proposes that the impoundment area be closed as a landfill under- the requirements of Section 2581, Chapter 15. If you wish to leave wastes in the subliner soils, closure as a landfill is required, and you may construct a compacted foundation layer that contains sc;me waste materials, but the final cap must be constructed of impermeable materials. Waste Discharge Requirements Order No.92-077, Discharge Specifications B.13 and B.14 require that the containment features of the impoundments meet the Chapter 15 standards and performance goals of a Class II landfill in order for the impoundments to be closed as a landfill. We recommend clean closure of the impoundment's. However, if you prefer to close them as a landfill in conformance with Chapter 15 standards,!you may wish to consider using the impoundment excavation as a repository for all ora portion of the worst-contaminated soils on the site, provided these wastes can meet the requirements of Class 11 wastes and the impoundments can meet the requirements of a Class II landfill, as stated above.