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The Department of Health Services has recently developed extensive technical guidance on cleanup level determination for the prevention of toxicologic impacts on humans and other "biological receptors of concern". The procedures in this guidance, entitled The California Site Miti ation Decision Tree Manual O reflect the legislative mandates that govern the site mitigation activities of DHS and, to a large extent, those of the U.S. EPA. However, the legislative mandate to protect all present and probable future beneficial uses of waters of the State, contained in the Porter-Cologne Water Quality Control Act and policies promulgated thereunder,require the State and Regional Water Boards to approach site mitigation from a somewhat different position. The three main areas of difference between the DHS and Water Board approaches to the cleanup question are summarized in Figure l 14. The State and Regional Boards must prevent even non-toxicologic endpoints of contamination(e g,taste and odor,recreation impairment) and endpoints that do not involve biological receptors (e g.,industrial use) The State and Regional Boards may require cleanup to levels below those that appear to protect all beneficial uses of water in order to account for the incompleteness of our current knowledge of environmental fate processes and the effects of contaminants as well as to address the allocation of water resources among potential users (i.e.if an individual is permitted to contaminate a body of water up to the water quality goal,there exists no room for additional contamination). Finally, the mandate to protect probable future uses of water require the State and Regional Boards to seek cleanup of even on-site waters to levels that will permit their future use. a For the reasons stated above,it is necessary for the State and Regional Boards to have a methodology to determine cleanup levels from a perspective of beneficial use protection. The Designated Level Methodology can fill this need. The threat to water quality posed by constituents of a contaminated soil is similar to the threat posed by constituents of a solid waste in an unlined landfill or waste pile, as seen by comparing Figures 12 and 15. These two situations share the same environmental fate processes that govern constituent attenuation and the same goal of beneficial use protection. By applying analytical procedures relevant to reasonable worst-case conditions at the site to determine the concentrations of constituents that are potentially available for migration to water(see Section 3.1 above) and by deriving site-specific Designated Levels for constituents of contaminated soils, the necessity for cleanup or mitigative measures for water quality protection should be apparent. In many cases,the exceedance of Designated Levels by constituents of contaminated soils does not necessitate soil removal and re-disposal. It does indicate that mitigation measures are necessary to prevent potential water quality impacts. As in waste classification, Designated Levels derived for use as cleanup levels should reflect site and constituent-specific characteristics whenever possible. The water quality goals used in the assessment must be applicable to the present and probable future beneficial uses of the water resource being protected or cleaned-up (see Section 3.2 above). To reduce the level of uncertainty,extractable constituent concentrations from the soils should be compared with Soluble Designated Levels whenever practicable. The selection of extractant(citrate buffer or deionized water) should reflect the potential for acidic conditions at the site. Finally, care should be taken to account for the combined effects of Page 46 Designated Level Methodology