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1-1 r � n mate environmental attenuation factors for specific waste constituents and site conditions. Computer models which are applicable to the waste constituents of concern and to the site's hydrogeologic conditions may also be used. If a waste discharger is unable or unwilling to ex- pend the resources necessary to develop detailed and specific attenuation factors for the site of waste dis- charge, the Central Valley Regional Water Board has provided guidelines for the selection of generic attenu- ation factors based on the depth to ground water"and the clay content of unsaturated zone soils. Designated Levels for Liquid Wastes stituents in the same manner as for liquid waste con- stituents, as shown in Figure 7. In this example, the Proposition 65 regulatory level for arsenic (0.005 mg/1) has been chosen as the water quality goal to protect ground water at this site for domestic supply, and the environmental attenuation factor has been estimated to be equal to "n". The Designated Level for arsenic in leachate at this site would be equal to (0.005 x n) milli- grams arsenic per liter of leachate. The goal in calculating Designated Levels for a solid waste is to determine concentrations of soluble constituents in the waste above which leachate may carry them to ground or surface waters in amounts that could cause water quality goals to be exceeded. There - Once the water quality goal is selected and an envi- fore, the next step in the methodology is to convert the ronmental attenuation factor is estimated, their values Designated Level for leachate into one which may be are multiplied together to obtain a Designated Level applicable to the specific liquid waste constituent and site of proposed waste discharge. If the concentration of a constituent in the liquid waste exceeds this level, the waste is classified as a "designated waste" and Class II containment is required if the waste is to be discharged to land for treatment, storage, or disposal at this site. Wastes having concentrations below the Des- ignated Level are assumed not to pose a significant water quality threat at the site and may be discharged to a waste management unit with less than Class II con- tainment. Due to the constant hydraulic head and mass load- ing of waste constituents presented by long-term im- pounding of liquid wastes at a site, most wastes which have a quality poorer than that of underlying ground water will eventually degrade water quality. This oc- curs as attenuative mechanisms in the vadose zone become saturated. For this reason, such wastes are most often classified as "designated waste" and are required to be discharged to Class II impoudments. Soluble Designated Levels for Solid Wastes As moisture from within a waste or infiltrating rainfall percolates toward the base of a landfill, soluble waste constituents are accumulated and leachate is formed. Constituents in leachate at the base of a land- fill pose a similar water quality threat to constituents in an impounded liquid waste. The processes of environ- mental fate which act to attenuate constituent concen- trations are the same in either case. Therefore, Designated Levels may be calculated for leachate con - Page 6 applied to leachable concentrations of constituents in a solid waste prior to disposal. Rationale presented by DTSC in the 1984 Statement of Reasons for the Hazardous Waste Identification Regulations indicates that the con- centrations of constituents in leachate could either be numerically higher or lower than the soluble constitu- ent concentrations in the solid waste prior to leaching. In the calculation of Designated Levels, these concen- trations are assumed to be numerically equal, as in DTSC's Statement of Reasons. Therefore, the Soluble Designated Level for a constituent in a solid waste is numerically the same as the Designated Level for the same constituent in leachate which forms at the base of the landfill—the water quality goal times the environ- mental attenuation factor. In the example of Figure 7, the Soluble Designated Level for arsenic in the solid waste is (0.005 x n) milligrams of soluble arsenic per jkilogram of waste. Soluble concentrations of constituents in solid wastes are determined by performing the Waste Ex- traction Test (WET) from Title 22 of CCR, Division 4.5, Chapter 11, Appendix II, or a variation of this test. [The Toxicity Characteristic Leaching Procedure (TCLP) from Title 22 of CCR, Division 4.5, Chapter 18, Appendix I, is used for volatile waste constituents.] The WET involves a ten -fold dilution of solid waste into an extract solution, agitation for 48 hours, followed by filtration and analysis of the liquid phase. Results are expressed in milligrams of extractable constituent per liter of extract solution. Therefore, the WET extract is expected to be ten -times more dilute than actual landfill leachate. The Soluble Designated Level for a Summary of The Designated Level Methodology