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