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:7 C quirements pursuant to Section 66310 of Title 22 of this code." [In July 1991, §66310 of Title 22 of CCR was repealed and replaced with §66260.210.1 The second part of this definition pertains to those wastes granted a variance by DTSC from Class I dis- posal, as discussed above. The first half of the "desig- nated waste" definition includes non -hazardous wastes which have the potential to impair water quality at the site of discharge. Due to their threat to water quality, "designated wastes" are to be discharged to Class II waste management units which have engineered con- tainment features—liners, leachate collection systems and caps—which act to isolate the wastes from ground and surface waters. The Chapter 15 regulations, how- ever, do not contain guidance on how to interpret the first part of the "designated waste" definition. The purpose of the Central Valley Regional Water Quality Control Board staff report, The Designated Level Method- ology, is to provide an interpretation of this definition. It may not be immediately apparent how a non- hazardous waste could pose a threat to water quality. A simple example will illustrate this point. Figure 4 shows an unlined surface impoundment which con- tains soluble arsenic at a concentration of 4.5 mg/l. The hazardous STLC for arsenic, the level above which a liquid waste becomes hazardous under Title 22 of CCR, is 5 mg/l. Therefore, the waste in this example is not hazardous. The Proposition 65 (Safe Drinking Wa- ter and Toxic Enforcement Act of 1986) no -significant - risk level for arsenic is 0.005 mg/1. If natural geologic materials between the base of the impoundment and the water table are unable to sufficiently filter out or attenuate the arsenic, the Proposition 65 regulatory level will be exceeded, adversely impacting the benefi- cial use of the water for domestic supply. Therefore, this waste at this site would be classified as a "desig- nated waste", and the impoundment would have to be designed to meet Class II containment standards to isolate the waste from ground water. THE DESIGNATED LEVEL METHODOLOGY As shown by the above example, the determination of whether a waste poses a threat to water quality must take into account factors relating to the waste and to the site of proposed discharge. In The Designated Level Methodology, this is accorhphshed by determining "Des- ignated Levels", concentrations of waste constituents Central Valley Regional Water Quality Control Board NON -HAZARDOUS WASTES CAN THREATEN WATER QUALITY DOMESTIC WATER WELL UNLINED SURFACE IMPOUNDMENT t-roposition ss reguiatory ievei,j WA [ t„ FLOW Figure 4 which provide a site-specific indication of the waste's water quality impairment potential. If measured con- centrations of constituents in a waste exceed these Des- ignated Levels, the waste is assumed to pose a water quality threat at the site in question. Because of the site-specific nature of the determination, the same waste may be classified as "designated" in one loca- tion, but not in another location which provides a greater degree of protection for water quality. Designated Levels are calculated by first determin- ing the bodies of water which may be affected by the waste management activity in question and the present and probable future beneficial uses of these waters, as shown in Figure 5. Next, site-specific water quality goals are selected, based on California's water quality standards to protect beneficial uses. Finally, the appli- cable water quality goals are multiplied by factors which account for the magnitude of environmental attenuation expected to occur under reasonable worst- case conditions at the proposed site of discharge. The result is a set of Soluble Designated Levels for waste constituents of concern which are specifically appli- cable to both the waste and the site and which, if not exceeded, should protect the beneficial uses of waters Page 3