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Subsistence Warehouse Soils -2- 12 March 1993 DDRW Tracy and surface waters. The State regulations on waste discharges to land (Chapter 15) , however, do not contain guidance on how to interpret the first part of the "designated waste" definition. The purpose of the RWQCB staff report, The Designated Level Methodology, is to provide an interpretation of this definition. A complete copy of "California's Water Quality Standards and Their Applicability to Waste Management and Site Cleanup" which contains the staff report The Designated Level Methodology, is included as an attachment. Two methods were used by the RWQCB to evaluate the leaching potential of the Subsistence Warehouse stockpiled soil . The first method presented is based on The Designated Level Methodology. The second method presented is based on the RWQCB staff draft report, The Water Quality Site Assessment Method. A copy of this report is included as an attachment. The units presented in The Designated Level Methodology and the DI-WET analytical results are expressed in milligrams per liter (mg/1 ) or parts per million (ppm) . The units presented in The Water Quality Site Assessment Method are expressed in micrograms per liter or parts per billion (ppb) . For consistency, units for the two methods presented in this memorandum will be expressed in parts per billion. The two methods and their results are presented in the following paragraphs: METHOD ONE: THE DESIGNATED LEVEL METHODOLOGY Using The Designated Level Methodology, Soluble Designated Levels (SDLs) were calculated for arsenic and lead. In this case, the Proposition 65 regulatory level for arsenic (5 ppb) , and the Primary Maximum Contaminant Level (MCL) for lead (15 ppb) , were chosen as the water quality goals to protect groundwater. The Proposition 65 regulatory level for arsenic was selected as a compromise between the MCL of 50 ppb and the one-in-million incremental cancer risk estimate of .3 ppb. In The Designated Level Methodology, the smallest degree of attenuation that would be expected to occur for the particular constituent at the specific site of discharge is approximated by an "environmental attenuation factor". The greater the amount of attenuation that is expected to occur; the larger the attenuation factor. In this case, the environmental attenuation factor for arsenic was assumed to be equal to "10". This attenuation factor was chosen because arsenic tends to be more mobile than metals like lead, copper and zinc. In addition, the depth to groundwater is shallow, at approximately 15-feet below ground surface. The environmental attenuation factor for lead was assumed to be equal to "100" . This attenuation factor was chosen because soils strongly buffer metals like lead, copper and zinc. However, the attenuation factor is not expected to be higher due to the shallow groundwater at this site. The SOL for a constituent of a solid waste, expressed in ppb of DI-WET extract, is equal to the water quality goal times the environmental attenuation factor divided by the ten-fold dilution of the test. Therefore, the SOL for arsenic is equal to (5 x 10 / 10) = 5 ppb of DI-WET extract. The SOL for lead is equal to (15 x 100 / 10) = 150 ppb of DI-WET extract. When the DI-WET analytical results for the 17 samples from the stockpiled soil are compared with the SOL for arsenic of 5 ppb, all of the arsenic DI-WET sample results are above the SOL for arsenic. The arsenic DI-WET concentrations are in a narrow range from non-detect (<50 ppb) to 79 ppb. Based on The Designated Level