SITE INFORMATION AND CORRESPONDENCE

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Subsistence Warehouse Soils -4- 12 March 1993 DDRW Tracy dissolved arsenic and lead analyses of 5 ppb, therefore, the detection limit of 5 ppb was selected as the site's background concentration in groundwater. ► The lowest arsenic and lead values in the DI-WET analytical results for the 17 samples from the stockpiled soils were assumed as background soil extract concentrations. Therefore, 50 ppb for arsenic and 55 ppb for lead were assumed as background soil extract concentrations. The Water Quality Site Assessment Method was used to evaluate the highest, lowest, average, and median DI-WET concentrations of arsenic and lead from the 17 stockpiled soil samples. Based on the assumptions presented above and as shown in the attached table, the attenuation required to protect background concentration levels and beneficial uses of groundwater from arsenic contamination ranges from 100 to 158. An attenuation factor of at least 100 would be required to protect groundwater quality, because of the relatively high mobility of arsenic and the shallow depth to groundwater at this site. Therefore, in this case, the arsenic concentration levels in the stockpiled soil pose a threat to groundwater quality. As shown in the attached table, the attenuation required to protect background concentration levels of groundwater from lead contamination ranges from 98 to 7299, and for beneficial uses the attenuation required ranges from 37 to 2720. The average DI-WET lead concentration from the 17 soil samples requires an attenuation of 855 to protect background level concentrations, and an attenuation of 318 to protect beneficial uses. An average attenuation factor of at least 318 would be required to protect groundwater quality, because of the shallow depth to groundwater at this site. The attenuation required to protect groundwater from the highest lead DI-WET concentrations detected in the 17 soil samples is significant. Therefore, in this case, the lead concentration levels in the stockpiled soil pose a serious threat to groundwater quality. CONCLUSIONS In the 26 January 1993 meeting, Mark Phillibur cited references that he felt demonstrated that arsenic concentrations found in the stockpiled soil were representative of background concentrations found locally and regionally. The references cited were: Telic Engineering Corporation. Project Summary Report for the Subsistence Warehouse Tracy Defence Depot. October 26, 1991. , and; Environmental Assistance and Information, Science Division. Arsenic Contamination Study for Sharpe Army Depot. April 1990. This position, based on a cursory review of the data seems reasonable. However, it is not a substitute for a detailed evaluation of background concentrations for all constituents of concern that will be required for the comprehensive site-wide remedial investigation. Regardless of the arsenic evaluation, it was agreed that the lead concentrations in the stockpiled soil pose a threat to groundwater. In addition, seven of the soil samples are "designated waste" based on The Designated Level Methodology for lead. The distribution of the seven "designated waste" sample locations indicates that lead contamination is extensive in the soil pile area, making separation of the lead contaminated soil impractical . Therefore, as a one-time removal action, DDRW, Tracy decided that the stockpiled soil should be removed and disposed of in a Class II landfill . This remedy satisfies State requirements in Chapter 15. The basis for this removal action is not intended to establish site-wide cleanup levels for existing Solid Waste Management Units (SWMUs) . DDRW, Tracy will be required to