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CATC California Maximum Contaminant Level (MCL) or below the EPA secondary standard (aesthetic) with the exception of total chromium(0.10 milligrams per liter[mg/1]) and hexavalent chromium(0.11 mg/1)in well MW-3. In order to assess the validity of the MW-3 results for chromium species, a verification round of sampling was performed in August of 1991 by WA. Water samples were collected from monitoring well MW-3 and Stocldon City supply wells WP-1 and WP-2. Each sample was analyzed for total and hexavalent chromium by Clayton Environmental Consultants. The results indicated both City wells were below method detection limits for total chromium (0.05 mg/1) and hexavalent chromium (0.01 mg/1). However, total and hexavalent chromium were detected in the water samples collected from MW-3. In October 1993, RESNA Industries Inc. (RESNA) attempted to evaluate the extent of chromium and hexavalent chromium in the soil in the vicinity of former Vault 8. Two soil borings, SB1 and SB2 were advanced north of the former Vault 8. The results indicated that the soil impact was limited to the area occupied by former Vault 8, encompassing approximately 450 square feet. It was concluded that the soil impact extended directly beneath the former vault to the water table. The findings and results were presented in RESNA's report titled Additional Soil Assessment Report and Fourth Quarter Monitoring Report, Underground Storage Vaults, dated January 1994. In November 2001 and January 2002, ATC advanced nine soil borings. Low concentrations of hexavalent chromium were detected in soil and groundwater samples collected during the investigation. The highest concentrations of hexavalent chromium in soil were detected in the samples collected from off-site boring S139. Hexavalent chromium concentrations at this location were well below the concentrations previously detected in soil boring B8-A, which was advanced beneath Vault 8. Hexavalent chromium was detected in groundwater at low concentrations in the samples collected from borings SB 1, SB2, SB3, SB4, SB5, and SB9. Concentrations ranged from 0.5 to 5.1 micrograms per liter (ug/1). The reported concentrations were below the MCL for total chromium of 50 ug/I and well below concentrations detected in monitoring well MW-3, which historically have been the highest detected levels at the site. These findings and results were presented in ATC's report titled Summary Report of Subsurface Investigation, dated February 2002. In September and October 2002, cone penetration testing (CPT) borings CPT1 and CPT2, Geoprobe (direct push) borings CPTla and CPT2a, and groundwater monitoring wells MW-4 through MW-8 were installed at the site. CPTI and CPT2 borings were advanced to 120 and 139 feet bgs, respectively, to provide continuous stratigraphic definition in the borings. Grab groundwater samples were collected from CPTla and CPT2a at depths of 109 and 114 feet bgs, respectively. Monitoring wells MW-4 through MW-8 were completed to a total depth of 65 feet bgs. Hexavalent chromium was not detected in soil samples collected from the boreholes of MW-4 through MW-8. Hexavalent chromium was detected in groundwater samples collected from wells MW-2 through MW-7 in September 2002, with the highest concentration detected at 4.0 mg/1 in the groundwater sample collected from well MW-3. Hexavalent chromium was detected at concentrations of 0.170 and 0.012 mg/I in grab groundwater samples collected from borings CPTla and CPT2a, respectively. These grab samples were collected from the deeper water-bearing zone.Total chromium was detected in the water samples collected from all the wells, including the facility well, with the highest concentration detected at 6.2 mg/l in the groundwater sample collected from MW-3. The grab groundwater samples collected from CPTla and CPT2a were not analyzed for total chromium. These findings and results were WA62583Veaers\Concept Design Outline.doc 2