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Pilot Study Workplan November 7, 2014 Project#54.62583.0001 Page 2 propylene glycol. The water sample collected from MW-3, adjacent to Vault 8, was analyzed for total soluble chromium, sodium and calcium, hexavalent chromium, and chlorides. All constituents were detected, but were below the California Maximum Contaminant Level (MCL) or below the EPA secondary standard (aesthetic) established at that time with the exception of total chromium and hexavalent chromium in monitoring well MW- 3, which had measured concentrations of 0.10 and 0.11 milligrams per liter (mg/L), respectively. Hexavalent chromium did not have an established MCL or Action Level at that time. To validate the January 1991 chromium results in MW-3, Weiss collected a round of verification groundwater samples in August 1991. Water samples were collected from monitoring well MW-3 and from two of four municipal water supply wells located at the comers of the facility (WP-1 and WP-2). All three samples were analyzed for total and hexavalent chromium, and the results indicated both municipal wells were below method detection limits for total chromium and hexavalent chromium. However, total and hexavalent chromium were detected in the water samples collected from well MW-3, both at a concentration of 0.7 mg/L. In October 1993, RESNA Industries Inc. (RESNA) was retained to evaluate the extent of total 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 soil impacts were limited to the area occupied by former Vault 8, encompassing an area of approximately 450 square feet. It was concluded that soil impacts extended directly beneath the former vault to the water table (RESNA, 1994). In November 2001 and January 2002, ATC Associates Inc. (now Cardno ATC) advanced nine soil borings across the site identified as S131 through S139. Low levels 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 from off-site boring SB9 located on the north property boundary of the facility on East Worth Street. Hexavalent chromium was detected in groundwater at relatively low levels in the samples from borings SB1, SB2, SB3, SB4, SBS, and SB9 at concentrations ranging from 0.0005 to 0.0051 mg/L. The reported concentrations were below the California Primary MCL for total chromium of 0.05 mg/L and well below concentrations previously detected in monitoring well MW-3, which historically have been the highest detected levels at the site (ATC, 2002a). In September and October 2002, cone penetration testing (CPT) borings CPT1 and CPT2, Geoprobe (direct push) borings CPT1a and CPT2a, and groundwater monitoring wells MW-4 through MW-8 were installed at the site. CPT1 and CPT2 borings were advanced to 120 and 139 feet bgs, respectively, to provide continuous stratigraphic definition in the borings. Groundwater samples were collected from CPT1a and CPT2a at depths of 109 and 114 feet bgs, respectively. Wells MW-4 through MW-8 were each completed to a total depth of 65 feet bgs. Hexavalent chromium was not detected in the soil samples collected during the drilling of wells MW-4 through MW-8. Hexavalent chromium was detected at 0.170 and 0.012 mg/L in the groundwater samples collected from borings CPT1 a and CPT2a, respectively. These 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 water sample collected from well MW-3. The groundwater samples collected from CPT1a and CPT2a were not analyzed for total chromium (ATC, 2002b). On April 29 and 30, 2003, ATC supervised the advancement and construction of deep monitoring well MW-9. The soil boring for MW-9 was advanced to approximately 113 feet bgs, and the well was completed with a screen interval from approximately 103 to 113 feet bgs in order to intercept the deeper water-bearing zone encountered in boring CPT1. Hexavalent chromium was detected in groundwater sample collected from well MW9 at 0.0049 mg/L (ATC, 2003). In May 2003, ATC conducted an additional subsurface investigation at the site that included producing a video log of the facility well (FW) located inside the facility building. The well was located within a concrete below-