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M%TC A S S O C I A T E S I N C , samples from off-site boring SB9. Hexavalent chromium concentrations at this location were well below the concentrations previously detected in soil boring B8-A, which was advanced Fbeneath Vault 8. Hexavalent chromium was detected in groundwater at low levels in the samples from borings SBI, 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 ugA 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 CPTI and CPT2, F Geoprobe (direct push) borings CPTla 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. (A brief description of the subsurface geology is included in the next section,) Grab groundwater samples were collected from CPTla and CPT2a at depths of 109 and 114 feet bgs, respectively. F, Wells MW-4 through MW-8 were 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 in the water samples collected from wells MW-2 through MW-7 in September 2002 with the highest concentration detected at 4.0 mg/l in the water sample collected from well MW-3. Hexavalent chromium was detected at 0.170 and 0.012 mg/1 in the 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 water sample collected from well MW-3. The grab groundwater samples collected from CPTla and CPT2a were not analyzed for F4' total chromium. These findings and results were presented in. ATC's report titled Summary Report of Additional Subsurface Investigation, dated November 1, 2002, 2.0 FIELD INVESTIGATION METHODS AND PROCEDURES 2.1 Drilling Activities ii On April 29 and 30, 2003, a geologist from ATC supervised the advancement and construction l of one monitoring well, MW9. A site plan with well location is shown on Figure 2. Mud rotary drilling was conducted by Pitcher Drilling, Inc. a subsidiary of Gregg Drilling (State F;. C57 license no. 717510). Soil boring MW9 was advanced to approximately 113 feet bgs and subsequently completed q Y P as groundwater monitoring well MW9. The .monitoring well was constructed of 2-inch inside diameter schedule 40 PVC well screen and casing. A 0.010-inch well screen slot and #3 sand pack was used. The well screen length extended from approximately 103 to 113 feet bgs to correspond with the deeper water-bearing zone encountered in boring CPTL No glue was used in the assembly of the groundwater monitoring well. A F- cement grout mixture was used to seal the well in accordance with California Well Standards (December 1981). To protect the integrity of the well, a locking, watertight well plug was E wAwordpro%2583Veportsldeep well inst.doc 3