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Stantec ,�i Results of Additional Assessment kAkLnIJ Former ARCO Service Station No. 443 September 4, 2009 2.1.2 Soil Boring Advancement Between December 1 to December 10, 2008, Stratus observed Woodward advance two on-site soil borings (MW-5 and MW-8). All boreholes were cleared to at least five ft below ground surface (bgs) using an air knife prior to drilling. MW-5 was advanced to an approximate depth of 65 ft. bgs using a B-61 type drill rig equipped with eight-inch hollow stem augers. A pilot hole for MW-8 was advanced to an approximate depth of 95 feet using B-61 type drill rig equipped with 10-inch hollow stem augers. MW-8 was then reamed by an 18-inch auger to a depth of 95 ft and a 10-inch diameter conductor case was installed. During grouting of the conductor case, the auger was not rotating per regulator request. Due to the presence of heaving sands, the drillerwas unable re80 ft of auger, which was abandoned in the bore hole. Boring MW-8 was then capped with—a—woo �lIug. Between February 9 and February 11, 2009, Stratus observed Woodward further advance MW-8 to 119 ft bgs using a BK-81 type drill rig equipped with eight-inch hollow stem augers. On April 30, 2009 Stratus observed Woodward advance one off-site soil boring (MW-7) to an approximate depth of 65 ft bgs using a BK-81 type drill rig equipped with eight-inch hollow stem augers. MW-7 was cleared to 6.5 ft bgs using an air knife prior to drilling. Well Installation. Stratus observed the installation of three groundwater monitoring wells in three soil borings. All groundwater monitoring wells were constructed using two- inch, schedule 40, flush-threaded polyvinyl chloride well casing. In MW-5 and MW-7, 20 foot lengths of 0.020-inch machine-slotted well screens were set at approximate depths of 45 to 65 ft bgs. In MW-8, a ten-foot length of 0.020-inch machine-slotted well screen was set at an approximate depth of between 103 to 113 ft bgs. MW-8 was installed with a ten-inch conductor case from an approximate depth of 95 ft bgs. Sand filter packs were installed in all wells from the bottom of the boring to approximately two ft above the well screen in MW-5 and MW-7, and approximately six feet above the well screen in MW-8. A two ft bentonite annular seal was installed above the sand filter packs MW-5 and MW-7, and five feet ft bentonite annular seal was installed above the sand filter pack in MW-8. Neat cement was used to seal the remaining annular space to the surface in each well. The surfaces were completed with a 12-inch flush-mounted well box. Well construction details are summarized in Table 1. Soil Sample Analysis. Selected samples collected from each boring were submitted by Stratus to Calscience Environmental Laboratories, Inc. of Garden Grove, California for analysis of gasoline range organics (GRO) by Environmental Protection Agency (EPA) method 80156; benzene, toluene, ethylbenzene, and xylenes, methyl tert-butyl ether (MTBE), di-isopropyl ether, ethyl tert-butyl ether, tert-amyl methyl ether, tert-butyl alcohol (TBA), 1,2-dichloroethane (1,2-DCA), 1,2-dibromoethane (EDB), and ethanol by EPA Method 82606; and lead by EPA Method 60158. The certified analytical reports, chain-of- custody documentation, and Stantec Lab Validation Forms for soil sampling are included as Attachment D. 443 Results of Additional Assessment.doc 2 1 1