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Stantec Site History and Previous Investigations Chevron and ARCO Commingled Plume No. CP0034 October 26, 2011 zone or depth intervals showing distinct pore pressure. Three to four depth-discrete groundwater samples per boring were submitted for analysis. Groundwater samples were generally collected at depths of ten to 12 feet (ft) below ground surface (bgs), 24 to 25 ft bgs, 35 to 40 ft bgs, and 68 to 78 ft bgs. GRO was detected in two of the 17 groundwater samples at concentrations of 73 micrograms per liter (µg/L) (CPT-5 at 37 ft bgs) and 130 gg/L (CPT-3 at 40 ft bgs). Methyl tert-butyl ether (MTBE) was detected in six of the 17 groundwater samples at concentrations ranging between 0.59 gg/L (CPT-4 at 10 ft bgs) and 2.6 gg/L (CPT-5 at 37 ft bgs). The site was demolished and five USTs were removed in June 2005. Two new USTs (30,000 and 15,000 gallons), dispenser islands, and a station building were constructed in June 2005. In preparation for site demolition, seven on-site groundwater monitoring wells (MW-1 through MW-5, MW-7, and MW-9) and ten SVE/AS wells (VW-1, GM-1 through GM-8, and GM-9) were destroyed in May 2005. Replacement monitoring wells were installed in February 2006 following completion of the station rebuild, as proposed in the SECOR International Incorporated (SECOR) Work Plan for Well Destruction and UST Removal, the Work Plan for Soil and Groundwater Delineation, and Addendum to Work Plan for Well Destruction and UST Removal (SECOR, 2005). On June 11, 2007, SECOR field staff observed WDC Exploration and Wells of Zamora, California (WDC) drill and install MW-10 to approximately 20 ft bgs using a CME-75 drill rig equipped with an 8.25-inch hollow-stem auger. MW-10 was constructed with two-inch diameter schedule 40 polyvinyl chloride casing extending from ground surface to approximately five ft bgs and two-inch diameter schedule 40 polyvinyl chloride casing with 0.020-inch slotted-screen extending from approximately five to 20 ft bgs. Petroleum hydrocarbons were not detected in any soil samples collected from MW-10. During the summer of 2011, the municipal well on the northeast corner of Ben Holt and Plymouth Road was decommissioned. Remedial Performance Summary An AS system was activated in April 1997 and an SVE system was activated in September 1997. The AS system consisted of a 1.5-horsepower (hp) rotary vane compressor, alternately injecting air into nine wells (GM-1 through GM-9). The SVE system was comprised of ten SVE wells (VW-1 and GM-1 through GM-9), a five-hp positive-displacement blower, and 100-cubic ft per minute (cfm) catalytic oxidizer. The SVE system was deactivated due to high groundwater levels on February 1998. Upon shutdown of the SVE system, the system extracted approximately 70 pounds (Ibs) of GRO from the soil and groundwater beneath the site. The SVE and AS wells were destroyed and the remediation equipment was removed from the site in May 2005. ' ARCO 2133 Facility Four USTs, associated product lines, and dispensersare present at the site. A release from a pipe leak was discovered in 1984. release of approximately 2,000 gallons of gasoline was reported in May 1989, and an unknown quantity of fuel was released at the northern dispenser island in 1995. ,pAq L5 S/261 clj 1 V (q3-"�N-) 94275 Site History and Previous Investigations.doc 2