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. S ECOR - • _. Work Plan for Monitoring Well Installation July 31, 2007 Former ARCO Service Station No. 4932 Stockton, California Page 5 Wells DMW-2, DMW-3, and EW-2 are located off-site north across E. Harding Way from the site. In August 2002, SECOR submitted a Problem Assessment Report (PAR), which evaluated various remedial alternatives for the site. SECOR recommended the installation of a GWE and SVE system. The recommendation included the installation of five additional GWE (EW-3 through EW-7) wells and one SVE well. SJCEHD approved the remedial recommendations in their letter dated November 7, 2002. Off-site extraction wells EW-2, EW-3, EWA EW-5, and EW-7 were pumped on a weekly basis, and the water was treated through the on-site GWE system, until the installation of the off-site ozone sparge system was completed during the second quarter 2005. The GWE portion of the DPE system is comprised of submersible pumps installed in wells EW-1 and EW-6 and "slurp" tubes installed in wells MW-4 and EW-7, which are connected to an air stripper and three 2,000 Ib-granular activated carbon vessels. Extracted groundwater is treated prior to discharge to a City of Stockton sanitary sewer line. From March 7 to 10, 2005, SECOR supervised the drilling of six soil borings and installation of six ozone sparging wells using a limited access drilling rig equipped with 8-inch-diameter hollow-stem augers. Wells S-1 through S-6 were drilled to a depth of approximately 40 feet bgs to accommodate ozone sparge wells. Soil samples were collected in the bottom five feet of each boring during well installation activities for logging purposes only. Ozone sparge wells OS- 1 through OS-6 were constructed of/< inch diameter schedule 80 PVC blank casing and a 3- foot long sparging micropore screen. The sparge tips were installed at an approximate depth of 40 feet bgs. A sand filter pack was placed within the annulus of the well from the bottom of the boring to approximately two to four feet above the top of the well screen. The annulus of the well was sealed with two feet of bentonite on top of the sand, and portland cement/bentonite slurry to the surface. An 8-inch-diameter, traffic-rated, watertight well box was installed. Remediation system construction began on May 2, 2005 and was completed on May 19, 2005. Ozone system installation included the placement of injection tubing (approximately '/2-inch diameter), doubly-contained in PVC tubing, connecting each ozone sparge well to a six-point ozone sparge panel located near the west end of the Taco Bell building. The panel was plumbed to a six-point injection within the ozone sparging unit, allowing rotation of the injection lines between the six ozone sparge points. The ozone injection system consists of Calcon model HiPro500 that produces 0.5 pounds per day of ozone. The ozone unit is equipped with a compressor rated to deliver four standard cubic feet per minute (scf/m) at 40 pounds per square inch (psi). On September 2005, the remediation system was shutdown due to electrical damage. The remediation system remained shutdown between September 2005 and June 2006. On August 2006, the remediation system was restarted and a source test was performed for the SJCEHD. The remediation system has been operational since August 2006. GEOLOGY AND HYDROGEOLOGY Lithology beneath the site generally consists of fine-grained silts and clay from ground surface to approximately 20 feet bgs, and from 30 to approximately 60 feet bgs. Silty sands and sands are generally present at the intervals of 20 to 30 feet bgs, and 60 to 85 feet bgs. Depth-to-groundwater has varied from approximately 25.83 feet bgs in MW-3 (5/2001) to 36.86 feet bgs in EW-7 (1/2004) since the installation of monitoring wells began in 2000. Depth-to- groundwater encountered during drilling has been approximately 30 feet bgs indicating that the 4932 WP for MW Installation 2007.doc SECOR International Incorporated