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WORKPLAN ADDENDUM ARCO Mini-Mart 4511 Pacific Avenue Stockton, CA 1.0 BACKGROUND Seven soil borings have been drilled since October 1992. Three of these were completed as groundwater monitoring wells, and one was completed as a vadose-zone remediation well The purpose of these borings was to provide data regarding the extent of hydrocarbon y contamination resulting from a broken coupling in a product line near the northern dispenser island The underground storage tanks have not been removed or investigated since they were installed in 1981, and it is unknown whether any unauthorized releases from the UST's have occurred or whether soil beneath the UST's is contaminated Groundwater conditions have been monitored quarterly since the wells were installed, and a vapor extraction pilot test has been conducted on the remediation well The results of these investigations have been reported in GeoAudit's Preliminary Investigation and Evaluation Report (November 1992) and Quarterly Reports (April, June, September, and December 1993) • In a letter to Mr Arthur Wyatt, dated September 16, 1993, Mrs Inane Hinson of PHS/EHD scheduled the removal of the existing single-walled underground storage tanks and replacement with double-walled tanks in February 1994 In accordance with Mrs Mnson's letter, GeoAudit has prepared this work plan to install a remediation system for interim and/or long-term in-situ soil remediation in the tank cavity in the event that soil contamination is discovered during tank removal 2.0 SYSTEM DESCRIPTION The system proposed by GeoAudit is illustrated in Figure 1 Its two main components are 1) a gallery of horizontal PVC pipe and vertical risers within the tank cavity and 2) a maximum of three remedial wells screened within the vadose zone The purpose of the gallery is to allow remediation of the near-surface contamination, and the purpose of the wells is to allow remediation of contamination below a depth of 15 feet The gallery will underlie the new tanks at a depth of approximately 12 feet, and will be encased in crushed rock or pea gravel and overlain by Geo cloth to reduce infiltration of soil particles into the slotted pipe Gallery pipe and risers will be 4-inch-diameter schedule 40 PVC with 020-inch screen The risers will be mamfolded together using PVC pipe in shallow surface trenches, for possible connection to a vapor extraction unit Excavated soil (with or without contamination) will be returned to the excavation, and additional clean fill material will be used to bring the cavity up to surface grade, as described 1n the tank replacement plan to be submitted separately