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i t i s . REMEDIAL SYSTEM ENHANCEMENT PLAN for Former Tosco(Unocal)Service Station No 6981 4707 Pacific Avenue Stockton,California Report No 140124 10 INTRODUCTION At the request of Tosco Marketing Company (Tosco), Gettler-Ryan Inc (GR) has prepared this Remedial System Enhancement Plan (RSEP)for the subject site This RSEP was prepared in response to a San Joaquin County Environmental Health Division(SJCEHD)letter dated June 18,2001 This document outlines various potential remedial enhancements for mitigating petroleum hydrocarbons in groundwater, and recommends a safe and cost effective ozone microsparging option in addition to replacing the existing vapor extraction system(VES)catox unit with an activated carbon abatement Targeted petroleum hydrocarbons addressed: by this RSEP include total petroleum hydrocarbons (TPH)as gasoline,benzene, toluene, ethylbenzene, and xylenes(BTEX)and methyl tert butyl ether(MtBE) - SITE DESCRIPTION ~ The subject site(parcel APN 108-160-04),,is a paved lot located within a Target Department Store parking lot at the northwest corner of the intersection of Pacific Avenue and March Lane in Stockton,Califorma The . site was formerly a Gemco Department Store and Service Station dispensing petroleum fuel prior to purchase by Target Operation of a facility dispensing 76 branded products was initiated in 1982 The subject site was most recently a 76 Service Station with two gasoline underground storage tanks(USTs), three fuel dispenser islands, and a station kiosk All aboveground and subsurface facilities related to the service station were removed in April 2000 A total of twelve groundwater monitoring wells, one groundwater recovery well, eight vapor extraction wells,a VES remediation system,and a compound exist at the site The nearest surface water is the Calaveras River,located approximately 2,000 feet southeast of the subject site Topography in the site vicinity is flat-lying, at an elevation of approximately 13 feet above mean sea level (MSL) Former locations of pertinent site features are shown on Figure 2 SITE GEOLOGY The subject site is located in the Central Valley geomorphic province of California,which is a northwestward- trending asymmetrical structural trough that has been filled with 6 to 10 miles of sediment ranging in age from Jurassic to Holocene The topographic surface is nearly flat with a slight westward slope Based on a geologic map prepared by March and Atwater(1975),sediments beneath the site consist of Quaternary alluvial deposits of the Modesto Formation,which were deposited by the Mokelumne River Previous subsurface studies indicate that the site is underlain by alluvium to the maximum explored depth of 86 feet below ground surface(bgs) The alluvium consists primarily of silts and clays with lenses of poorly graded sand and silty sand Historically,depth to groundwater has risen from approximately 51 to 53 feet bgs in 1993 to 27 to 29 feet bgs in March 2001 Groundwater flow has been to the northeast with the hydraulic gradient ranging from 0 002 to 0 013 feet/feet