ARCHIVED REPORTS_XR0012617 - Laserfiche WebLink

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n. 1. PROBLEM kSSI SSNIENT SU11IMARY 1.1 INTRODUCTION As requested by Exxon Company, U.S.A., EA has prepared a work plan to remediate petroleum hydrocarbons in the soils and groundwater at fonder Exxon RS 7-39=31. Petroleum hydrocarbons may have been released from [lie former underground storage tanks when the groundwater level was much lower than it is at present, leaving tthc hydrocarbons in the soil column above and stranded below the water table. LA proposes to remediate the soils by extracting; vapor from existing monitoring wells and from dedicated vapor extraction wells to be installed. The groundwater will be rerncdiated by extracting groundwater from existing recovery wells and a proposed recovery well. 'Fite groundwater will be pumped from the recovery wells at a rate sufficient to dewater the area beneath and downgradient of the former tank field allowing the vapor extraction system to effect cleanup below the present water level. The extracted ground- water will be treated by stir stripping and activated carbon polishing. Both the vapors extracted from the soils via the vapor extraction wells and the effluent from the air stripper will be treated with a catalytic oxidizer to remove benzene and other petroleum hydrocarbons. Treated groundwater is proposed to be,reinjected in three wells along the western boundary of the site to limit the radius of influence of the pumping wells and to hydraulically isolate the site. Data on transmissivity of the aquifer and yield of the wells indicate that the effect Of Pumping can extend upgradient beyond the former Exxon site but can be limited by reinjecting treated water into three wells along the western boundary. Wells will also be used as venting wells, to reduce the influence of the vapor extraction system to the immediate site. 1.2 SITE DESCRIPTION 1 'f ire site, located at 4444 Pershing Avenue in the northwest part of Stockton, is near the north bank of the Calaveras River, about 2.5 nniles from its confluence with the San Joaquin River (Figure 1). According to maps of the San Joaquin County Flood Control and Water Conserva- tion District(FCWCD), the groundwater gradient near the site is to the northeast,paralleling the Calaveras River, but groundwater pumping in the city forces water to flow in the upstream direc- tion, At present the site,on the southeast corner of Nortlr Pershing Avenue and Rosemarie Lane, is an unimproved grassy lot; all buildings and installations have been removed. The areas to the northeast and soutinvest of the site lure occupied by apartment buildings(Figure:2). Across Pershing Avenue incl Rosemarie Lane, about 100-200 feet from the site boundaries, there are Shell and 7-Eleven service stations incl office buildings. North of the site, across Rosemarie Lane,is the Chateau Convalescent llospitu!. The well inventory of the California Department of Water Resources in Sacramento indicates four wells within one-11,111'mile of the site and 10 wells within one mile. There are no wells S'S