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I I3.3 Local Groundwater Flow Direction and Gradient I The groundwater surface undulates slightly but has a gentle slope toward S50W with a gradient of 0 001 (Figure 4) This flow direction and gradient are consistent with published groundwater data from the City of Lodi(Environmental Forensic Investigations, Inc, 2002)and San Joaquin County Department of Public Works(2004)groundwater surface elevations for the North San Joaquin Water Conservation District 3.4 Petroleum Hydrocarbon Groundwater Plume The areal or horizontal extent of the petroleum hydrocarbon groundwater plume at the site extended throughout the site All monitoring wells except MW4 had reportable concentrations of petroleum hydrocarbons Significant concentrations of TPH-gasoline and benzene were present in well MWI, 2, 3, and 6 at concentrations above maximum contaminant levels(MCLs) Minor concentrations of 1,2- DCA and 1,2-EDB were reported to be present in monitoring wells MW1 and 3 MTBE was reported to be Ipresent at low concentrations in MW2 and MW3 A review of past groundwater petroleum hydrocarbon concentrations was made and included in the Itabulations presented in Table 4 No horizontal limit of the petroleum hydrocarbon contamination plume has been identified to date The past maximum areal extent of the groundwater petroleum hydrocarbon plume as determined from previous soil boring investigations suggest the site to has had a minimum areal extent of approximately 200 by 225 feet(45,000 square feet) We believe the plume to be somewhat larger in areal extent lie Updated Site Conce tual Model The Site Conceptual Model(SCM) is a written and/or graphical representation of the unauthorized release I scenario, site characteristics (geology, hydrogeology, etc)and the likely distribution of constituents of concern at the site The SCM lmks potential sources to potential receptors through transport of chemicals in air, soil, surfacewaters, and groundwaters It also provides a technical framework for the entire project I and a communications tool for regulatory agencies, responsible parties, and other parties involved in the investigation and ultimate cleanup of the site A Preliminary SCM was presented in the Report of Continued Investigation of Petroleum Hydrocarbon Soil and Groundwater Contamination(Muir, 2003d)that was submitted to PHSEHD in November, 2003 The overall goals of the Site Conceptual Model are listed below a Identify how the distribution of chemical is changing in space and time, b Identify potential current and future receptors, c Identify environmental issues that need to be addressed The Parmar Texaco Preliminary SCM includes the following basic site information parameters I 1 Site Description Parmar Texaco station, 521 North Cherokee Lane, Lodi, , California 2 Land Use Former Service Station Equipment Supplier Current laird use is anautomotive tire service shop ' 3 Water Use City of Lodi supplied water and City of Lodi sewer and storm dram service are provided to property No release of liquids to ground surface Several water supply • Maln\DTnvuonmentaIT&n arTexaco\Repom\123104QMR wN 5 I