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L Stantec Consulting Inc. <br /> West Shaw Avenue Suite 104 <br /> Fres <br /> Fresno 93711 <br /> Tel: (559)9)271-2671-26 <br /> 50 <br /> Fax: (559)271-5108 <br /> Stantec <br /> December 17, 2009 <br /> Ms. Lori Duncan <br /> San Joaquin County Environmental Health Department <br /> 600 East Main Street <br /> Stockton, CA 95202 <br /> Dear Ms. Duncan: <br /> Reference: Results of Hydrogeologic Modeling <br /> Former 76 Service Station #6981, 4707 Pacific Avenue, Stockton, California <br /> Project Number: 211402667 <br /> Stantec Consulting Corporation (Stantec) has prepared this letter to summarize the results of <br /> hydrogeologic modeling of total petroleum hydrocarbons as gasoline (THPg) flow and transport for <br /> the subject site. The purpose of this modeling is to assess the migration of TPHg in groundwater at <br /> the Site and off-site. The modeling was conducted in response to a letter from the San Joaquin <br /> County Department of Environmental Health dated July 21, 2009 regarding the necessity to <br /> conduct fate and transport modeling in order to move the site toward No Further Action. <br /> GROUNDWATER FLOW AND TRANSPORT MODELING <br /> Groundwater flow and transport modeling was conducted in the vicinity of former 76 Station #6981 <br /> in Stockton, California to assess the northeast migration of contaminants from the Site (Figure 1). <br /> The FLOWPATH II groundwater flow and transport model was used to provide numerical <br /> simulations of flow and transport of TPHg at the site. The model is a two-dimensional (x-y plane) <br /> finite-difference model that provides groundwater flow simulations and contaminant transport. The <br /> flow and transport parameters include those specific for the aquifer(hydraulic conductivity, effective <br /> porosity, and longitudinal and transverse dispersivity) and specific for TPHg transport (first-order <br /> decay coefficient or half-life, and diffusion coefficient.) <br /> Model Setup <br /> The FLOWPATH II model was established as a rectangular area with sides of 600 feet north-south <br /> and 400 feet east-west and using 10-foot grid spacing. Linear constant head boundaries were <br /> developed along the upgradient, downgradient, and crossgradient boundaries based on <br /> extrapolation of the groundwater elevation data from July 28, 2009 to these boundaries. Generally, <br /> the groundwater flow direction appears to be toward the northeast. <br /> The hydraulic properties were estimated by determining the predominant water-bearing material <br /> (silt/sands and silt) and obtaining a uniform hydraulic conductivity from published values for that <br />