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I I GROUND ZERO ANALYSIS,INC. 1714 Main Street Escalon,California 95320-1927 ' Telephone (209)838-9888 Facsimile (209)838-9883 September 15, 2005 ' CONTAMINANT TRANSPORT MODELING FOR WILBUR-ELLIS/STOCKTON ' At the request of the San Joaquin ounty Environmental Health Department, Ground Zero Analysis modeled the transpopt of dissolved contaminants in groundwater at the site ' using the computer program RBCA Tool Kit for Chemical Releases Version 13a (O Groundwater Services, Inc , 2000) The program uses a Domenico method to model the ' advective transport of dissolved contaminants in the direct downgradient direction of a contaminant source The model is very conservative in that the source is assumed to remain constant (concentrations downgradient will never diminish with time) Contaminant concentrations at various distances downgradient of the source can be ' calculated at specified times following a release as well as at a final steady state lie Approach The approach to modeling the contaminant transport at the Wilbur-Ellis site was to ' initiate the exercise by entering lithologic, hydraulic and contaminant parameters as known from site specific data or as estimated based on the range of expected values for similar soil types The model was run and actual contaminant concentrations In ' groundwater at two known downgradient monitoring points (MW3 and SBB) were compared to the predicted values The model was then calibrated by manipulating the hydrogeologic/contaminant source parameters (within a tight range of values) until the ' model closely approximated the actual concentrations at the monitoring points When the modeled concentrations and the measured concentrations were in agreement the data were deemed usable to predict concentrations further downgradient than the monitoring ' points and at times in the future Initial Assumptions and Parameters Used for Modeling 2 The release began ��ween 1965 when the UST was installed and 1987 when the UST was removed (18�� years of transport time) The maximum contaminant concentrations ' detected in MW I, P5 W and B-2 are reflective of maximum impact and will be used to approximate the "source" concentration The current concentrations in MW3 (about 50 feet downgradient of source) and SBB (about 120 feet downgradient) will be used to ' calibrate the model Benzene will be the contaminant modeled since this is the only compound present in any significant concentration • I