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Y U � OEOMATRIX • Mr Jeff Wong San Joaquin County Department of Public Health Services August 10, 2004 Page 5 An Evaluation of Threats to Beneficial Uses of Groundwater and Water Supply Wells. A well survey was conducted by ATC in 2000 that reportedly searched Department of Water Resources (DWR) files, performed a site and vicinity reconnaissance, and contacted the City of Stockton and California Water Service for wells within 2000 feet of the site vicinity (ATC, 2000) The CAP will document the results of the well survey and also will evaluate the current and future probable beneficial uses of site groundwater This task will also include an evaluation of the stability of the area of affected groundwater, in terms of occurrence and mass flux, if the concentrations of target analytes in groundwater samples remain above WQOs for the site This evaluation would be important for assessing the timeframe that remedial or management actions would be necessary for WQOs to be achieved Evaluation of the Completeness of Site Characterization The current lateral extent of petroleum fuel constituents in groundwater is believed to be constrained, based on historical . results from perimeter monitoring wells MW-1, MW-5 and MW-6 However, recent anomalous detections of TPHd in wells MW-1, MW-5, MW-6 and MW-7 require further evaluation It is our understanding that site wells will be re-sampled in August 2004 The results of the August 2004 sampling event will be used to evaluate the representativeness of the June 2004 groundwater sample analytical results by comparison with historical information Should the June 2004 analytical results be considered invalid, then site characterization will be considered complete If the August 2004 analytical results indicate that petroleum fuel constituents are present in groundwater samples collected from wells MW-1, MW-5, MW-6, and MW-7, additional site characterization activities may be proposed to define the lateral and vertical extent of petroleum fuel constituents in groundwater Additionally, a forensic analyses of the product that may be present in a project well (such as well MW-2) will be performed to characterize the product for weathering, soluble fraction, and other pertinent characteristics important for developing an appropriate management plan for the site Evaluation of"TPHd"in Groundwater. The TPHd analyses is based on gas chromatography-flame ionization detection and does not reliably distinguish between dissolved petroleum hydrocarbons and other organic materials, often present in groundwater as a result of biodegradation of the petroleum or other factors A silica-gel preparation procedure, based on U S Environmental Protection Agency (EPA) Method 3630B, when performed on the sample extract prior to analyses, will remove non-petroleum hydrocarbons from the sample Therefore, analyses of future groundwater samples will include analyses for TPHd both with and without a silica-gel preparation procedure to assess whether or not detected "TPHd" is representative of petroleum hydrocarbons or is due to other organic I IProjec11900009631CAP Work Plan doc