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..r DISCO PROPERTY,STOCKTON,CALIFORNIA expedited sampling procedure. These samples will be obtained at locations where the CPT survey indicates preferential groundwater flow pathways may exist. To the extent feasible,groundwater screening will be undertaken in the following order: • The first sample location will be near NP-1,where the highest TPH and benzene concentrations have been detected in existing wells, suggesting that this is near the original release area. Available data indicate that elevated concentrations extend to greater than 90 feet below ground surface (bgs). Based on CPT results,groundwater grab samples will be collected at the first encountered groundwater (50 to 60 feet bgs) and depths of 110, 130, and 150 feet below grade,if accessible. Screening for the presence of hydrocarbons in groundwater grab samples will be performed using a field test kit. Samples will also be collected for analysis by a state-certified laboratory for benzene, toluene,ethylbenzene, and xylenes (BTEX),TPH—gas, and gasoline oxygenates. • Two more groundwater screening locations will be selected based on the CPT data and field screening results from the first sample near NP-2. These will be selected to provide preliminary data on the vertical and horizontal extent of hydrocarbon constituents upgradient and downgradient of NP-2. Appropriate utility clearances and permits for access and drilling activities will be obtained for the above additional activities as described in the 1997 workplan. A report summarizing and tabulating the results of the Workplan Addendum activities and recommending any appropriate modifications to 1997 workplan activities will be prepared for review and approval by PHS/EHD prior to initiating soil boring and monitoring well construction. Modifications to 1997 Workplan Activities The1997 workplan is modified as follows: • The location and depths of additional monitoring wells proposed in the 1997 workplan will be reevaluated based on the results of the CPT and groundwater screening investigations. The precise well locations and depths must be approved in writing by the PHS/EHD prior to construction. _ • All soil boring and groundwater monitoring well samples will be analyzed for TPH-gas, BTEX, and the additives MTBE,TAME,DIPE, tertiary butyl alcohol, and ETBE. • Soil samples will be collected from all borings following the applicable sections of the DTSC guidance document "Drilling,Coring,Sampling,and Logging at Hazardous Substances Release Sites'. • Groundwater samples will be collected following the applicable sections of the DTSC guidance document "Representative Sampling of Ground Water for Hazardous Substances Release Sites" • Monitoring wells will be constructed following the applicable sections of the DTSC guidance document"Monitoring Well Design and Construction for Geologic Characterization". 2