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November 24, 1997 Harding Lawson Associates 39960 1 Margaret Lagorio, Supervising REHS San Joaquin County Public Health Services Environmental Health Division Page 3 Task 2.0: Field Investigation Based on the concentrations found in soils beneath the site, additional subsurface soil sample will be required to sufficiently evaluate the lateral and vertical extent of petroleum impacted soils. In reviewing the existing analytical data from the three borings drilled in 1990, it appears as if the lateral extent of impacted soils is adequately defined north of the tank locations. The No. 1 Reservoir is present northeast of the tank locations and will preclude any additional sampling in this direction. TPH-d and TRPH concentrations in the shallow sample from NSA-1 were elevated. A boring is proposed east of NSA-1 to try and establish the limits of the impacted soils in this direction. Vertically, impacted soils have been detected to the maximum depth previously explored at the site of approximately 16 feet bgs. The highest concentrations of contaminants were reported directly beneath the sump at a depth of approximately 15 feet bgs. were reported from Boring NSA-1. A boring is proposed between NSA-1 and the sump to establish the vertical extent of impacted soils. This boring will need to extend to the bottom of the impacted soils which may extend to 25 feet or more. Soil Borings Two approximately 25 foot soil borings/probes are proposed at the locations shown on the attached Plate 1. Soil borings will be advanced using a bobcat mounted (or equivalent) Geoprobe sampling technique. The Geoprobe is a direct push technology(DPT). Boreholes will be continuous cored or if hollow-stem auger equipment is required to reach the desired depths, drive samples will be collected at 2.5 foot intervals. Soil samples will be retained at 5-foot intervals for lithographic description, volatile organic compound (VOC) screening, and potential laboratory(chemical) analysis. Soils encountered in the borehole will be monitored for the presence of volatile organic vapors using a photoionization detector (PID). The samples will be collected in either clean 6-inch long brass tubes or Lucite core barrels. The samples collected will be removed from the sampler and immediately capped with Teflon-lined plastic end caps, labeled, and placed in an ice-chilled cooler for transport to a State-certified analytical laboratory. Samples will be recorded on a standard HLA chain-of-custody form to document sample identification and handling. The adjacent soil sample will be checked for hydrocarbon stains and monitored for VOC vapors using a photoionization detector(PID) which is calibrated daily. The PID will be equipped with a 10.2 electron-volt lamp and calibrated to a 100 part per million (ppm) isobutylene standard. The soils will be visually classified using the Unified Soil Classification System (USCS); soil classifications and other field observations are recorded on standard HLA field logging forms. After drilling is terminated, the borings will be tremie grouted with neat cement in accordance with PHS/EHD requirements. Soil cuttings and decontamination water generated during drilling activities will be placed in barrels and labeled with boring number and date. The cuttings will be retained on site pending analytical results after analytical results are received, the cuttings will be disposed in an appropriate manner.