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GENERAL DYNAMICS Additional Investigation —Ro*3; SWMU21 Workplan, Revised Novem&003 samples will be collected in order to further investigate petroleum hydrocarbons following refusal encountered during the March 2003 event, and to define the vertical extent of TPH particularly downgradient of S-26. Selection of which soil samples collected during the monitoring well installation will be analyzed will be based on field test kit data. 1 2.0 SITE DESCRIPTION AND PREVIOUS INVESTIGATIONS ' The site is located on approximately 500-acres of land at 32727 Corral Hollow Road, approximately 3.2-miles south of the intersection of Interstate Highway 580 SW and ' Corral Hollow Road, near Tracy, California. The majority of the surface area of the site is located in a water gap (Elk Ravine) transecting the northeast slope of a southeast- northwest trending anticlinal ridge of the Altamont Hills. A northeast trending ' intermittent stream, Corral Hollow Creek, is located approximately 150-ft northeast of Corral Hollow Road in the bottom of the water gap. Outcrops of very coarse-grained (conglomerates) to fine-grained (sandy siltstones) sedimentary strata, obscured by ' undifferentiated coarse to fine-grained detrital deposits at the foot of the ridge, are present at higher elevations in the area of the site. Grazing of livestock (cattle) occurs north and east of the site. In 1999 a DTSC approved investigation of SWMU 21 found TPHd above 1,000 mg/kg in the soil. In 2001 a DTSC approved follow up investigation found additional TPH in soil above 1,000 mg/kg resulting in the removal and off site disposal of 1,200 tons of soil. Continued site assessment in 2002 resulted in the drilling of 9 "step out", and two borings to evaluate the depth of petroleum hydrocarbons. Step out boring S-26 ' approximately 35' southeast of SWMU21, recorded TPHd above 1,000 mg/kg to a depth of 40' bgs; sampling of the BH-4 boring measured TPHd and TPHo&g above 1,000 mg/kg at 45' bgs. ' In March of 2003 a DTSC approved workplan was implemented to evaluate three primary areas: ' 1. the extent of petroleum hydrocarbon southeast and down the topographic slope from boring S-26. 2. groundwater below boring BH-4 ' 3. soil below the storm drain outfall approximately 100' east of SWMU21. ' The March 2003 field investigation and sampling event found TPHd in groundwater below boring BH-4 at 45' bgs (from monitoring well S-33) at 30 mg/I. Soil samples collected from boring S-31 southeast and down the topographic slope from S-26 at 5' ' intervals to a maximum depth of 50' bgs found all samples to be well below 1,000 mg/kg. Soil samples below the storm drain at depths of 1' and 3' bgs found 10 mg/kg TPHd at the 3' bgs location. ' The following work scope will provide the specific approach CE will follow in further assessing TPH at the site. CHOW ENGINEERING, INC. 2 1