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5.1 Soil Plume Definition Impaction to the soil by TPH-g above 100 mg/Kg and benzene above 1 mg/Kg remains in a portion of the overexcavated tank pit at locations "S-Bottom" and "N-Wall" as listed in Table A. These locations are depicted on the Site Plan (Figure 2), the Map of Benzene in Soil at 15 Feet (Figure 4), and the Map of Benzene in Soil at 20 Feet (Figure 5). The activities performed on July 20, 1993 were terminated prior to removal of all impacted soil because the extension required of the backhoe to reach the depths necessary in the south-bottom portion of the overexcavation had been maximized. Although no field notes could be located in the files, it is our understanding that a representative of the U.S. Army Corps of Engineers responsible for maintaining the integrity of the levee upon which overexcavation work was performed had ordered the cessation of digging in order to preserve levee integrity. Regarding outbound soil plume limits, detectable, primarily fractional BTEX analytes were revealed only in soil samples collected from HP-7 at 15 feet, and from HP-9 at 15 feet. The highest TPH-g result was 3 mg/Kg, revealed in the sample collected from HP-9. TRPH-d results were wholly nondetectable. 5.2 Ground-Water Monitoring Data and Plume Definition On July 7, 1995, the average depth to the water table underlying the project area was 15.73 feet. Using the three-point method, the calculated local direction of ground-water flow that date was approximately N77°E, a shift of 36° since January 10 when the calculated flow direction was =~ S67°E. Overall, the flow direction has remained easterly. The gradient has flattened slightly from 0.023 foot/foot to 0.009 foot/foot between January 10 and July 7. Ground-water samples collected from the five monitoring wells (MW-4 through MW-8) on July 7, 1995 were received on July 7 and analyzed by chemists at Sherwood Labs. The analytical results have revealed wholly nondetectable concentrations of TRPH-d analytes and nondetectable results for TPH-g and BTEX analytes in samples collected from wells MW-4, MW-5, MW-7, and MW-8. The results of the analyses of the samples collected from MW-6 indicate that the ground water underlying that vicinity remains impacted with TPH-g and benzene analytes well above the Maximum Contaminant Levels (MCLS) set forth by the California Department of Health Services. A sample of surface water collected from the slough just upgradient of MW-6 on January 13, 1995 and delivered to the laboratory on that date, revealed wholly nondetectable analytical results for TPH-d, TPH-g, and BTEX analytes. The sample collected from HP-3 on April 8, 1994 was impacted with TPH-g at 360 tcg/P, and benzene at 160 ,ug/f. HP-3, bordering the slough, revealed known TPH-g/benzene concentrations on the west side of the overexcavation which was backfilled in May 1994. The sample collected from HP-9 on July 13, 1995 revealed 196 lugIf of TPH-g and 124 ,ug/1' of benzene. W.W.Irvin, Inc. Project No. 33016.05&.06 August 1995 14