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5. 0 CONCLUSIONS & RECOMMENDATIONS 5 . 1 Excavation Soil samples collected from the excavation indicate that the contaminated soil has been removed from the southern area of the excavation. Contaminated soil still exists at the north, east and west sides of the excavation. Soil was excavated to the west as far as the property lane. Further excavation to the north and east is limited by the presence of MW-1 and the building, respectively. Therefore, further excavation would probably not result in the removal of a significant quantity of contaminated soil. We recommend that the excavation be backfilled to grade with clean fill. 5 . 2 Stockpiled Soil Laboratory analysis revealed that approximately 400 cubic yards of stockpiled soil are contaminated with petroleum hydrocarbons. Several on-site remediation techniques are available to mitigate this contamination. However, because of time and space limitations at the site, we recommend that the soil be transported and disposed of at a licensed landfill facility. A Soil Remediation Plan should be prepared and submitted to PHS/EHD. 5 . 3 Groundwater ITwo rounds of groundwater monitoring have been completed at this site. A consistent groundwater flow direction west of north has been established. The depth to groundwater has fluctuated since the first wells were installed in 1988 , but has always been within 15 feet of surface grade. A plume of contaminated groundwater extends northwestward from the former tank- cavity (Figure 4) . Concentrations of TPH as gasoline rose sharply in MW-1 from 80 ppb in January 1990 to 4, 000 ppb in ' March 1993 , but have decreased to 2 , 300 ppb at the present time. Other contaminants are also present in this well. MW-2 is east of the plume, and no water samples from MW-2 have been contaminated in I any of the monitoring events. Toluene was detected in MW-3 in March, but no contamination was detected in September. It is probably near the up-gradient margin of the plume. MW-4 is probably near the down--gradient edge of the plume, as indicated by 1) the presence of low concentrations of gasoline-range hydrocarbons and volatile aromatics in capillary-fringe soil samples at 10 and 15 feet and 2) gasoline-range hydrocarbons, ethylbenzene, and xylene ' in the water sample from this well. The Central Valley Regional Water Quality Control Board (CVRWQCB) will not grant site closure when groundwater is contaminated with benzene. Typically, quarterly groundwater monitoring must demonstrate that groundwater remains uncontaminated for one year ' 16