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Supplemental Investigation Report,Problem Assessment Report and Work Plan Clemua and Abatement Order No 5-00-707 EAI Proiect No 1232 • Unocal excavated approximately 6 000 cubic yards of impacted soil at its site in 1988 Pp Y � Y p , to a maximum depth of 53 feet bgs Through aeration, it was reported that the hydrocarbon concentrations in 5,800 cubic yards of the excavated soil were reduced to acceptable levels, and that 5,800 cubic yards of the excavated soil was reused on-site as backfill The remaining 200 cubic yards of excavated soil was transported off-site for processing • Significant ground water fluctuation, over 25 feet, has occurred beneath both the Wards and Unocal sites, which may have resulted in the formation of a smear zone 0 Vertical extend of ground water contamination has been observed to the maximum extent explored to date, i e , approximately 65 feet bgs a Highest historical concentrations of dissolved hydrocarbons were detected in former Unocal well MW-1 (see Figure 4 and Table 4) A building is currently located over the area where well MWA was formerly located The area between former Unocal well MWA and existing Unocal MW-11 is probably impacted with petroleum hydrocarbons Proposed wells MW-23S and MW-23D (see Section 5 1) will provide data on this supposition • Petroleum hydrocarbons were present in ground water samples collected from the RIW, when the RIW was in use a Ground water flow direction is to the east when the RIW is not used, and to the southwest when the RIW is in operation, o The area between the Unocal and Wards sites and the RIW is impacted with petroleum hydrocarbons The vertical extent of this impact is currently unknown Proposed CPT locations (see Section 5 2) will provide data on this issue a Based on available analytical data for the Wards site, dissolved hydrocarbons are subjected to biodegradation by natural microorganisms 63 1 Institutional Control Shut down of the existing RIW (this occurred on August 22, 2000) and relocation andro er p P construction of a new RIW is probably the most efficient effort which will meet the ultimate threshold criteria, such as overall protection of human health and environment and compliance with applicable regulatory guidance Based on the results of hydrogeological investigation completed by ERI (see ERI, 1999), the new RIW location should be at least 1000 feet from the existing RIW location Further, the new RIW must be properly constructed, e g, slots beginning at depths not less than 150 feet bgs, and zones properly sealed against pollution A separate Work Plan will be submitted to the RWQCB by November 6, 2000, addressing _ relocation of the RIW HS 1232P 1RW_WP -20^ ENVIRONMENTAL AUDl7,INC