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• 0 Work Plan -Former Unocal Station No. 0187 October 9, 1997 Water levels have risen steadily since August 1994. It is not possible to evaluate the range over which groundwater levels might fluctuate, as water levels have not dropped from one quarter to the next since the start of quarterly monitoring. Soil with the highest TPHg and benzene concentrations are submerged. The zone of sandy soil is submerged. Soil in the remaining unsaturated zone consists predominately of clay. Sandy strata in the unsaturated zone do not appear to be laterally continuous. A cross section depicting the subsurface conditions is included as Figure 5. Based on these observations, site conditions do not appear to warrant implementation of the soil vapor extraction feasibility study at this time. The highest concentrations of residual hydrocarbons are below water, which indicates that vapor extraction is not an effective remedial alternative at this time. We anticipate that the proposed offsite well will delineate the dissolved hydrocarbon plume. Historical groundwater monitoring and sampling data indicates that the highest dissolved hydrocarbon concentrations are in the samples from wells U-1 and U-3. The highest concentrations in well U-1 were 5,000 ppb of TPHg and 960 ppb of benzene (August 1994) and have decreased to a low of 310 ppb of TPHg and 0.77 ppb of benzene (May and August 1997). The highest concentrations in well U-3 were 53,000 ppb of TPHg and 9,700 ppb of benzene (August 1994 and May 1995) and have decreased to 810 ppb of TPHg and 35 ppb of benzene (August 1997). Wells U-5 and U-6 have never contained detectable concentrations of TPHg or benzene. Wells U-8 and U-9 located downgradient of the Unocal site have contained detectable concentrations of TPHg and benzene. Well U-8 contained concentrations of TPHg (ranging from 110 to 380 ppb) in four of the eight sampling events and benzene was detected (ranging from 0.78 to 75 ppb) in three of eight sampling events. Well U-9 was recently installed and has been monitored and sampled only once. This well contained 930 ppb of TPHg and 130 ppb of benzene. Samples from well U-7, located upgradient of the former Unocal site, has indicated that TPHg and benzene concentrations range from less than 50 to 12,000 ppb and less than 0.5 to 44 ppb, respectively. As previously reported, there is a potential upgradient source from the Unocal site. To date, it is unknown whether any environmental investigations have been performed at this potential upgradient. source. To date, the highest MTBE concentration was detected in well U-1 at 260 ppb. The maximum concentrations of MTBE in the other eight wells has been 12 ppb. The groundwater flow direction has consistently been to the northwest. The most recent (third quarter 1997) Quarterly Data Report is attached. To summarize the groundwater conditions beneath this site, the highest concentrations within the dissolved plume are near the western portion of the Unocal site (wells U-1 through U-3). These concentrations have decreased over the past three years. The concentrations in the other wells (U-4 through U-8) have either been non-detectable are have also decreased over the past three years. Only one sampling event has been performed with well U-9. The groundwater plume does not appear to be migrating at this time (stable plume), MTBE in the downgradient well (U-9) is only 12 ppb, the nearest water supply well is approximately 850 feet upgradient from the site (nearest downgradient water supply well is approximately 1,575 feet), and there appears no vapor pathways. This information suggests that this site does not pose an immediate risk. 4827.07-1 5