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ENSR AECOM 1 5.0 Conclusions and recommendations 5.1 Status of response actions Soil and groundwater beneath the site has been adequately assessed. Some additional delineation downgradient could be performed as part of a commingled plume investigation in conjunction with the Vintage Car Wash site. However, because the data from U-20 and U-21 suggest that Unocal plume is a relatively minor component of the commingled plume, and because downgradient concentrations approach MCLs, downgradient delineation is not considered as high a priority, as remediation operations such that additional oxygen is introduced to the system to enhance the natural attenuation processes that are ongoing at the site. Permeability testing has not been performed at the site. Some permeability testing, including evaluation of pilot test data, is recommended to allow better analysis of plume fate and transport and to getter evaluate remedial alternatives at the site. 5.2 Implications of the conceptual site model The complex, interbedded alluvial deposits at the site and the history of groundwater fluctuations make remediation extremely challenging. Sparging relies on generating contact between injected gases and source area contaminants. While petroleum has been detected in both fine-grained and coarse-grained soils, ' sparged gases are preferentially passed through coarse-grained materials. Accordingly, most ozone sparge points have been installed in coarser-grained lenses within the formation. Given the near forty-year age of the release,the plume is almost certainly at_stea�state with the natural attenuation processes us further mi ration of the plume is not a valid concern. In addition, natural �. attenuation parameter data are consistent with some meas re o anaerobic biodegradation occurring at the site. Because anaerobic degradation is much less efficient than aerobic degradation and because the ' downgradient wells are approaching MCLs, remediation will likely incorporate a method that introduces oxygen to the aquifer to enhance the natural attenuation processes that are already occurring. ' 5.3 Existing remedial system Given that the evidence indicates that the ozone sparge system has not been operating as intended, a reevaluation of technical feasibility and potential system effectiveness is necessary. This reevaluation should be performed in a phased fashion. The first phase will involve rehabilitating the sparge wells on site and performing necessary system repairs, collecting some new baseline data and performance testing the repaired sparge system to determine whether it is adequately raising oxygen concentrations in the aquifer. Because ' replacing lines beneath the street will be an involved process,the decision regarding the disposition of these wells will be made following evaluation of the initial performance testing results. New baseline data would include dissolved oxygen, oxidation-reduction potential measurements and VOC concentration data, including a search for tentatively identified compounds (TICS)that are indicative of biodegradation of gasoline hydrocarbons (e. ., alcohols and phenolics). These data would be obtained at g , wells in the vicinity of, and downgradient o eon-Mea sparge sys em (i.e., U-3, U-8, U-9 and U-10 and the NP-1 and NP-2 well clusters). Performance testing should be conducted over a period of three months with field chemistry parameters monitored monthly and TIC samples obtained during quarterly monitoring events. The next phase of remediation evaluation and testing activities will depend on the results of the initial performance test. If the rehabilitated system works as intended, then the lines and wells across California Street associated with sparge clusters SP-5 and SP-6 should be rehabilitated and repaired. However, if the results of the performance test indicate that raisin ox levels in thea uifer is not feasible with the given i• . JARem_EngTroject File51P60(6000- ' 6999)16940-UNOCAL Portfolio Closure%940 -1 January 2006 UNOCAL Portfolio Closure\Site 0187-Stockton East MinoACSM FleporACSM Report_Final.doc