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i ENSR AECOM Monitored natural attenuation parameters have been analyzed on three occasions. The data are presented in Table 3-6. These data, as well as the dissolved oxygen data presented in Table 3-5, are indicative of reducing conditions near the center of the groundwater plume. These reducing conditions are apparent in the low dissolved oxygen readings observed at wells U-3, U-10, NP-1-22/65/111 and NP-2-22!60/116 and in lower nitrate concentrations In monitoring wells U-3, NP-1-65, NP-2-60, U-12, U-13, U-20 and U-21, which are ' located along the centerline of the groundwater plume,than in background monitoring wells U-5, U-6, and U- 14. Reduced sulfate concentrations relative to background were also observed at U-3, NP-1-22, NP-1-65, NP- 1-111, NP-2-60, NP-2-116, U-8, U-9, U-12, U-13, U-20 and U-21. These data are indicative of anaerobic metabolism of hydrocarbons. This metabolism is less efficient than aerobic metabolism, and thus biodegradation might be enhanced by the addition of oxygen to the aquifer. ' 3.6 Ozone sparging system operational status While contaminant concentrations have been declining at the site,the rate of decline has not increased subsequent to the installation of the ozone sparging system, and concentrations have declined at wells upgradient of the treatment zone of the ozone sparge system. Furthermore, dissolved oxygen concentrations have not risen in site monitoring wells, subsequent to the installation of the ozone sparging system. An inspection and test of the ozone sparging system was performed in June 2005. The results of this test indicate that the system was in a state of disrepair and was not adequate to perform as intended. All indications are that the onsite wells had not operated as intended and that the offsite wells had not operated as intended for an extended period of time. Most sparge lines were broken in several places, and after the lines were replaced,the compressor generated inadequate pressure to sparge the on-site sparge wells with the exception of well SP-413. It is possible that well or valve fouling is responsible for the inability to sparge the majority of on-site ozone wells. Additional assessment, repair, and well redevelopment or rehabilitation activities (e.g., citric acid injection and withdrawal)are recommended. Following repair of the system, additional system evaluation should be performed (see Section 5.3). 3.7 Data gaps and recommended activities The existing dataset largely characterize the nature and extent of contamination related to the site. However, a few data gaps still exist. These data gaps include the following: a The extent of the likely commingled plume downgradient of the Vintage Car Wash site needs to be further delineated; • The permeability of the subsurface needs to be better characterized; and • The applicability of the interim remedy as an ongoing measure or as an upgraded system needs to be assessed. Based on the concentrations detected at monitoring wells U-20 and U-21, it is highly likely that the Unocal Station #187 plume and the Vintage Car Wash plume commingle. Characterization of Zone C impacts beneath the Vintage Car Wash would provide an indication of the extent to which the two plumes commingle ' at that depth and of the relative impact of each site to groundwater concentrations in that area. Vintage Car Wash should install a single additional monitoring well, coupled to Vintage Car Wash monitoring well MW-1,to assess dissolved-phase petroleum hydrocarbon concentrations at depth. However, based on the existing data, it has been concluded that the bulk of the impacts beneath and downgradient of the Vintage Car Wash site are associated with the Vintage Car Wash release. Because dissolved-phase impacts in Zone A do not extend as far downgradient as the Vintage Car Wash site, the downgradient assessment would include a monitoring well cluster screened in Zones B and C. The J:\Rem_EnglProject F11es\P60(6000- 6999)\6940-UNOCAL Portfolio Closure\6940. 3-4 January 2006 UNOCAL Portfolio Closure\Sile 0187-Stockton East Minor\CSM Report\CSM Report_Final.doc i