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ENSR ; I 1 5.0 Conclusions and recommendations 5.1 Status of response actions Contaminant delineation of the soil and groundwater is complete Concentrations are declining in wells MW-2, MW-10, MW-11, and MW-17 located immediately downgradient and crossgradient of the source area, but still remain above MCLS within Zone A at these wells Concentrations have been below MCLs at all other wells both on-site and off-site in Zones A and B 52 Implications of the Conceptual Site Model ' A release of petroleum hydrocarbons (i e , gasoline) happened at the site sometime prior to 1988 when the gasoline USTs were removed and evidence of impacted soils were detected in the UST cavity ' Subsequently, the impacted soils in the UST cavity were removed down to the groundwater table at approximately 30 feet bgs Fallowing the excavation, residual levels of TPH (maximum of 18 ppm) and BTEX (maximum of approximately 12 ppm) were reported in the sidewall samples and lower concentrations in the bottom sample In addition, residual levels of TPHg and BTEX were also detected in the vadose zone at two borings near the two former pump islands—KB-3 and KB-5 Soil concentrations detected at KB-3 were 239 ppm of TPHg and 11 2 ppm benzene and 370 ppm of TPHg and 138 ppm of total BTEX in KB-5 at 20 5 feet bgs The concentrations are considered to be a source of residual soil contamination A significant flux in the groundwater table of 17 feet had been observed in MW-1 between 1988 and 1993 Since 2000, depth to groundwater in MW-1 has fluctuated approximately 10 feet from approximately 21 feet bgs (June 2000) to 31 feet (October 2004), and was most recently encountered at approximately 28 feet bgs (January 2006) During 1993, depth to groundwater at the site was reported to be approximately 47 feet bgs (RESNA, 1993) and most recently is typically 28 feet bgs This indicates that the groundwater has subsequently risen into the zone of soil impact creating a significant smear zone Groundwater quality data has been collected over 18 years at the site (i 988—2006) during which time no separate-phase petroleum product has been detected in any monitoring well The groundwater data ' indicates that the groundwater plume is relatively stable and is centered around three on-site wells MW-2, MW-10 and MW-11 with some downgradient migration off-site The plume stability can be attributed to the low permeability silts and clays, the shallow horizontal gradient of approximately 0 002 feet/foot, and biodegradation The complex, interbedded alluvial deposits (predominantly clays and silts) at the site make groundwater remediation extremely challenging The ozone sparging system at the site relies on generating contact ' between injected gases and source area contaminants at three separate depth intervals —30 feet, 60 feet, and 90 feet to provide chemical oxidation and stimulation of aerobic biodegradation The ozone sparge points have been installed in fine-grained lenses within the formation limiting their zone of influence Given ' that the ozone sparge system has had limited effectiveness, a re-evaluation of the system performance should be conducted 5.3 Existing remedial system The remedial system at the site consists of eight ozone sparge wells installed at three locations (SP-1, SP-2 and SP-3) and three vapor extraction wells (VW-1, VW-2, and VW-3) in the area of highest petroleum concentrations, near the eastern property boundary of the site The ozone was injected at low flow rates of 2 to 6 cubic feet per minutes into each point and a design criteria for the radius of influence to be 5-1 .� �r