SITE INFORMATION AND CORRESPONDENCE

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f *� October 11, 2010 — d Messrs Altwies and Palma Page 3 of 4 'A Former Manna Pro Site 4004 S. EI Dorado Street, Stockton, CA sand unit will migrate laterally, as that requires less energy, until equilibrium conditions are p achieved and the air components dissolve into the water. It would be very fortuitous for the air to migrate to one or more,of the-vapor wells that penetrate the sand. It therefore seems quite unlikely to the EHD that any significant portion of the sparged air, and the strippedhydrocarbon contaminants in it, ever made it to the SVE system for removal: The main beneficial effect of the sparged solved oxygen content of the water column and air may have been to increase the dis enhance biodegradation of the contaminants if low oxygen concentrations were the only limiting condition on the biodegradation process. It appears to the EHD that sparging probably had only minimal effect on the reduction of the sorbed hydrocarbon contaminant mass in soil in the saturated `zone. The EHD is unaware of any data collected that demonstrates recovery of sparged air and the associated stripped hydrocarbons. Because SVE is not normally effective for impacted soil in the saturated zone and it seems unlikely that IAS has transferred, hydrocarbon contaminants to the SVE system, the EHD interprets the estimated 15,000 pounds of recovered hydrocarbons to have originated from unassessed areas of the vadose`;zone and possibly along the capillary fringe, with a minor contribution.from'the uppermost groundwater. It appears to the EHD that most of the vadose zone soil (soil less than 30 feet deep) is not assessed; other than excavation area samples and boring.MP-1, there is no analytical'.data for vadose.zone soil samples. Of the estimated 22,000 pounds estimated to be sorbed to soil,in the saturated zone, the EHD is of the opinion that except for that portion removed via biodegradation and fluxing into groundwater, most of the mass remains in the saturated zone. The EHD also has some doubt on.the removal of the estimated 15;000 pounds of hydrocarbon contaminants. GPE'states that verylittle water vapor produced as part of the SVE process is retained by the-carbon, hence the weight of the carbon when exchanged is a measure of the 'hydrocarbon mass removed; GPE cites several articles to support this, The -EHD is not familiar with the:organization that.produced the articles, which appear to be a trade group and an equipment and technology vendor; while the research and conclusions may be valid, the EHD is uncertain that it is peer-reviewed .and generally accepted in the scientific community. GPR states that granulated activated carbon (GAC) absorbs 25 to ,35% of its weight in hydrocarbon. The EHD notes that the 6,000-pound carbon unit switched out on 7/10/2007 weighted an extra 3;475 pounds, which would be an increase of 57.9%, far`exceeding the 25 to 35% absorption range for hydrocarbons. It seems likely that a significant portion of the extra weight maybe due to water in the carbon. Based on the discussion above, the EHD cannot concur with the low-risk closure request at this time. It is .the opinion of the EHD that additional remediation would be beneficial,-and will accelerate'"achieving WQOs in a timely manner. To address the issues identified in this letter, the EHD recommends the following actions: f Reevaluate the remediation system in the.context of the site hydrogeological model, and if appropriate, recommendan alternative remedial technology to address the residual mass and reduce the impact to groundwater; • Using site data, demonstrate the recovery of sparged air by the SVE system; • Provide .additional lines of data that support the contaminant mass estimated to have been captured by the GAC; and • Advance several confirmation borings to demonstrate a lack of impact in vadose zone I soil and reduction of contaminant mass in the saturated zone soil. Closure Request Response Letter 1010 I J