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Revised Remedial Action flan McBillin Real Eslate Investment Property 2154 South El Dorado Street,Stockton,California Page 4 believes that air sparging with SVE or dual phase extraction (DPE) is not an effective method of remediation at the Site. On February 8, 2021, Apex conducted a DPE test over a five-day period on two extraction wells at the Site(VW-2 and V W-3). Apex's goals for this phase of the investigation were: • To assess if hydrologic control could be imposed on the aquifer to halt the offsite migration of petroleum hydrocarbons. • To design an appropriate system for mitigating residual soil and groundwater contamination at the site. The results from the DPE pilot test indicate that this method of remediation will be effective at controlling and recovering the hydrocarbon plume at the Site. Test results showed adequate vapor flow rates from wells VW-1 and VW-2 as high as 102 standard cubic feet per minute(SCFM)and vapor concentrations reported at a maximum of 100 pounds per day TPIIg. During the testing, vapor radius of influence (ROI) was not measured due to limited well screen available. The groundwater extraction yielded hydraulic influence at a maximum distance of approximately 40 feet from the source well, which dewatered the contamination zone and enhanced the overall recovery of contaminants given the conditions found at this Site, For the purpose of sizing the equipment, Apex will use approximately 35 feet as the functioning ROL On November 5, 2021, Apex submitted a RAP detailing the installation of a DPE system. The DPE within the November 2021 RAP proposed use of a series of drop tubes or stingers to simultaneously extract groundwater and soil vapor to reduce hydrocarbons concentrations in soil beneath the Site and reduce/cease hydrocarbon concentrations migration in groundwater. In response to the November 2021 RAP submittal, the CVRWCB issued a letter titled Response to Remedial Action Plat),AlIcBillin Real Estate Investment, 2154 South El Dorado Sheet, .Stockton, San Joaquin County, LUST Case #391184, GeoTracker Global 1D4TI0000002615. Within the letter,the CVRWQCB did not concur with DPE as proposed and had the following comments. 1. Apex proposes to destroy existing monitoring wells MW-3 through MW-6 via over-drilling and replace each as 4-inch casing DPE wells. However, this would leave no dedicated monitoring wells on-Site, except for deep well MW-1 and shallow well MW-2. MW-I is screened from 90-95 feet below ground surface (bgs) and MW-2 is located well north of the source area cross-gradient to the plume. During and after active remediation has been performed in a well it is no longer representative of general plume conditions.Additionally, this project has significant Cleanup Fund budget, and over-drilling and replacing existing wells would provide no cost savings as compared to installing new dedicated DPE wells, and new wells could provide new on-site soil and groundwater data. As such, Central Valley Board staff requests that you propose additional DPE wells rather than destroying and replacing on-Site monitoring wells.