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Revised Remedial Action flan Mcnillin Real Estate Investment Property 2154 South El Dorado Street,Stockton,California Page 5 2. Apex's Proposal in unclear as to which wells they intend to utilize as DPE wells. In the text of the RAP, Apex does not specify that existing wells VW-1 through VW-3 will be utilized as DPE wells. However, on Figure 3,each of these wells has a circle depicting the radius of influence. Inversely, in the text Apex specifies that the MW-6 replacement well will be utilized as a DPE well, but Figure 3 does not depict a radius of influence circle around the well. Additionally, in the Operation and Maintenance section of the RAP,Apex states that"all three VW wells"will be sampled,which is confilsing as the total of 10 DPE wells are discussed in some way in the document. 3. Apex proposes deep DPE wells screened between 10-45 feet bgs to target deeper hydrocarbon impacts when groundwater levels are low. However, as shown in Table 2 above,45 feet bgs is approximately 25 feet below the average recent depth to groundwater. Additionally, with the screen interval of the deep DPE extending up to 10 feet bgs, the effective extraction zone will be roughly the same as that of the shallow wells.To properly target depth discrete extraction zones,the shallow and deep extraction zones should be set so that the top of the deep screen interval should be at or near the depth of the bottom of the shallow screen interval. 4. 111 a 7 July 2021 meeting with Apex staff, and in a follow-up same-day directive letter, Central Valley Water Board staff discussed the possibility of using submersible pumps and wellhead vapor extraction to separate the extraction streams(multi-phase extraction;MPE) and allow better dewatering of the subsurface. Due to concerns regarding the cost if initial installation and future system expansion of MPE, in the RAP Apex proposes to use stingers which will remove vapor and groundwater in a single combined extraction stream(DPE), though they have not made any conclusions as to whether they believe the system will be able to properly dewater the Site in order to remediate significant petroleum impact present between 20-30 feet bgs, Recent average depth to groundwater is approximately 20 feet bgs. During the 2021 DPE pilot test, Apex extracted from one DPE well at a time and reported a groundwater removal rate of 1.08 gallons per minute (gpur) over the course of the 96-hr test. In the RAP, Apex states that full-scale DPE renediation will extract from 10 DPE wells utilizing the same remediation system as the pilot test, with an anticipated maximum water flow of 3 gpm. With a combined extraction stream DPE setup, high volumes of water may overload the system's extractions capacity and limit the system's ability to extract vapor, which is the primary purpose of the system. Apex has not provided sufficient evidence that the proposed system will provide much dewatering capability or be able to effectively extract vapor-phase hydrocarbon mass from the subsurface. 5. Connecting each DPE well to its own valve on the manifold would allow for more extraction to be turned on/off in well individually, which would allow for the targeting of extraction in specific areas as mass is removed. In the RAP, Apex does not specify the manner in which the wellhead piping will be connected to the system manifold, and thus the targeting flexibility of the proposed system is unclear. Additionally,while Apex states that the manifold construction will allow for system expansion, they do not specify the intended expansion capacity.