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i McBillin Real Estate Investment - 3 - 19 January 2022 Central Valley Water Board staff does not concur with DPE remediation as proposed, and has 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-1 is screened 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 remaining 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 Water Board staff requests that you propose additional DPE wells rather than destroying and replacing on-Site monitoring wells. 2. Apex's proposal is 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 this well. Additionally, in the Operation & Maintenance section of the RAP, Apex states that "all three VW wells" will be sampled, which is confusing as a 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 wells 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. In 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 for better dewatering of the subsurface. Due to concerns regarding the cost of initial installation and future