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OUR <br /> Comment 3: <br /> Substrate Injection Plans <br /> We are concerned that the proposed substrate injection plans presented in Section 4.1 of the <br /> Detailed Plan, utilizing a line of injection wells spaced 30-ft apart, with relative long (15-ft) well <br /> screens, will not result in sufficiently uniform distribution of the injected substrate. A lack of <br /> uniform distribution of injected substrate will reduce the effectiveness of ERD and slow the <br /> progress toward complete remediation. <br /> The proposed 30 ft well spacing appears too large to ensure uniform delivery of the substrate. We <br /> recommend that the well spacing be reduced to a maximum of 20 ft. Alternately, the currently <br /> planned line of wells could be installed initially, and the actual distribution of injected substrate <br /> could be monitored during the injection process to evaluate the extent and uniformity of injections <br /> through the planned set of injection wells. Where the injected substrate distribution is found to be <br /> inadequate (e.g., as evidenced by total organic carbon monitoring in adjoining wells), additional <br /> in-fill injection wells should be installed and operated. <br /> A common problem with the use of relatively long-screened wells for substrate injection is that the <br /> injectate is distributed only into the more permeable sediments. There is evidence that HS <br /> contamination exists in the finer-grained sediments (e.g., CPT-104 HS = 1,100 ug/L at 63 ft bgs, <br /> and CPT-219 HS = 1,300 ug/L at 80 ft bgs), and therefore it is important to treat the fine-grained <br /> sediments along with the more permeable sediments during the implementation of the Phase V IRA. <br /> All reasonable efforts, including the use of more injection wells with shorter well screens, should <br /> therefore be taken by LFR and LCERT to ensure a more uniform distribution of injected substrate <br /> during the Phase V IRA implementation. <br /> Finally, the conceptual well layout shown on Figure 8 indicates that a vertical gap of <br /> approximately 5 ft would exist between the bottom of the shallow A-zone and the top of the deeper <br /> A-zone injection-well screens. Because HS contamination exists throughout the entire A-zone <br /> profile in the vicinity of the BOS, we recommend that the entire A-zone receive direct substrate <br /> injections, with no vertical gap between injection-well screens. <br /> Response 3: <br /> LFR recently completed similar injections at other sites within similarly permeable sediments and <br /> found the effective lateral radius of influence at the injection wells to be generally greater than 15 <br /> feet in each direction or greater than 30 feet overall. As such, LFR believes that a line of injection <br /> wells spaced 30 feet apart will be appropriate for this project. Please note that the Phase V Plan <br /> contemplates injecting approximately 7,000 gallons of substrate and water into each of 22 wells; <br /> injection rates and volumes will be monitored during the injection process. In addition, existing <br /> monitoring wells MW-108A, MW-20513, and MW0301C and proposed wells MW-121A, MW- <br /> 122A, and MW-214B (located at or immediately downgradient of the treatment area), will be <br /> monitored for a full suite of parameters during injection. This data will be used to evaluate the <br /> extent and uniformity of injections. If it is determined during monitoring that portions of the <br /> sediments are not being treated, then fill-in wells would likely be recommended. <br /> Itr-Fart-RTC-141m_LPL-Phv-06750(fimDAmd 3 <br />