SITE INFORMATION AND CORRESPONDENCE_2001-2005

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DEER extraction on Inglewood Avenue, is designed to capture this PCE-affected groundwater to the extent practicable, and to reduce concentrations of hazardous substances in groundwater. COMMENT 2 In addition to the important tasks of aquifer testing and updating the groundwater flow model to serve as a design tool for the proposed Phase III IRA as proposed in the Phase III Plan, it would be appropriate and worthwhile to complete a thorough evaluation of the subsurface effectiveness of the existing Phase I groundwater extraction system. The extensive data(e.g., pumping rates, drawdown in pumped wells and monitoring wells, and water-quality data)that have been collected in association with historical operations of the Phase I IRA groundwater extraction system should be analyzed to assess the effectiveness of plume capture, and to further understanding about the three-dimensional response of both A- and B-zone groundwater to the Phase I IRA pumping from the A-zone. It is important to assess plume capture at this time, especially considering the hydraulic response observed in the B-zone due to Phase I IRA pumping from the A-zone. Such response was not anticipated when LFR designed the Phase I IRA system in 1997,and the original groundwater flow model used in the design process was a single-layer, two dimensional model which was not capable of representing or evaluating the effect of B-zone response on plume capture effectiveness. The effectiveness evaluation that LPL is recommending should be integral to the task of expanding and updating the groundwater flow model into a three dimensional design tool. In this way, the resulting model could serve as a powerful tool for assessing plume-capture effectiveness under varied conditions, now and in future,and would provide enhanced confidence in design decisions regarding the proposed Phase III IRA. The model could also be used with confidence to optimize operations of the existing Phase I IRA system and the combined Phase I and Phase III system. RESPONSE LFR routinely evaluates the response of the A- and B-zone aquifers to the performance of the Phase I IRA, which includes extraction and treatment of A-zone groundwater from 17 extraction wells located on Lincoln Center along Pacific Avenue. A- and B-zone groundwater potentiometric surface maps are prepared and included in quarterly groundwater monitoring reports currently being submitted to the RWQCB. Recent A-zone groundwater potentiometric surface maps demonstrate a pattern of capture by the groundwater extraction wells that validates the former groundwater flow model and that indicates the GWETS is effectively preventing downgradient migration of hazardous substances in A-zone groundwater. An evaluation of the most recent A-zone groundwater potentiometric surface map indicates that the full lateral extent of PCE-affected A-zone groundwater is currently being captured by the GWETS and that a stagnant zone exists downgradient from the capture zone (Figure 1). A-zone groundwater extraction is also influencing B-zone groundwater in the vicinity of Pacific Avenue, as demonstrated by the recent B-zone groundwater potentiometric surface map (Figure 2). This preliminary evaluation was used to prepare a conceptual design of the groundwater extraction network proposed in the Phase III IRA plan. Existing hydrogeologic information, along with the new data to be collected during the proposed B-zone aquifer pumping, will be used to update LFR's existing one layer MODFLOW model and to simulate three-dimensional groundwater flow regimes beneath the Site. This updated model will then be used to evaluate optimum groundwater extraction scenarios for the upcoming Phase III MAA REVISIONS-rsp-Phaselll-IRA-06752:DEH 3