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- - -- • ~~ ~~~ ~ ~~ -~-~= Hpr 5 2000 13:45 P. 03 <br />i <br />IQ.ng? ~~~ 9a1 ~ Rom 9.0 Identification of the Selected Remedy <br />• 9.0 Identification of the Selected Remedy <br />The selection of the preferred remedy for remediarion of groundwater, structure, and soil for the Oo-Pose <br />Operable Unit was based an the NCP evaluation mteria, which are desmbed in'Fgwe 8.0.1 and discussed <br />with respect to each of the alternatives evaluated in Sections 8.1 duough 8.3. As a result of these evaluations. <br />the selected remedy for the On-Post Operable Uait eonsisu of implementing Groundwater Alternative 4, <br />Structures Altenarive 2, and Soil Alternative 4. These selected'altemarives are described in detail in Section 7, i ~' , <br />Rtmediation goals for rite selected remedy satisfies the evaluation of staanory regnvemenu under CERCI.A as ~ ~ <br />described in Section 10. `r~ <br />~. i <br />-. <br />9.1 Groundwater Aftematrvs 4 -Boundary Systems/IRASllntercept Systems ~, ;' <br />The saletted groundwater alternative is Alternative 4. This alternative includes operation of all existing <br />boundary systems and on-post groundwater IRA systems, installation of a new extraction and piping system, <br />and development of an extended monhoting program. The specific components of the altemadve are as <br />follows: <br />• Operation of the three boundary rystems, the hBCS, NWSCS, end 1CS, continues, These systems <br />include extraction and recharge systems, slurry walls (NBCS and NWBCS) for hydraulic wntrals, and <br />carbon adsorption for removal of organics. The systems will be operated anal shut-off criteria, as <br />described below, arc met <br />• Operation of existing oa-post groundwater IRA systems continues. T'he Motor Pool and RaH Yard <br />IRA systems, which pipe watt/ to ICS for treatment will be shut down when shut-off criteria, u <br />described below, arc met The Basin P extraction system wntinues to extratt water that is treated at the <br />Basin A Neek ryetem and the Bnsia A Neck rystem continues to exacter end trot water from Basin A ' <br />until shut-off Muria arc rget. <br />• Anew exttaetitm rystem will be installed in the Section 36 Bedrock Ridge area. Extracted watef will <br />be piped to the Basin A Neck system for treatment (e.g., by air snipping or carbon adsorption). <br />• Water levels in Lake Ladora, Lake Mary, and Lower Derby Lake will be maintained to support aquatic <br />ecosystems. The biological health of the ecosystems will continue w be monitored. <br />Lake-level maintenance or other means of hydraulic containment or plume control will be used m <br />prevent South Plants plums from migrating into the lakes at wacentrations exceedin Cg BSGs in ~- <br />grouadwa[er at the point of discharge. Groundwater monitoring will be used m demonstrate <br />compliance. <br />._~ Confined aquifer wells are monitored in the $ourh_ Plants, Basin A, and Basin F ar_eas_ Specific <br />monitormg we wrQ 6e x etted-during remedial design. <br />• Those monitoring wells installed in the eonfined aquifer that may represent pathways for migration <br />' from rite unconfined aquifer (approximately 30-40 wells) ere closed and sealed; replacement walls <br />will be installed if the Parties jointly de[ermine that specific wells to be closed are necessary for future <br />monitoring. <br />• Chloride and sulfate are expected to anmuase naturally to the CSRGs. t <br />• Monitoring and asxssment of NDMA contamination will be performed in support of design <br />refutemenddesign cha2cterization to azhieve remediation goals specified for the boundary <br />groundwater treatment systems. <br />FOSTER ©MM1IEELER <br />rout)~93G.DOC ro•ran wruaata ewWrorMrirrK CWroawT10M 9-1 <br />6 <br />v <br />