Conceptual Site Model - Argonne National Laboratory

WVDP Phase 1 CSAP• There is a significant amount of groundwater data for WMA 2 available within ELIMSfrom historical characterization work addressing the North Plateau Groundwater Plume.These data were obtained from monitoring wells and from groundwater samples obtainedby GeoProbe. Figures B.15 and B.16 show the locations of groundwater data collectioncolor-coded by Sr-90 and tritium results, respectively. The detection limits for Sr-90 andtritium varied widely among ELIMS samples. 10 pCi/L and 500 pCi/L were selected asthresholds for Sr-90 and tritium, respectively, because based on ELIMS data resultsabove these activity concentrations were consistently above method detection limits andso indicative of contamination impacts rather than simply measurement error.Groundwater contamination was encountered throughout WMA 2.B.6 Planned Phase 1 DP ActivitiesFor a more complete description of planned Phase 1 DP activities, see the Phase 1 DP, Section3.1 and Section 7.4.In WMA 2, the five lagoons, the Interceptors, the Neutralization Pit, the LLW2 Building, theSolvent Dike, the Maintenance Shop Leach Field, the remaining concrete slabs and foundations,and the underground wastewater lines within the large excavation will all be removed as part ofPhase 1 decommissioning activities. Figures B.17 and B.18 show the location of the largeexcavation and provide a cross-sectional view of the planned excavation, respectively. The largeexcavation is intended to address the bulk of discrete releases and heavy subsurfacecontamination within WMA 2 associated with the lagoons and lagoon support facilities, but notthe deep North Plateau Groundwater Plume that originated from a release underneath the ProcessBuilding and that has migrated into WMA 2’s subsurface.The DP Phase 1 activities will not include the North Plateau Pump and Treat System, the PilotScale Permeable Treatment Wall, the Full-Scale Permeable Treatment Wall, and undergroundlines not within the excavated areas.B.7 Conceptual Site ModelBased on the limited available information, the conceptual site model (CSM) for WMA 2 is asfollows. A number of potential releases within the WMA 2 footprint are known to have occurred,Rev. 1 B-16