Conceptual Site Model - Argonne National Laboratory

WVDP Phase 1 CSAPThe deep excavation currently planned for WMA 1 would cover approximately 3 acres. Thelateral extent of the excavation is not expected to deviate significantly from the design (FigureA.18). In fact, along its eastern and northeastern edges, the excavation will be constrained by animpermeable barrier slurry wall; and to the south, west, and northwest, the excavation will beconstrained by sheet piling that will serve as a groundwater barrier wall. In the case of theimpermeable slurry wall, the excavation floor will extend to the wall, and excavation mayactually continue into the wall to address contamination. In the case of the sheet piling,excavation will be sloped up to the sheet piling at an angle of 45 degrees to provide stability.Figure A.18 shows a plan view and cross section of the planned excavation. The slurry wall willremain in place after the WMA 1 excavation is complete and backfilled; however the sheet pilingwalls will be removed after backfilling is complete.Excavation will continue vertically into the Lavery Till at least one foot until cleanup goals havebeen achieved. The Phase 1 DP has specified cleanup goals to be achieved prior to excavationhalting.Beneath the Process Building there are about 476 foundation pilings that extend into, and in somecases through, the Lavery Till. These will be cut below the final excavation grade and left inplace (Figure A.19).A.7 Conceptual Site ModelBased on the available information, the conceptual site model (CSM) for WMA 1 is as follows. Anumber of releases within the WMA 1 footprint are known to have occurred, or may haveoccurred, and likely impacted surface and subsurface soils and groundwater. The most significantof these were the 1968 stack release that produced the cesium prong and the 1968 subsurfacerelease that produced the North Plateau Groundwater Plume. In addition to these two majorreleases, there were several known or suspect releases associated with building infrastructure andprocesses that likely contributed to surface soil, subsurface soil, and groundwater contaminationand that may have resulted in more localized contamination at significant levels.After initiation of the WVDP, portions of the surface of WMA 1 underwent significantreworking. This reworking occurred primarily along the western and northern portions of WMA 1and included the construction of the Vitrification Facility. This reworking may have coveredRev. 1 A-11