Conceptual Site Model - Argonne National Laboratory

WVDP Phase 1 CSAPThe limited Cs-137 data that are available are, in general, consistent with the historicaldirect reading data for this area. The same spatial pattern observed in direct readings forErdman Brook is replicated in sediment samples, with the highest Cs-137 activityconcentrations occurring at the northern fence line and coincident with the highest directreadings. Erdman Brook sediment Cs-137 activity concentrations drop as one moves upthe stream channel, mirroring the direct exposure readings.• A hot spot was identified in 1990 about 75 feet below the confluence of Erdman Brookand Franks Creek. As described in the footnote on page 4-49 of the Phase 1 DP, in 1990,a sample from a hot spot in Erdman Brook that measured 3000 μR/h during the groundlevelsurvey showed 0.01 μCi/g (10,000 pCi/g) Cs-137. This was a screening analysisthat may have been performed on a wet sample; it was not validated. This area oflocalized contamination was described as about six inches by six inches located onemeter from the edge of the water. Limited investigation indicated that the contaminationextended more than seven inches below the streambed surface (WVDP-EIS-007,Appendix C).• There are sixteen historical groundwater sampling locations within WMA 12 South(Figure J.11) that have historical sample results within ELIMS. These include eight alongthe WMA 2 border on the north side of WMA 12 South, and another six along the WMA7/WMA 8 border on the south side of WMA 12 South. 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.All of the shallow groundwater sampling locations along the WMA 2 boundary showtritium impacts, and most show low levels of Sr-90 as well (although not all wereanalyzed for Sr-90). In general, Sr-90 activity concentrations increase as one moves eastalong the border. Likely sources of this contamination are the lagoons and the NorthPlateau Groundwater Plume. Along the southern boundary, one well, WNNDADR, hadelevated Sr-90 impacts. Shallow groundwater movement within WMA 12 South is likelycontrolled by surface topography and Franks Creek and Erdman Brook. In general oneRev. 1 J-6