be 10° 7m. presumably caused by the migration of However, the total inte~ the initial surface-deposited material grated air activity (from t = 0 to o& for into the soil, 239 py would be changed only by Attempts to use the resuspension wm AX 104 [ exp (- 0, 693t/45d) dt factor approach to derive acceptable Oo levels of soil surface coniamination have included this "attenuation factor" as a exp (- 0, 693t/24, 400y) dt oO simple exponential function with half- times of 35 or 45 days”? 4 . eo +axio!? _= 6.5AX10°+1,3AX10°%, -3 -3 There are major uncertainties in such a formulation, which is an increase of 20%, and more however, importantly, cannot be accumulated dur- The longest study of this de- crease with time extended to only11 mo ing an individual's life span, Because the functional nature of the following the initial deposition®, which is extremely short compared to the half- decrease in resuspension rate with time life of a radionuclide such as 239 pu, cannot be confidently extrapolated, pre- There are also published reports which viously published models should not be indicate on experimental and theoretical applied to the reoccupation of areas many bases that the decrease with time will years after the contaminating event. The resuspension-factor approach can not be adequately represented by a single exponential function, but that the rate of be applied in an approximate way, how- decrease itself will also decrease with ever, if resuspension factors are used Fortunately, the exact nature time!? 6 which were derived from measurements of this time dependence is not critical in over aged sources, determining the integrated exposure from relevant data are unpublished results the time of initial deposition due to the from current resuspension experiments fairly well-documented rapid decrease at at the GMX site in Area 5 of the Nevada early times, Test Site. However, it is obviously Perhaps the most The 239 Pu at this location the controlling factor for questions con- was deposited following 22 high-explosive cerning the reoccupation of areas many detonations during the period from years after the contaminating event. December 1954 to February 1956, As an illustration, the most conserva- Measurements of resuspended air activity tive published model (Kathren?) may be levels at this site curing 1971-1973 used to calculate a resuspension rate for appear to be the only available data con- material 15 yr aiter deposition: cerning resuspension of 2395, from a K-10 -4 m -n -.. _ ony ( 0.693 X liv X 365d ‘ 45d Vv \ source of this age, Data from two types or measurements } ~ 10° 74/m, are available and can be used to derive average resuspension factors, The first If, however, the resuspension rate tvpe of measurement? was accomplished asymptotically approached some finite by placing five high-volume cascade value 19° of the original, then the resus- impactors © within the most highly con- pension rate 15 yr later would obviously taminated area, and running them for I-17