transuranics in environmental soils is lacking. on transuranic chemistry Although the literature in soils was not discussed, many of the papers reviewed either hypothesized, concluded, or in isolated cases measured the fact that plutonium had migrated into the soils and probably was in particulate form, either as plutonium oxide particles, hydrated plutonium oxide colloids, or plutonium particles aggregated to soil parti- cles. particles. However, the migration of plutonium as soluble complexes, or in conjunction with soil particles also occurs to a sufficient degree, and some plutonium does migrate rapidly and to great depths, as evidenced by the ground water study at Enewetak. Close examination of many of the published profiles indicates that the true front of plutonium migration was not observed due either to inThe sufficient depth of sampling or inadequate detection capability. Even though the plutonium distributions in soil are quite variable, there has been an attempt at a rough categorization. It appears that irrespective of the source, plutonium profiles in soil can be categorized into three groups: normal, disturbed, and abnormal. Normal profiles exhibit no obvious profile development and the plutonium distribution appears to reflect a complex exponential function. This type of profile is more likely to be associated with an undisturbed soil. Only a limited number of observed plutonium profiles can be categorized as normal. The disturbed plutonium profiles result from activities associated with construction, burrowing animals, water and wind erosion, and redeposition after the initial deposition of plutonium, The plutonium distributions in these disturbed soils are quite varied. a uniform distribution of plutonium with depth, samples were collected, as was They might show that is, as deep as found on beaches and open areas of implication is that a small amount of plutonium may have migrated to greater depths than those measured. The few inferences that can be drawn from observations of the duration of migration suggest that for a short period of time after deposition onto soil there is a rapid penetration of plutonium into the soil. In- dications are that a smal) amount of plutonium moves rapidly, and to some depth, into the soil as though the plutonium were complexed and relatively uneffected by the attractive forces of soil particles. For large concentrations of freshly deposited plutonium this effect may ultimately present a significant environmental hazard. Studies should be conducted to assess the magnitude of the possible mechanisms involved. The turnover rate of fallout plutonium has been calculated to be 5-6 years for each 5 cm of depth giving rise to the exponential depth proinfiles in undisturbed soils. This observation appears to be somewhat dependent of the total rainfall of the area. Enewetak and Bikini Atolls, or in a desert wash area where runoff from intermittent but severe rainstorms actively mix the top layers of material. Many of the early scil profiles were collected by coring techniques Abnormal plutonium distributions are generally unexplainable and a problem was recognized and much of the later work used techniques (such variable. as the "trench method") which provided some control over that series of hypotheses have been used to suggest the possible mechanisms responsible for individual observed distributions. These possibilities range from faulty analyses of some fraction (or fractions) to unidentified mechanical disturbances, to the physical and chemical interactions of plutonium with the soil constituents. Understanding of the abnormal distributions may be a key chemical predicting which may have introduced a cross-contamination variable, However, this higher-than-real concentrations of plutonium at depth, the coring One cannot argue at this point with definitive data that remains method is faulty, only that the variable does exist and unanswered. to describing the physical and/or interactions effective in the redistribution of plutonium or transuranic nuclides within soils. For instance, the accumulation of ACKNOWLEDGEMENT Plutonium in specific soil zones which are related to soil forming factors, nized. such as A and B horizons or carbonate The filtering action of specific soil lenses, horizons has been recogfor small colloids containing plutonium or plutonium particles, and the potentially different chemistry among the factors zones, for plutonium migration rates may be the long term governing in soils. The authors are aware of no study currently in progress to address these problem areas. The authors wish to thank Mr. T. Ensminger, Early in the history of plutonium distribution studies, it was thought that plutonium was so tightly bound to soil, that plutonium could not penetrate into the soil in the short time that has elapsed since first introduction into the soil. of soluble ionic plutonium since its This may be true for the migration it may be 70 tightly bound by Oak Ridge National Labora- tory (ORNL) for his assistance in querying the Nevada Applied Ecology on of Information Data Base, for references dealing with the distributi ‘ transuranic nuclides in soils. soil 71