moisture content as important soil factors; and cover, roughness, obstructions, temperature, and topographic features as important ground factors in soil The structure of the soil in turn is affected by organic matter, resuspension. lime content, and texture. Although from the standpoint of inhalation the fine sizes (generally limited te the less than 7 pm diameter at 1 g/cm? density) are considered most hazardous, Chepil (1945a) the coarser soil particles cannot be ignored in resuspension. reported that the resuspension of fine dust in an air current is mainly the Saltation occurs mainly in result of movement of larger grains in saltation. He also found that single grains grain size of 100 to 500 um (Chepil, 1945b). or aggregates greater than 3 mm in diameter were not moved by wind velocity of 30 miles per hour at 6-inch height; and that in soils containing a range of particle sizes, the surface became covered with a bed of aggregates too coarse to be moved by the wind as the finer sizes eroded. At the Nevada Test Site (NTS), the occurrence of gravels (2 mm to 3 inches in diameter) is common as a surface cover; and they serve to stabilize the surface from wind erosion. If any decision is made to remove the contaminated soil such as at NTS, considerations must be given to minimize the erosion of the new surface which has yet to become stabilized. At Rocky Flats (RF) in Colorado, the soil surface is covered with varying amounts of grassy vegetation. The resuspension potential of the soil is influenced by the "protection" afforded the surface (Chepil, 1945a). These examples are cited to emphasize the importance of those factors which were not considered in developing the index. On the other hand, when reliable resuspension factors are determined for the contaminated soils from the different sites, the index may be used to evaluate the potential hazard by way of the inhalation pathway. Three considerations were included in developing the soil plutonium factor. The first considers the concentration of plutonium relative to the soil particle size concentration; the second considers the depositional character of the soil-associated plutonium in the pulmonary compartment of the respiratory tract; and the third factors the fraction of the total soil plutonium in the resuspendible size fractions. Two soil properties associated with plutonium and which have not been factored into the index should be mentioned since they can greatly modify the final index. One is the erodibility of a soil which is dependent on several properties including soil structure, specific gravity, and moisture content. The other is the potential solubility of the plutonium associated with the soil particles. Inclusion of these properties into the derivation of the index must await results of further research. The derived soil plutonium index (SI) is the product of the soil activity factor (SA), lung depositional factor (LD), and the resuspendible activity factor (RA). Thus one can define it as: SI = SAX LD X RA In the following sections, each of the factors is discussed and examples of the factors derived for samples from the four sites are given. sites include (ML), and the be considered each site was These four the Nevada Test Site (NTS), Rocky Flats (RF), Mound Laboratory Oak Ridge National Laboratory (ORNL). The examples given should as preliminary since the number of samples investigated from limited. Soil particles which undergo resuspension are those Soil Activity Factor. Particles greater than 100 ym move by saltation less than 100 ym in diameter. and creep; Chepil (1945b) reports that particles between 100 to 500 ym move mainly by saltation. Of the particles less than 100 pm in diameter, a further distinction is made for particles less than 10 - 7 ym mass mean aerodynamic These latter sizes are based on the diameter (mmad) (Volchok et al., 1972a). In terms aerodynamic diameter which is based on particle density of 1 g/cm3. of silicate mineral density, the diameters would correspond to 6 - 4 ym. commonly are Particles less than 10 — 7 mass mean aerodynamics diameter (mmad) referred to as respirable particles. This delineation was made to segregate that portion of the particles which deposits in the nonciliated part of the lung (Volchok et al., 1972a). As discussed above, several investigators have made soil particle size segrega— tions and determined the plutonium in the different size fractions (Tamura, 1976b; Muller and Sprugel, 1977; Bernhardt, 1976). In terms of potential resuspension, the higher the plutonium activity in size fraction relative to the fractional mass of the soil particles, the higher the activity of the particles when resuspended. For example, a soil with 1% of the soil mass in the less than 2 um size and 1C% of the activity associated with 1t will have a higher activity per unit mass in the air than a soil with 1% mass of < 2 um particles containing 1% of the total soil activity. Conversely, a soil with 10% of the soil mass in the < 2 um fraction and 1% of the activity will have less activity in the air when equally resuspended than a soil with 10% soil mass and 10% of the activity. To obtain the relative activity as distributed in the soil sizes, the activity fraction is divided by the mass fraction and identified as the soil activity factor. . The different particle size ranges of the resuspendible fractions and the fractions of the activity associated with the respective size fractions are given in Table 5. The size ranges are derived in terms of the silicate density for the 5 - 2 ym and the less than 2 pm sizes. Since the sieve used to segregate the particles had 125 um openings, the upper limit is approximately 25 um larger than 100 um normally considered. The ratio of the activity to the mass is defined as the soil activity factor (Table 5). The sum of the soil activity factor ranges from 3.14 for the sample from Oak Ridge National Laboratory (ORNL) to 7.27 for Rocky Flats (RF). The Nevada Test Site (NTS) sample and the Mound Laboratory (ML) sample have values of 4.27 and 4.62, respectively. Note, however, that the ML sample is assumed to have all particles less than 125 um. Data from Muller and Sprugel (1977) reported particle sizes up to 45 ym, and the larger particles were reported as being greater than 45 ym. If the soil activity factor for ML is limited co particles between 4-45 ym, the factor is 0.64 for that size range. Another interesting observation is the distribution of plutonium in the less than 2 pm fraction of NTS and RF. Although only 3% of the activity is in this fraction of the NTIS sample and 28% in the RF sample; the soil activity factor is only 3 times larger in the RF sample rather than 10 times based only on activity distribution. Thus if a particle in this size range is resuspended, then the particle from RF will have 3 times the activity as the same size particle from NTS. If the soil activity factor alone is considered, then the 107