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A model to predict annual‐average, order‐of‐magnitude leaching constants for solutes in agricultural soils is given. The leaching constants, specific by geographic location and solute, are for use in exposure, dose, or risk assessment models to account for removal from the root zone via leaching in a manner analogous to radiological decay. The model presented here relates annual‐average water infiltration (Vw) with soil bulk density (ϱ) and porosity (θ) and the distribution coefficient (Kd) for the solute. Annual infiltration is determined from site‐specific estimates of total precipitation (P), irrigation (I), and evapotranspiration (E). A review, discussion, and determination of generic default model input parameters is included. The parameters ϱ and θ vary within a factor of three, and are lognormally distributed with estimated geometric mean values of 1.35 g cm−3 and 0.48 cm3 cm−3, respectively. Geographic distributions of U.S. county‐averaged P, I, and E were determined from historical weather station records, the 1974 Agricultural Census, and an evapotranspiration model, respectively. Variability in site‐specific annual‐average estimates of these parameters is expected to be within a factor of three, also. The most variable and unpredictable parameter is Kd, which may range from one to many orders of magnitude, depending on solute and soil characteristics, including pH. Estimated distributions of Kd for 27 elements in soils 4.5 ≤ pH ≤ 9.0, and correlations between Kd and pH for Cu, Zn, Cd, and Pb are also included. Finally, eight comparisons of model predictions with leaching constants, determined directly from observed data, are made for Pu, Sr, and Tc. In all comparisons, except one study of Pu, the model predicted leaching constants within an order of magnitude of observed values. For Pu the model underpredicted, and for Sr and Tc the model overpredicted leaching rates.

Concentration ratios of aluminum to calcium, magnesium, and other divalent cations in increment cores obtained from red spruce and eastern hemlock trees growing in the Great Smoky Mountains National Park in Tennessee and North Carolina have increased in an unprecedented manner during the last 15–40 years. These trends, which also occur in other trees growing in eastern North America and Germany may reflect the mobilization of exchangeable aluminum by [Formula: see text] and [Formula: see text] deposition. The soil chemistry and plant physiology bases for this hypothesis are presented. Many of the spruce and hemlock cores from the Great Smoky Mountains also showed an inverse relationship between radial growth and aluminium:alcium ratios in the wood. Changes in the availability of soil cations, as recorded in tree rings, may provide an ecosystem-level method of evaluating the historical response of forest soils to atmospheric deposition.

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Work en the model evaluation proj'ect is continuing at this time. As this work proceeds, modification of the recommendations presented in this report may be made to reflect the latest ii'ndings of the project staff. L Xll 1. mathematically simple and flexible, 2. in accord with much, but not a'l, diffusion theory, and 3. a reliable framework for the correlation of both field studies and mathematical and physical modeling studies of atmospheric diffusion. 2.1.1.1 Dispersion parameters. It has been found that the dispersi parameters, s and s , need to be carefully specified when using the Gaussian plume model. ' A number of empirically determined graphs of s and s as a function of downwind distance and atmospheric stability 8 9 have been proposed. GeiB et al. have published a useful set of com parisons between some of these sets of graphs.