A Proposal for Estimation of Soil Leaching and Leaching Constants for Use in Assessment Models

Abstract

AbstractA 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.