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

Abstract

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.

Solid/liquid partition coefficients to model trace element critical loads for agricultural soils in Canada

“…The definition of λ leaching can be a fairly simple formulation (Baes and Sharp 1983), with one element-specific parameter. That is the solid/liquid partition coefficient (Kd) used to define the resistance to leaching.…”

“…Apart from such empirical support for lognormal, the Central Limit Theorem provides an a priori basis for assuming a lognormal distribution for ratio data such as Kd (Sheppard 2005a). These data for Kd were compared with compendia by Baes and Sharp (1983), Thibault et al (1990) andIAEA (1994). Overall, the soil Kd values obtained in this study were 5.6-fold higher than the compendia values (ranging from 160-fold higher for Fe to 9-fold lower for Zn).…”

A randomized controlled evaluation of absorption of silver with the use of silver alginate (Algidex) patches in very low birth weight (VLBW) infants with central lines

“…The distribution coefficient (Kd: ratio of the concentration in soil to the concentration in water) for silver in a number of soils ranged from 10 to 1,000 (Baes and Sharp 1983). Factors that affect the Kd include soil pH, clay content and particle size distribution, organic matter content, and free iron and manganese oxide content.…”

BLT-EC (Breach, Leach and Transport-Equilibrium Chemistry) data input guide. A computer model for simulating release and coupled geochemical transport of contaminants from a subsurface disposal facility

“…It is often a critical parameter in estimating transport. A number of compilations of values for distribution coefficients in soil systems exist [Baes and Sharp, 1983;Looney et al, 1987;Thbault et al, 19901. For most radionuclides, the compilations typically show a several order of magnitude range in distribution coefficient.…”