Behavior Assessment Model for Trace Organics in Soil: I. Model Description

A mathematical model is introduced for describing transport and loss of soil-applied organic chemicals. The model assumes linear, equilibrium partitioning between vapor, liquid, and adsorbed chemical phases, net first order degradation, and chemical movement to the atmosphere by volatilization loss through a stagnant air boundary layer at the soil surface. From these assumptions and the assumption of steady state upward or downward water flow, an analytic solution is derived for chemical concentration and volatilization flux. This model, which is intended to classify and screen organic chemicals for their relative susceptibility to different loss pathways (volatilization, leaching, degradation) in the soil and air, requires knowledge of the organic carbon partition coefficient (Koc), Henry's constant (KH), and net, first-order degradation rate coefficient or chemical half-life to use on a given chemical. Illustration of the outputs available with the model is shown for two pesticides, lindane (7-1,2,3,4,5,6-hexachlorocyclohexane) and 2,4-D [(2,4-dichlorophenoxy)acetic acid], which have widely differing chemical properties. Lindane, with a large Koc, large KH, and small degradation rate coefficient, is shown to be relatively immobile, persistent, and susceptible to volatilization. 2,4-D, with a small Koc, small KH, and large degradation rate coefficient, is mobile and degrades rapidly, but is only slightly susceptible to losses by volatilization. Additional Index Words: pesticide, chemical movement, volatilization, diffusion, leaching. Jury, W. A., W. F. Spencer, and W. J. Farmer. 1983. Behavior assessment model for trace organics in soil: I. Model description. J. Environ. Qual. 12:558-564. Beginning with DOT in the late 1960's, several established pesticides with effective weed or insect control were removed from the market because of undesirable 1 Contribution of Dep. of Soil and Environmental Sciences, University of California, Riverside, CA 92521 and USDA, Riverside. Received 10 Sept. 1982. 'Professor of Soil Physics, Univ. of California-Riverside; Soil Scientist, USDA; and Professor of Soil Science, Univ. of CaliforniaRiverside, respectively. environmental characteristics. Unfortunately, these undesirable characteristics (such as excessive mobility, persistence, or volatility) were determined or observed only after the chemical had been widely used. Because a certain amount of mobility and persistence is essential for proper management and performance of a pesticide, its pollution potential can only be minimized subject to its effectiveness for weed or insect control. For this reason, it is clear that environmental screening tests are needed at the time of development of the chemical when screening for toxicity is being performed. It is equally clear that this screening procedure cannot involve excessive experimentation, because of the massive numbers of chemicals involved. Instead, what it needed is a model that is able to make predictions of behavior of one chemical relative to another from a standard set of easily obtainable chemical benchmark properties. Ideally, the result of this screening procedure would be a classification of large numbers of chemicals into a smaller number of groups whose members display similar behavior. From these groups could be selected prototypes for more extensive experimentation under natural conditions. Pesticide simulation models are not new. Lindstrom et al. (1968) proposed a mathematical model for describing leaching of pesticides through soil columns. Oddson et al. (1970) and Davidson and McDougal (1973) reported a theoretical leaching model for use with chemicals whose solid-liquid adsorption was linear, but for which the liquid and solid phases were not in equilibrium. Van Genuchten and coworkers, in a series of papers (Van Genuchten et al., 1974, 1977; Van Genuchten & Wierenga, 1976, 1977), described a model for predicting pesticide movement in soil, including effects of pore bypass and hysteresis in the adsorption isotherm, and validated the model in soil column experiments using 2,4,5-T herbicide. They also investigated the effect of nonequilibrium adsorption. Leistra, in a series of papers (Leistra, 1973, 1978, 1979), has devel5S8 J. Environ. Qual., Vol. 12, no. 4,1983 Published October, 1983