A mathematical screening mode! of the pesticide leaching process is used to estimate the potential for a pesticide to reach groundwater at significant concentrations. The model assumes steady water flow, equilibrium linear adsorption, and depth-dependent first-order biodegradation and predicts groundwater travel times and residual concentrations that depend on soil and environmental conditions as well as pesticide adsorption and decay constants. When groundwater protection is expressed as a condition that the residual undegraded pesticide mass remaining below the surface layer of soil must be less than a specified fraction of the initial mass added in a pulse application at the surface, the model prediction is shown to reduce to a linear inequality between the organic C partition coefficient Koc and the biochemical half-life, r. The screening model is illustrated on 50 pesticides and two scenarios representing low and high potential for groundwater contamination. The calculations reveal a significant dependence on sitespecific soil and environmental conditions, suggesting that regulations restricting pesticide use should take soil and management factors as well as chemical properties into account when screening for groundwater pollution potential. Additional index words: Screening model, Chemical transport, Leaching. Jury, W.A., D.D. Focht, and W.J. Farmer. 1987. Evaluation of pesticide groundwater pollution potential from standard indices of soil-chemical adsorption and biodegradation. J. Environ. Qual. 16:422-428. 422

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 (γ‐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.

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In this paper, the organic chemical transport screening model developed in Jury et al. (1983) is simplified by dividing chemicals into volatilization and mobility categories. The volatilization classification is based on whether or not the predominant resistance to volatilization loss lies in the soil or in the boundary layer above the soil surface. This categorization reduces to a condition on the Henry's constant (KH) and organic C partition coefficient (Koc) when standard values are used to represent soil and chemical parameters. The mobility categories are based on the calculated time to convect or diffuse a given distance through the soil.Simulations are conducted for chemicals falling into one or another of these volatilization or mobility categories to examine the sensitivity of these processes to variations in water evaporation, water content, organic C fraction, and boundary layer thickness. The dependence of both volatilization flux and leaching flux on these parameters is summarized.

The soil chemical screening model developed in Jury et al. (1983) is applied to a set of 35 chemicals for which benchmark properties (organic C partition coefficient, vapor pressure, solubility, half‐life) have been obtained. Environmental screening tests are conducted on the chemicals to de{ermine their relative convective mobility, diffusive mobility, volatility, and persistence with the results presented in a series of classifications rating the susceptibility of the chemical to a given loss pathway.The convective mobility tests estimate the time required for a pulse of chemical to travel a distance of 10 cm through an ideal soil of uniform water content and organic C content while being subjected to a water application rate of 1 cm/day. The diffusive mobility tests determine the time required for a chemical to diffuse 10 cm through the same ideal soil. In the volatilization screening tests, each chemical is applied at a uniform concentration of 1 kg/ha to a standard depth in the soil with uniform properties and is allowed to volatilize through a stagnant air boundary layer during a specified time period in the presence or absence of water evaporation. The resulting volatilization fluxes and cumulative losses for a standard time period are used to categorize the relative susceptibility of the chemical to loss to the atmosphere. The persistence tests are used to determine the amount of chemical left after a specified time period when it is free not only to degrade but also to volatilize.

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This study was conducted to reconcile an apparent inconsistency between the simazine laboratory sorption isotherm data and the field lysimeter transport experiment reported by Poletika et al. (this issue). In this investigation, linear and nonlinear one-and two-stage simazine sorption models were fitted to the sorption and desorption isotherm laboratory data to obtain parameter estimates for use in the transport model. Once obtained, the calibrated sorption model was combined with the parameterized outflow concentration record from a mobile Br tracer to represent rate-limited sorption and transport of the simazine added simultaneously with the Br. The calil•rated model did an excellent job of representing the final simazine profile in the soil, particularly with the nonlinear model. This is in contrast to a single-stage adsorption model tested by Poletika et al. (this issue), which reached poor agreement with the field profile when laboratorymeasured sorption parameters were used. The results demonstrate the compatibility of field and laboratory experiments on pesticide movement and also indicate that sorption isotherms may require substantially longer to reach equilibrium than is customarily allowed in current protocols. Pignatello, J. J., Sorption dynamics of organic compounds in soils and sediments, in Reactions and Movement of Organic Chemicals in Soils, edited by B. L. Sawhney and K. Brown, pp. 31-44, Soil Science Society of America, Madison, Wisc., 1989. Poletika, N., and W. A. Jury, Effects of soil surface management on water flow distribution and solute dispersion, Soil Sci. Soc. Am. J., 58, 999-1006, 1994. Poletika, N., W. A. Jury, and M. L. Yates, Transport of bromide, simazine, and MS-2 coliphage in a lysimeter containing undisturbed, unsaturated soil, Water Resour. Res., this issue. Press, W. H., B. P. Flannery, S. A. Teukolsky, and W. T. Vetterling, Numerical Recipes in C, The Art of Scientific Computing, Cambridge University Press, New York, 1988. Rao, P.S. C., and J. M. Davidson, Estimation of pesticide retention and transformation parameters required in nonpoint source pollution models, in Environmental Impact of Nonpoint Source Pollution, edited by M. R. Overcash and J. M. Davidson, pp. 23-67, Butterworth, Stoneham, Mass., 1980. Seber, G. A. F., and C. J. Wild, Nonlinear Regression, John Wiley, New York, 1989. Selim, H. M., J. M. Davidson, and R. S. Mansell, Evaluation of a two-site adsorption-desorption model for describing solute transport in soils, paper presented at Summer Computer Simulation Conference, Simul. Counc., Washington, D.C., 1976. Susarla, S., G. V. Bhaskar, and S. M. R. Bhamidimarri, Adsorptiondesorption characteristics of some phenoxyacetic acids and chlorophenols in a volcanic soil, I, Equilibrium and kinetics, Environ. Technol. Lett., 14, 159-166, 1993. Swanson, R. A., and G. R. Dutt, Chemical and physical processes that affect atrazine and distribution in soil systems, Soil Sci. Soc. Am. Proc., 37, 872-876, 1973. Talbert, R. E., and O. H. Fletchall, The adsorption of some s-t...

Picloram (4‐amino‐3,5,6‐trichloropicolinic acid) adsorption‐desorption isotherms were derermined on six western USA soils. Adsorption isotherms were determined as a function of temperature, soil/solution ratio, and solution ionic strength. Special attention was given to the effects of pH and organic matter content in the analysis of the result. Adsorption increased with increasing organic matter with soils varying in organic matter content from 0.94% to 4.2%. Variations in adsorption between soils were not correlated with clay content or soil pH in the pH range 5.6 to 7.4, but increased adsorption resulted with decreasing solution pH of any individual soil. Sorption was described by the Freundlich isotherm at equilibrium concentrations less than 20 µg/ml. Temperature had only a slight effect on adsorption by the three soils examined. Increasing temperature from 10 to 20 to 30C generally resulted in decreased adsorption. Increasing the soil/solution ratio from 1:5 to 1:2 increased the Freundlich k value for the five soils tested. Freundlich k values were higher for the desorption isotherms than for the adsorption isotherms for all six soils. Increased adsorption with increasing ionic strength was predictable from associated pH changes and the dissociation constant for picloram.