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

Abstract

Abstract 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 ( K oc ), Henry's constant ( K H ), 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 K oc , large K H , and small degradation rate coefficient, is shown to be relatively immobile, persistent, and susceptible to volatilization. 2,4‐D, with a small K oc , small K H , and large degradation rate coefficient, is mobile and degrades rapidly, but is only slightly susceptible to losses by volatilization.