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Predicted Distribution of Organic Chemicals in Solution and Adsorbed as a Function of Position and Time for Various Chemical and Soil Properties¹

ABSTRACT

Equations were developed to describe the mass transfer of organic chemicals through soil and evaluated for various soil conditions. Movement due to diffusion was assumed to be negligible. The model assumed the relationship S/t = (Kc – S) where S is the adsorbed concentration [mass per total volume], c is solution concentration [mass per total volume], t is time, and K and are constants. The model also considered the effect of applying various amounts of chemical to the soil surface and allowed for a prior adsorbed concentration in the soil ahead of the wetting front. The following are conclusions drawn for the case when the soil is initially free of organic chemical.

K influences the depth of maximum concentration of organic in solution but does not affect the value of that concentration. The organic chemical will move in solution as a wave through the soil. The lower K the more spread out the wave will be. The depth of movement of maximum concentration is equal to the depth of water penetration divided by K.

The concentration of material adsorbed on the soil also moves down as a wave. The position of maximum adsorbed concentration is about the same as for the maximum concentration in solution. Increasing K causes an increase in concentration of adsorbed material.

Application of greater amounts of organic chemicals to the soil surface has the effect of increasing the concentration of organic both in solution and adsorbed but does not influence the depth of movement greatly except at initial time periods.

Increasing the value of has the effect of making the wave of chemical moving through the soil narrower and increasing the concentration of the organic in solution as it moves through the profile as compared to a lower value of .

¹ Contribution of the Departments of Mathematics and Soils and Plant Nutrition, University of California, Riverside. The study was supported by matching fund project B-072-CAL of the Office of Water Resources Research, United States Department of the Interior as authorized under the Water Resources Research Act of 1964 and implemented by the University of California Water Resources Center as Project W-206.

² Associate Professor of Mathematics, Professor of Soil Physics, and Laboratory Technician, respectively.

Received for publication August 22, 1969. Accepted for publication February 17, 1970.