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A One-dimensional Model of Chemical Diffusion and Sorption in Saturated Soil and Aquatic Systems¹

ABSTRACT

A one-dimensional model for chemical diffusion in saturated porous media systems is presented. The one-dimensional diffusion model provides for adsorpfion-desorption reactions either Freundlich or Langmuir, and both solid and liquid phase first-order chemical degradation under isothermal, water-saturated conditions. Finite-difference solutions to the model which simulate the one-dimensional diffusion of an inorganic/organic chemical from the sediment of an aquatic system into the overlying water are provided for a variety of initial and boundary conditions. Analogous applications to flow processes within a slructured soil can be made for the diffusion of an inorganic/organic chemical into and out of saturated planar soil aggregates. The initial and boundary conditions are designed to simulate a variety of aquatic situations: ponds, lakes, streams, rivers and oceans. Both explicit and implicit (i.e., Crank-Nicolson) finite-difference schemes are presented for four different saturated systems. The model provides a means of assessing the physicoehemical dynamics of chemical-sediment-water interactions and of assessing the pollution potential of chemicals to plants and animals as determined by their propensity to migrate to different compartments of the aquatic environment. The models can also determine the influence of diffusion processes within planar soil aggregates upon solute flow in saturated structured soils. Various simulations are presented for boron (B), diquat (1,1'-ethylene-2,2'-dipyridilium dibromide), and bromacil (5-bromo-3-sec-butyl-6-methyluracil).

Key Words: movement • finite-difference model • structured soils • soil aggregate

¹ Contribution from the U.S. Salinity Laboratory, USDA-ARS, 4500 Glenwood Drive, Riverside, CA 92501. Author is currently employed at Morrison-Knudsen Engineers, Inc., 1700 Broadway, Suite 1600, Denver, CO 80290.

² Soil Scientist.

Received for publication December 3, 1984.