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Predicting Anion Movement in Disturbed and Undisturbed Soils¹

ABSTRACT

Prediction of anion displacement in soils has proven difficult owing to anion exclusion and the occurrence of anion sources or sinks within the soil system. In addition, variations in bulk density and texture with depth cause mean pore water velocity and soil water content to be functions of depth. In this paper a numerical solution of a modified convective diffusion equation, which accounts for apparent anion exclusion and variations in soil water content and mean pore water velocity as functions of depth, is discussed. Moreover, a zero-order rate term is incorporated to account for nitrification occurring in the upper portion of the soil profile.

The model was tested using previously published NO₃ and ³⁶Cl displacement data for disturbed and undisturbed cores of Aberdeen loam. The model predicted ³⁶Cl displacement well. Incorporation of the source term improved prediction of the nitrate breakthrough curve, but agreement of the predicted curve with observed data was only fair. When average values for water content and bulk density were used throughout the entire column, instead of depth dependent values, the description of anion movement was not as good. It is suggested, however, that the improvement in accuracy of prediction of anion displacement may be less than the variation in solute movement observed among replicate soil cores.

¹ Contribution from the Department of Soils, North Dakota State University, Fargo, N.D. 58102. Journal Paper 566.

² Associate Professor, North Carolina State Univ. and former Graduate Research Assistant and Associate Professor, New Mexico State University, respectively. The senior author is presently with the Department of Soil Science, North Carolina State Univ., Raleigh, N. C. 27607.

Received for publication May 20, 1974. Accepted for publication July 28, 1975.

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