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The Upper Limit of Solute Dispersion and Soil Hydraulic Properties

Dep. of Soil Science, Massey Univ., Palmerston North, New Zealand

P. J. Ross

CSIRO, Division of Soils, Private Bag, P.O Aitkenvale, Australia 4814

*Corresponding author.

ABSTRACT

Current theory inadequately describes the effects on solute movement of local variations in pore-water velocity. A satisfactory theory for solute flow in structured soil has yet to be developed. As a possible starting point for such a development, we present a simple analytical description of the maximum amount of solute dispersion that can occur during steady water flow in a vertically uniform soil. The distribution of soil water velocities is found from the derivative of the unsaturated hydraulic conductivity-water content relationship. With just this information, given the water flux and solute boundary conditions, the solute distribution can be described, assuming no mixing between water (and solute) flowing at different velocities in the soil. The analytical solution describes not only the upper limit of the dispersion that can occur, but also the maximum depth to which the leading edge of a surface-applied pollutant can travel, given a steady water flux for a certain time. It is shown that the steeper the hydraulic conductivity curve, the greater the expected difference between the soil solution flux concentration (measured with a lysimeter) and resident concentration (measured by soil sampling). Transverse solute diffusion between water moving at different velocities reduces the dispersion below the upper limit described here. Incorporating a description of such diffusion into the approach given here may be a fruitful alternative to considering dispersion as enhanced longitudinal diffusion in soil water moving at a uniform velocity.

Received for publication March 12, 1993.

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