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Hydrodynamic Dispersion of Solutes in the Soil Moisture Stream¹

ABSTRACT

The soil moisture stream is characterized, even in the simplest cases, by a heterogeneous velocity pattern that affects the movement of dissolved solutes. The solutes are carried by the moving stream at diverse rates, causing a spreading action whose amount depends upon the detailed pattern of the flow.

The amount of dispersion (_D) in unidirectional flow is proportional to the square root of the linear displacement of fluid. The proportionality constant is a characteristic of the flow system and can be determined experimentally. The amount of dispersion obtained for a given amount of linear displacement is theoretically independent of the velocity of flow. Experiments supporting this conclusion are cited.

The experiments with ion exchange resin described herein indicate that the free ions are translocated in accordance with Scheidegger's probability theory. One concludes that the movement of dissolved solutes in the soil moisture stream cannot be determined adequately from the average fluid velocity unless the hydrodynamic dispersion effect is taken into account.

¹ Contribution from Department of Soils and Plant Nutrition, University of California, Berkeley. Presented before Div. I, Soil Science Society of America, Nov. 15, 1956 at Cincinnati, Ohio.

² Associate Professor of Soil Physics and graduate student, respectively.

Received for publication November 23, 1956. Accepted for publication May 10, 1957.