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Miscible Displacement of Gases Through Soil Columns¹

ABSTRACT

The mixing that occurs as one gas displaces another within columns of soil or glass beads was examined by using O₂, He, and air for selected combinations of gaseous flow rates and column lengths. After columns were initially flushed with air or He, a second gas pumped at a constant flow rate was introduced at one end of the column. The concentration of the second gas in the gaseous effluent at the opposite end of the column was monitored during the displacement process. Elution curves (concentration of second gas versus volume of effluent) were analyzed by using a mathematical model that includes mixing by molecular diffusion, mass transport at a constant velocity, and sorption (interaction of gas with soil particle surfaces and solution of gas in water present in air-dry soil) by a linear sorption isotherm. Although apparent diffusion coefficient values were not constant with flow velocity, the general behavior of the mixing process was adequately described by the model. Difficulties stemming from the varying gas velocity from point to point within the porous media are discussed in light of previous miscible displacement studies with liquids.

¹ Contribution from the Dep. of Agronomy, Iowa State University and the Dep. of Water Science and Engineering, University of California, Davis. Journal Paper no. J-6101 of the Iowa Agr. & Home Econ. Exp Sta., Ames, Iowa. Project 1235.

² Research Assistant and Professor of Soils and Physics, Iowa State University, and Professor of Water Science, University of California. Senior author is now Laboratory Technician, Dep. of Water Sci. & Eng., University of California, Davis.

Received for publication December 10, 1968. Accepted for publication February 21, 1969.