Solute movement through two unsaturated soils

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Abstract
Simple and inexpensive apparatus is described for studying the solute movement during unsaturated water flow through intact soil columns. A preset pressure head is maintained at the top and bottom of the soil. The applied solution is easily changed, and there is negligible 'dead volume' both above and beneath the soil. Columns 37-150 mm long, carved from the A horizons of a well structured silt loam and a weakly structured fine sandy loam, were used in the apparatus. After the soil was preleached with calcium sulfate solution, solid potassium nitrate was applied, followed by a solution of potassium chloride. Nitrate and chloride concentrations in the effluent, and in the soil after leaching, were measured. With the imposed pressure heads of between -40 and -150 mm, flow rates ranged from 0 . 5 to 9 mm/h. The convection dispersion equation (CDE) with the appropriate boundary conditions was found to adequately describe the flow of chloride and nitrate in both soils. The outflow concentration data indicated all the soil water participated in the solute transport, and the chloride breakthrough curves scaled with column length as predicted by the CDE rather than by stochastic-convective theory. The dispersivity values for the well structured silt loam were 15 and 19 mm. The values for the weakly structured fine sandy loam were 68 and 27 mm. The dispersivity may be a useful structure index, as well as being the key parameter needed to describe solute movement. The resident concentration data indicated all the water in the well structured soil was mobile, but about 15% of the water in the weakly structured soil was relatively immobile.

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