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Solute Discharge During Steady Water Drainage from a Sand Tank¹

ABSTRACT

An alternating direction implicit (ADI) numerical method was used to simulate transient solute movement resulting from a step-function increase of solute concentration in water applied to a shallow sand tank system with a subsurface drain. Steady water flow in the saturated portion of the tank was simulated using an iterative alternating direction implicit (IADI) numerical method. The saturated-unsaturated condition in the actual sand tank was approximated by a smaller rectangular flow region of saturation in the model simulation. Isopleths of hydraulic head and solute concentration within the soil demonstrated a decrease in the rate of infiltration from the point vertically above the drain to the point vertically above the midpoint between the drains. Consequently, solute transport was much faster in the surface soil overlying the drain than in the soil near the mid-point between drains. When cumulative effluent volume was expressed as the number of pore volumes, breakthrough curves for solute concentration in the drainage water revealed early arrival of solute compared to a one-dimensional system. In spite of the different boundary conditions imposed at the soil surface in the tank and in the model, reasonable agreement was obtained between simulated breakthrough curves and experimental data from the sand tank.

¹ Contribution from the Univ. of Florida, Gainesville. Published as Journal Series #6167.

² The first and third authors are Professor and formerly Laboratory Technologist, respectively, in the Soil Science Dep. at the Univ. of Florida. The second authors is Associate Professor in the Mathematics Dep.

Received for publication April 30, 1984. Accepted for publication January 3, 1985.