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Leaching of a Surface Layer of Sodium Chloride into Tile Drains in a Sand-Tank Model¹

ABSTRACT

A series of experiments was conducted to study the leaching of NaCl through tile drains from a surface layer of salinized sand in a sand-tank model. The effects of two recharge rates, three salinities in the salted-sand layer, and three drain spacings were evaluated. Each experiment was conducted with a steady-state, arch-shaped water table maintained by recharge from a battery of capillary tubes that gave a uniform infiltration over the model. All the salt was leached from the model. The salt was dyed blue so that the leaching front could be photographed. The salinity of the tank solution and of the drainage water was measured at regular intervals. Intense fingering developed in all experiments in which a salinized layer was used. Fingering caused a rapid and erratic redistribution of salt within the model and an erratic variation in the salinity of the drainage water until about 1 pore volume had drained. The higher the initial sand salinity, the more intense was the fingering, the longer were the fingers, and the higher were the finger velocities. The behavior of the fingering transition zone was controlled by the interaction between the density gradients and the hydraulic gradient; this latter depends on the recharge rate and the drain spacing. The salinity of the outflow drainage water rose quickly to a peak and then declined almost as quickly for a short period, after which it reduced more gradually. After about 1 pore volume had drained, the salinity of the drainage water declined exponentially in all salinized experiments. Experimental curves fitted closely to an exponential relationship except initially and where fingering resulted in erratic variations.

¹ Journal Paper no. J-6886 of the Iowa Agr. & Home Economics Exp. Sta., Ames. Project no. 1888. This research was supported chiefly by the W. K. Kellogg Foundation; the Agricultural Institute, Dublin, Ireland; and in part by the US AEC Contract AT(11-1)-1269, report no. COO-1269-25.

² Formerly Kellogg Fellow, Iowa State Univ., Ames; now at Agricultural Institute, Ballinamore, Co. Leitrim, Ireland.

³ Professor of Soils and Physics, Iowa State Univ., Ames 50010.