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Transport of Exchangeable Cations in an Aggregated Clay Soil

Agronomy Dep., Sturgis Hall, Louisiana State Univ., Baton Rouge, LA 70803

* Corresponding author.

ABSTRACT

The objective of this study was to assess the applicability of the classical and mobile-immobile transport models for describing cation transport in soils. The transport of exchangeable cations for binary and ternary systems in an aggregated clay soil using miscible displacement methods was examined. Packed columns of aggregates of Sharkey soil (Vertic Haplaquept) were saturated with Ca. Pulses of Mg or Mg plus Na were applied to these columns, then eluted with Ca. Chloride was the accompanying anion. These leaching experiments were conducted at constant and variable total solution concentrations. Two pore-water velocities were used in these experiments. Effluent was collected in fractions and analyzed for concentration of cations. Exchange isotherms were determined for the cation pairs Ca-Mg, Ca-Na, and Mg-Na. Experimental leaching data were compared with predicted breakthrough curves obtained using the classical (or one-region) and mobile/immobile water (or two-region) transport models. The two-region model offered no improvement over predictions obtained using the one-region model. The locations of all breakthrough peaks were well described by the models; however, peak heights for Mg breakthroughs were consistently over-estimated. Extension of the model to include the occurrence of CaCl⁺ and MgCl⁺ and adsorption of these complexes did not improve model predictions.

Approved for publication by the Director of the Louisiana Agric. Exp. Stn. as Manuscript no. 89-09-3340. This work was supported in part by Grant no. INT-8713283 of the National Science Foundation.

Received for publication April 29, 1989.