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Ion Exchange and Diffusive Mass Transfer During Miscible Displacement Through an Aggregated Oxisol¹

ABSTRACT

Effluent breakthrough curves (BTCs) for four tracers (³H₂O, ¹⁴CH₃OH, ³⁶Cl, and ⁴⁵Ca) were measured over a wide range of porewater velocities (3 to 164 cm/hour) using water-saturated columns of Ione soil, an aggregated Oxisol. Increasing asymmetry of the measured BTCs for all tracers with increasing pore-water velocity was attributed to decreasing residence time for solute transfer by diffusion between inter- and intra-aggregate regions. The measured BTCs for ³⁶Cl and ⁴⁵Ca illustrated the role played by the pH-dependent soil surface charge in determining the retention and transport of ionic species.

Measured BTCs were compared with those calculated using a simulation model where (i) convective-dispersive solute flow was limited to the inter-aggregate regions, (ii) solute transfer into and out of equivalent spherical aggregates was described by Fick's second law of diffusion, and (iii) isotopic exchange was considered to be an instantaneous reversible process and was described with a linear exchange isotherm. For the slightly sorbed solutes (³H₂O and ¹⁴CH₃OH), all model parameters were independently estimated. Owing to the strong pH-dependence of isotopic exchange and the inability to exactly duplicate in batch experiments the chemical environment (pH and ionic strength) of the soil columns, exchange coefficients (K_D) for ³⁶Cl and ⁴⁵Ca were estimated by curve-fitting the model to BTCs measured at the low pore-water velocity (3 cm/hour). These K_D values and all other independently estimated parameters were then used to simulate ³⁶Cl and ⁴⁵Ca BTCs for the high pore-water velocity (120 cm/hour). Good agreement was found between measured and calculated BTCs for all tracers. These data and simulations demonstrated the coupling between physical processes (convective-dispersive flow, diffusive mass transfer) and chemical processes (ion exchange, pH-dependent charge) in determining the shapes and positions of BTCs obtained from water-saturated aggregated soils with a variable surface charge.

¹ Contribution from the Soil Science Dep., Univ. of Florida. Florida Agric. Exp. Stn. Journal Series no. 3770. Presented before S-1 at the American Society of Agronomy meetings, Detroit, Mich., on 1 Dec. 1980.

² Research Associate, Assistant Professor, Scientific Programmer, and Professor, respectively, Soil Science Dep., Univ. of Florida, Gainesville, FL 32611.

Received for publication May 22, 1981. Accepted for publication July 30, 1981.

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