Mass Transfer Studies in Sorbing Porous Media: II. Experimental Evaluation with Tritium (3H2O)1

A comparison is made between observed tritium effluent concentration distributions and those calculated with a previously published analytical solution for the movement of chemicals through unsaturated, aggregated sorbing media. In the analytical model the liquid phase of the soil is divided into mobile and immobile regions, with transfer between the two regions diffusion controlled. Effluent data obtained from several displacements of tritium through 30-cm long columns of Glendale clay loam were used to determine the different parameters in the analytical solution by curve fitting. The data indicate some adsorption or isotopic exchange of tritium during its flow through the soil columns. The amount of immobile water increases with decreasing flow velocity and increasing aggregate size, and varies between 6 and 45% of the total volume of water in the columns. The analytical solution provides an excellent description of the experimental effluent data, and shows that tailing can be explained satisfactorily by diffusional exchange of tritium between mobile and immobile regions of the soil. Additional Index Words: intra-aggregate diffusion, immobile water, miscible displacement I PART i OF THIS STUDY (van Genuchten and Wierenga, 1976), a mathematical model was developed to describe the movement of chemicals through unsaturated, aggregated sorbing porous media. The medium was thereby analyzed by dividing the liquid phase into mobile and immobile regions. Transfer between the two liquid regions, presumably a diffusion process, was assumed to be proportional to the concentration difference between the mobile and immobile liquid phases. Sorption processes in both the stagnant and dynamic regions of the medium were further assumed to be instantaneous. The resulting differential equations were solved analytically assuming a linear equilibrium adsorption isotherm. The experiments to be discussed in this study were carried out to evaluate the model of van Genuchten and Wierenga (1976), and to obtain estimates of the different parameters in the adsorption model. The effects of flow velocity, bulk density, and aggregate size on the amount of immobile water (and hence, on the amount of tailing and asymmetry) were also studied. In a subsequent paper (van Genuchten et al., 1977) the theoretical model will be used to describe the movement of 2,4,5-T (2,4,5-trichlorophenoxy acetic acid) through the same soil columns. MATHEMATICAL DESCRIPTION We refer to the original paper of van Genuchten and Wierenga (1976) for a more extensive discussion of the physical system. The following general system of equations describes the mobile (Cm) 'Journal article no. 565, Agric. Exp. Stn. , New Mexico State Univ., Las Cruces, NM 88003. The work upon which this report was based was supported in part by funds obtained from the U.S. Dep. of the Interior, Office of Water Resources, as authorized under the Water Resources Act of 1964. Received 7 July 1975. Approved 9 Nov. 1976. Formerly Graduate Student, Dep. of Agronomy, New Mexico State Univ.; presently Research Associate, Dep. of Civil Engineering, Princeton Univ., Princeton, NJ 08540. 'Associate Professor, Dep. of Agronomy, New Mexico State Univ., Las Graces, NM 88003. and immobile (Cim) concentration distributions in a semi-infinite column, resulting from a pulse input of solute. [ dCml f VmC0 0 lim vmCm D -r— = \ z—»oL "£ J t ^ t -