assuming the presence of a linear equilibrium isotherm. „ . j u . *. ., _, . . . ., «. These solutions were subsequently used in part II (van Comparisons are made between observed and calculated effluent ,-, , , .... .„!_ ,. . . , ., . . concentration distributions for the movement of 2,4,5-T (2,4,5Genuchten and Wierenga, 1977; this issue) to describe tntrichlorophenoxyacetic acid) through 30-cm long unsaturated soil coltlum movement through unsaturated, aggregated Glendale umns. The comparisons are made using both analytical and numerical ^y loam. In this paper, part III of the study, the theoretical solutions of a previously published model, which included the effects of model will be used to describe experimental data on the intra-aggregate diffusion and adsorption. The results in this study inmovement of the herbicide 2,4,5-T (2,4,5-trichlorophenoxdicate that intra-aggregate diffusion and adsorption/desorption are the yacetic acid) through the same Glendale soil. As was menmain mechanisms responsible for effluent tailing. An estimated 60% of tioned in Part I, no analytical solutions exist when either the the adsorption was found to occur in the stagnant region of the soil. adsorption isotherm is nonlinear, or when the adsorptionWhen intra-aggregate diffusion was included in the model, the obdesorption process is not single-valued (hysteretic). For served adsorption/desorption hysteresis phenomenon found to be sig.1. j-.• i » u • L. j TU .„ .. F . . . . . . u . u ui . mese conditions numerical techniques must be used. The mficant in several earlier studies, was shown to be much less important . . . . , . , . , . . in describing the observed concentration distributions. numerical solutions used in this analysis were programmed in IBM S/360 CSMP as described by van Genuchten and Additional Index Words: intra-aggregate diffusion, adsorptionWierenga (1974; 1976b). desorption, hysteresis, miscible displacement, 2,4,5-T. 'Journal article no. 568, 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 IN PART I OF THIS STUDY (van Genurhrpn and Wierenaa of Water Resources Research, as authorized under the Water Resources 7 TM , L • , J V uenucmen ana Wierenga, Ac, of ,%4 Received 8 March 1976 Approved 9 Nov. 1976 iy/oa), a theoretical model was developed to describe the "Formerly Graduate Student, Dep. of Agronomy, New Mexico State movement of chemicals through unsaturated, aggregated Univ.; presently Research Associate, Dep. of Civil Engineering, Princeton sorting porous media. Analytical solutions were derived for ^g£%£^. of Agronomy, New Mexico state Univ., Las both the mobile and immobile liquid regions of the medium, Cruces, NM 88003. VAN GENUCHTEN ET AL.: MASS TRANSFER STUDIES IN SORBING POROUS MEDIA: III 279 Table 1—Soil-physical data for data for various displacements through Glendale clay loam. 6 r-