Abstract
la recent years wide-spread observation of ground water con-lamination has focused attention on solute transport through soil. Three widely used solute transport models, namely has convection-dispersion equation (CDE), the mobile-immobile water model (M IM) and the transfer function model (TEM) are reviewed. The CDE model is valid if the residence time of the solute is much longer than the time required for transverse mixing by m]lecular diffusion. Conversely, if the mean residence time of the solute in the soil is short compared to the transverse mixing time, thou the TFiVI can do a better job for the area-averaged concentration of the solute than the CDE model,. The MiM model is good for describing solute transport in aggregated soil but it introduces ne iv parameters into the CDE model which cannot be independently estimatecl.
The concept Prediction or measurement of solute movement throigh th, soil is of inte. rest for improving the efficiency of leaching of saline soils and the distribution of surface - applied fertilizers and pesticides. Chemical transport models could be useful for estimating the movement of various dissolved chemicals through the unsaturated zone. Most field solute transport models used in the unsatura-ted zone are based on the convection-dispersion equation which was developed and tested almost exclusively in laboratory experiments. Convection refers to the average bulk movement of solute within flowing solution while dispersion describes solute spreading and mixing a'bout the mean displacement position caused by local irregular convective displacements, In a system comprised of water and dissolved chemicals, the only mixing occurring is by molecular diffu-sion which results from collisions occuring at the molecular scale. However, in porous media, convective flow is expressed as an effective flux which has been
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Present Address : Depti. of Soil Science, University of Agriculture, Faisalabad.
