Solute Movement in large Soil Columns under different Water Flow Velocities

Soil-water flow velocity can be a major factor affecting ion movement in the soil matrix. The transport of both cations and anions and their interactions with soil surface affect many aspects of soil management such as wastewater disposal, saline soil reclamation, remediation of sodic soils, and soil decontamination (HENG et al., 1999a). Therefore it is important to understand the processes that govern downward and upward solute movement through soil, and the influence of flow velocity on the concentration of different cations and anions. The reclamation of saline soils depends upon the leaching of excess salt from the root zone. Insufficient leaching may cause extensive crop loss, while, excessive leaching, besides wasting water, may create some drainage problems, and, possibly, subsequent resalinization of the soil. The effects of slow and fast flow velocities on ion transport have also significant impacts on ion behaviours that allow to avoid these problems. Downward moving soil-water enters and leaches salt from smaller soil pores to a greater extent at lower moisture contents. TERKELTOUB and BABCOCK (1971) reported that slower rates of irrigation leached salt more efficiently than higher rates. Solute movement experiments have shown that a readily soluble salt moves at a different rate through the soil than the water in which it is dissolved (BIGGAR and NIELSEN, 1962). This behaviour can be explained by anion repulsion or negative adsorption of salt on soil particle surfaces. Most studies on solute transport have been performed assuming that the distribution of ion concentration within a soil pore is uniform, and that the mixing of salt primarily takes place as a result of variations in the water velocity within and between soil pores (KNUPP et al., 1972).