Movement of solutes in structured soils during intermittent leaching : a theoretical and laboratory study

Abstract

Soil salinity is one of the major problems in and and semi-arid zones, affecting up to 50% of arable land in Syria. Salt-affected soils are usually desalinized by leaching the excess salts out of the soil profile. Some studies have shown that applying the leaching water intermittently instead of continuously may result in more efficient leaching. This thesis aims to investigate, theoretically and experimentally, the benefits and limitations of intermittent leaching and to develop mathematical models able to simulate solute transport through structured soils under such conditions. Laboratory leaching experiments were conducted on bi-continuum media, as an analogue of structured soils, created by packing porous aggregates (ceramic spheres or soil aggregates of uni- or multi- diameters) in glass columns. The columns were either leached continuously or intermittently and with different pore-water velocities. Intermittent leaching was undertaken either under saturated or drained conditions. Under "saturated conditions" the column remained saturated throughout the experiment, while under "drained conditions" the column was allowed to drain at the beginning of each rest period and remained like this until being saturated again for the next leaching period. The solute concentration in the leachate was monitored continuously (either using a flow-through conductivity cell, or by using ion-selective electrodes for Ký and Br' ) to produce breakthrough curves. These curves were used to investigate solute transport through such media and validate the developed models. The experiments showed that water savings of up to 22% under intermittent leaching from a soil aggregate column were possible under saturated conditions. Such saving increased with aggregate size, flow velocity and duration of rest period. Under drained conditions, for ceramic spheres, 12% more solute was leached with the same amount of water under intermittent leaching. Two models were developed, the SIL (Saturated Intermittent Leaching) and the DIL (Drained Intermittent Leaching) models, for saturated and drained conditions respectively. The SIL model simulated solute transport in structured soils under intermittent leaching. The governing equations during displacement period were the mobile-immobile convection-dispersione quations. During the rest period the flow is stopped, and the solute transfers only by diff-usion between immobile and mobile water regions. The DIL model simulated solute transport when the soil drained. Here, during the displacement period, the mobile water was drained. The model simulated this using the equations of the SIL model by assuming that air displaced the solution in a piston-type displacement. During the rest periods the solute difluses within the aggregates establishing a more uniform concentration in the immobile water across the aggregate. The models can be used with a wide range of column conditions and for both sorbed and non-sorbed solutes. Both models were verified against experimentarel sults.