PhD ThesisSoil 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.Al-Baath University, Syri