Agriculture Reference
In-Depth Information
immobile region of stagnant pore water. Furthermore, the diffusion of reac-
tive solutes inside soil matrix increases their contact with reaction sites on
the surfaces of minerals and organic matters. The time-dependent process
can significantly impact the extent and rate of geochemical and microbial
reactions with the solid phase.
Matrix diffusion can be manifested in the laboratory by examining the
asymmetric breakthrough curves from miscible displacement experiments
with nonreactive tracers. Experimental and modeling studies have dem-
onstrated that the mass transfer process can be influenced by aggregate
shape, particle size distribution, and pore geometries. Brusseau (1993) per-
formed column experiments using four porous media with different physi-
cal properties to investigate the influence of solute size, pore water velocity,
and intraparticle porosity on solute dispersion and transport in soil. It was
concluded that solute dispersion in aggregated soil was caused by hydro-
dynamic dispersion, film diffusion, intraparticle diffusion, and axial diffu-
sion at low pore water velocities, whereas for sandy soils, the contribution
of intraparticle diffusion and film diffusion is negligible and hydrodynamic
dispersion is the predominant source of dispersion for most conditions.
The flow interruption technique has been employed to detect and quan-
tify the rate and extent of physical nonequilibrium during solute transport
in heterogeneous porous media. A flow interruption stops the injection of
fluids and tracers, allowing more time for reactive tracers to interact with the
solid phase and for all tracers to diffuse into or out of the matrix. Reedy et al.
(1996) conducted nonreactive tracer (Br
-
) miscible displacement experiments
with a large, undisturbed soil column of weathered, fractured shale from a
proposed waste site on the Oak Ridge Reservation. The tracer flow was inter-
rupted for a designated time to quantify the diffusive mass transfer of a non-
reactive solute between the matrix porosity and preferential flow paths in
fractured subsurface media. Decreased and increased tracer concentrations
were observed after flow interruption during tracer infusion and displace-
ment, respectively, when flow was reinitiated.
8.2 Preferential Flow
Preferential flow is primarily due to the naturally occurring soil structure,
which consists of pores having different diameters, cracks formed by soil
shrinking during drying and wetting cycles, various macropores, and chan-
nels created by decaying plant roots and earthworms. Luxmoore (1981) sug-
gested that macropores are those with diameters greater than 0.1 mm in size.
In shrink-swell clay soils such as Sharkey clay, cracks 2 to 4 cm in width and
12 to 15 cm deep are not uncommon during drying periods. Preferential or
macropore flow is of particular significance in heavy textured soils because
Search WWH ::
Custom Search