Mobile, Immobile, and Multiflow Domain Approaches - Transport and Fate of Chemicals in Soils: Principles and Applications - page 259

they can increase infiltration and may result in bypass flow where water and
solutes move rapidly through the soil profile with limited interaction with
the soil matrix.
Macropores are essentially considered as noncapillary pores, and it can be
assumed that flow occurs in macropores only in the presence of free water at
the soil surface. However, several observations indicate that macropore flow
occurs in very dry soils at the onset of a rain. This is of significance in the
movement of reactive chemicals to lower depth due to water flow in macro-
pores. In general, preferential flow occurring via distinct flow pathways in
fractures and macropores can significantly influence the movement of heavy
metals through soils. Preferential and nonequilibrium flow and transport
are often difficult to characterize and quantify because of the highly irregu-
lar flow field as a result of the high degree of spatial variance in soil hydrau-
lic conductivity and other physical characters (Biggar and Nielsen, 1976). A
significant feature of preferential flow is the rapid movement of water and
solute to significant depths while bypassing a large part of the matrix pore-
space. As a result, water and solutes may move to far greater depths, and
much faster, than would be predicted using averaged flow velocity (Jury and
Flühler, 1992). The consequence of the preferential flow is the nonequilib-
rium transport processes where solute in fast flow regions does not have
sufficient time to equilibrate with slowly moving resident water in the bulk
of the soil matrix. Therefore, the breakthrough curves of the solute trans-
port through preferential flow often exhibit a sharp front indicating early
breakthrough in the rapid flow region, and long tailing as a result of the rate-
limited transfer of solutes from the slow flow region to the fast flow region
(Jury and Flühler, 1992).
Since macropores in structured soils can conduct water rapidly to deeper
soil horizons, while bypassing the denser, less permeable soil matrix (Jarvis,
Villholth, and Ule´n, 1999), an application of reactive chemicals during dry
seasons can cause their rapid transport and to an extensive depth in the soil
profile. It is commonly accepted that reactive chemicals move down the soil
profile through these preferential flow pathways after application of sew-
age sludge, wastewater, or smelter residues. Sterckeman et al. (2000) reported
that concentrations of Cd, Pb, and Zn increased down to a 2 m depth in soils
near smelters. They suggested that earthworm galleries were the main path-
ways for accelerated particulate metal migration. Conventional laboratory
transport studies using column leaching techniques with homogenized soil
samples are not capable of capturing the increased mobility of heavy metals
through preferential flow. Studies using undisturbed soil samples where the
pore structure is kept intact are useful in evaluating the potential of trans-
port through preferential flow. For example, the column leaching experiment
of Cd, Zn, Cu, and Pb conducted by Camobreco et al. (1996) showed that the
metals were completely retained in homogenized soil columns. In contrast,
the preferential flow paths in the undisturbed soil columns allowed met-
als to pass through the soil profile. Distinctively different characteristics of