ABSTRACT
Recent experimental research in soil structure and nonideal chemical transport is forcing scientists to refine their analysis of the movement and fate of contaminants in the soil and consider processes that have become important when small concentrations of highly toxic chemicals must be accurately accounted for to protect groundwater reserves from contamination (Stagnitti et al., 1995). The mobility of the
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chemicals and contaminants in well-structured soils is affected by the continuity as well as the size of the pores. Networks of interconnected, highly conductive pathways that result from biological and geological activities, such as subsurface erosion, faults and fractures, shrink-swell cracks, animal burrows, wormholes, and decaying roots may be responsible for transmitting moisture, solutes, and contaminants to groundwater at faster velocities than those predicted by the theory based on the convective-dispersive equation (CDE) in which the water velocity is represented by the local average values (Bear, 1972; Sposito et al., 1986). In most cases, the volume of water within the preferential paths is much lower than the volume of the stagnant fluid, and only a small part of the solution (but very significant when the solute is toxic at low concentrations) may be moving within the well-defined preferential paths ahead of the main flow. For example, Smettem and Collis-George (1985) found that a single continuous macropore can conduct more water than a surrounding soil sample 10 cm in diameter. In structured soils, the convectivedispersive model gives unsatisfactory results under field conditions, and the interchange of fluids between the matrix and the fracture or large pore must be modeled explicitly (Steenhuis et al., 1990; Stagnitti et al., 1995; Parlange et al., 1996, 1998).
