ABSTRACT
Concern over chemical loadings to unconfined aquifers and into the surface water sources through drain tiles and subsurface groundwater flow has directed researchers to focus on the pathways that speed up the pollutant arrival to such sources. Preferential flow of water and chemical transport though porous media has attracted the attention of scientists and engineers working in the environmental field. Research has indicated that structured soils promote bypass flow (a form of preferential flow induced by macropores formed due to shrinking and swelling of soils) that results in fast movement of solutes, whereas ‘‘piston flow’’ is mostly responsible for the flow of water and solutes in nonstructured homogeneous (e.g., homogeneous sand-textured) soils (Skopp, 1981; Schumacher, 1864; Bergstro¨m and Shirmohammadi, 1999). Nieber (2001) stated that preferential flow includes macropore flow, gravity-driven unstable flow, heterogeneity-driven flow, oscillatory flow, and depression-focused recharge. Flow systems such as fingering caused by a sequence of texturally different layers (Hill and Parlange, 1972), hydrophobicity (Ritsema et al., 1983), and funnel flow due to texturally different lenses (Kung, 1990) are examples of preferential flow mechanisms. Such multiple transport behavior has created a multitude of difficulties in modeling solute leaching in vadose zone.
