Role of macropore continuity and tortuosity on preferential transport of water and solute through soils

Although many researches have used tracers and models to study preferential flow, little is known on the effect of macropore characteristics such as continuity and tortuosity on contaminant movement in soils. The objective of this research was to improve the knowledge on the importance of these characteristics on contaminant transport through macroporous soils.; Tracer breakthrough curves were run in a plate columns containing macropores of different continuity and tortuosity levels. The macropores studied for continuity effects were: Control (no macropore), open at both ends, open at the surface--closed at the bottom of the column, closed at the surface--open at the bottom, and closed at both ends. The chosen tracers (bromide, Rhodamine WT, FD&C Blue #1) had various levels of retardation. Macropores open at the soil surface drastically changed water flow pattern and all characteristics of tracer movement. Macropores closed at the soil surface also influenced the solute movement but only in presence of other macropores. Macropore tortuosity levels studied were: 1.0, 1.1, 1.3, 1.5, and 3.0. Macropore tortuosity progressively influenced breakthrough time and anisotropy of tracer distribution in the profile. The importance of continuity and tortuosity increased with an increase in adsorption coefficient of the tracer. These results suggest that macropore continuity and tortuosity, usually neglected in modeling macropore flow, are important.; A finite element model based on the convective-dispersive equation was tested against laboratory data on breakthrough curves and tracer distributions in soil columns containing macropores. Model predictions were in close agreement with laboratory measurements. Simulations showed that macropores open at the surface favored rapid movement of contaminant when it was applied at the surface, but delayed the movement of contaminant when it was initially incorporated into the soil. Macropores closed at the soil surface favored movement of chemical only when a constant head was applied at the surface. Macropores had no impact on tracer movement at low intensity rainfall. Based on the simulation results, guidelines are provided to simplify macropore description in computer models. The study also discusses the results of an experiment on the pro and cons of using multiple tracers as surrogates of contaminants in soils.