WHY PREFERENTIAL FLOW IS IMPORTANT?
In 1980, residents of eastern Long Island were dismayed to learn that approximately a thousand wells, from which they got their drinking water, had been contaminated with Aldicarb, a pesticide used to fight the Colorado potato beetle. Regulators familiar with local conditions were as surprised as anyone, for they did not expect the pesticide to get into the ground water so rapidly, nor in such high concentration. Their conception of the fate of pesticides was clearly inadequate.
Long Islands problem is not unique. During the last two decades, many studies have found high concentrations of pesticides in tile lines or shallow ground water shortly after application. Contamination of ground water with agricultural chemicals is becoming a serious threat. Modern agriculture relies on a broad range of fertilizers and pesticides to assure reliable crop yields, and although integrated pest management may reduce the amount of chemicals needed, a total ban is not currently feasible. A long-term hazard is involved: once the chemicals get into groundwater, they may remain there for hundred of years.
The high pesticide concentrations are associated with preferential flow phenomena, which was initially described in 1882, in a landmark paper by J.B. Lawes, J.H. Hilbert, and R. Warington. During field drainage experiments, the authors distinguished between preferential and matrix flows, and pointed out that the relative importance of the two kinds of drainage depends on soil type and rainfall intensity. Preferential flow refers to the uneven and often rapid movement of water and solutes through porous media, typically soil, characterized by regions of enhanced flux such that a small fraction of media (such as wormholes, root holes, cracks) participates in most of the flow (Figure 1). Matrix flow is a relative slow and even movement of water and solutes through the soil while sampling all pore spaces, obeying the convective-dispersion theory which assumes that water follows an average flow path through soil (Figure 2). These two types of flow affected solute transport differently. The chemical composition of preferential flow reflects the concentration of water near the surface, while matrix flow represents the concentration of water around the drain. Thus, when a salt is equally distributed throughout the soil, the salt content of matrix flow is higher for water in preferential flow paths, characterized by the rainfall composition. The opposite was true (i.e., preferential flow had a higher solute content than the matrix flow) when a fertilizer or tracer were recently surface applied. Consequently, the solute concentration of drainage water depends on the ratio of water in preferential and matrix flow paths.
Due to its rapid movement, preferential flow allows much faster contaminant transport and creates significant consequences for ground-water quality and has direct impacts on drinking water and human health, animal waste management, nutrient and pesticide management, and watershed management. In the mining sector, the phenomenon of preferential flow has been used in designing the best configuration for tailings left behind by mining operations. By including clean material at strategic places where preferential flow is directed, the toxicity of leachates can be greatly reduced.