Funneled flow mechanisms in layered soil: field investigations

Abstract

The movement of water and potential pollutants in the vadose zone of fluvial deposits is often difficult to predict because fine-over-coarse layers may behave as capillary barriers, funneling water and dissolved solutes into concentrated preferential flow paths. Capillary barriers have been studied in laboratory experiments and by mathematical analysis with well-defined boundaries but little is known about water and solute movement in naturally layered soils. This paper demonstrates how naturally occurring capillary barriers affect water and solute movement. Experiments were carried-out in a river valley near Amherst, MA and on a prehistoric beach near Cornell University in Ithaca, NY. Dye tracer and chloride were initially applied near the soil surface, followed by intermittent rainfall over several weeks. After 2–6 weeks the blue dye, soil water content, and chloride concentration distributions in the soil profile were examined by excavating a trench and photographing and sampling the exposed soil face. At both sites, the infiltrating water was diverted and flowed laterally over a relatively coarse layer. At the Cornell site, flow broke through the fine–coarse interface into the coarse layer at a few points along the barrier. The primary breakthrough at the Cornell site occurred at a horizontal section of the interface, 2 m down-slope from the point of dye application. The capillary diversions generally agreed with current theory, however, the flow patterns were difficult to predict accurately without more detailed information about the soil layering characteristics.