Abstract

Five laboratory experiments were conducted to study solute transport in the capillary fringe in a sand-filled glass tank containing an artificial groundwater zone, an unsaturated zone, and a capillary fringe in between. Dye-stained water, applied at the soil surface, moved downward through the unsaturated zone and then horizontally in the capillary fringe. The horizontal velocity of the dye plume front was calculated using optical image analysis and electrical resistivity tomography (ERT) measurements. Both methods gave similar velocities if an appropriate value of the threshold ratio describing the dye front was used. The hydraulic model HYDRUS-2D was used to simulate dye movement in the glass tank. After calibrating the dispersivity value in the hydraulic model, the horizontal velocity was found to be in the range −10 to 17% compared with the values measured using image analysis and −12 to 24% compared with the values measured by ERT; the differences could probably be attributed to uncertainties in the hydraulic parameters and soil heterogeneities. Both experimental and numerical results showed that the horizontal velocity of the capillary fringe is more or less identical to the one in the saturated zone. Thus, from a water transport perspective, the results suggest that the capillary fringe should be treated as a part of the saturated zone in hydrologic modeling.

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