Experimental Investigation of Direct Connectivity between Macropores and Subsurface Drains during Infiltration

Laboratory experiments are commonly utilized for investigating the effect of macropore flow on contaminant transport. Most research to date has focused on lateral water and solute diffusion between macropores and the soil matrix with controlled bottom flux or specified pressure head boundary conditions as opposed to simulating local drainage flux to tile drains. Furthermore, recent research indicates immediate breakthrough of solutes and pesticides in subsurface drainage by extraordinarily efficient transport through directly connected macropores. Macropores, such as those created by earthworm burrows, have been documented to transfer water and solutes directly to subsurface drains. In this study, this "direct connectivity" phenomenon was verified by conducting infiltration experiments in a laboratory column (28 cm by 50 cm rectangular cross-section with length of 95 cm) with an artificial macropore directly connected to the subsurface drain. A novel design of the experimental setup allowed open surface and buried macropore lengths to be varied from the subsurface drain to the surface without unpacking/disturbing the soil column between experiments. Experiments were completed for various buried macropore lengths ranging from zero (no macropore effect) to 75 cm (surface connected macropore). The column was packed with a sandy loam soil with bulk density of 1.6 g/cm³. For each experiment, a 1-cm ponded boundary condition was maintained at the soil surface. Breakthrough curves were plotted for both matrix and macropore flow at the outlet. The movement of the wave front down the column was observed with pencil size tensiometers mounted on the side of the column at various depths. The macropore and soil matrix outflow revealed hydraulic nonequilibrium between soil matrix and macropore domains. It was observed that the longer the buried macropore length (i.e., as the macropore approached the soil surface), the more rapid response occurred at the drain outlet in addition to an increased percentage of total drain flow through the macropore. Breakthrough times with the surface connected macropore decreased significantly compared to buried macropores. This research also suggests that "drain connected" macropores may not necessarily have to possess a direct physical connection to subsurface drainage.