Experimental investigation of direct connectivity between macropores and subsurface drains during infiltration

Recent research indicates immediate breakthrough of surface-applied contaminants in subsurface drainage by transport through macropores directly connected to the surface. This "direct connectivity' phenomenon was verified and investigated by conducting infiltration experiments (1-cm ponded water at the soil surface) in a laboratory soil column (sandy loam soil with bulk density of 1.6 g cm(-3)) with a vertical artificial macropore placed directly above or shifted away from a lateral subsurface drain. The experimental setup allowed surface-connected and buried macropore lengths to be varied from the surface to the subsurface drain depth without unpacking or disturbing the soil column between experiments. It was observed that the longer the buried macropore length (i.e., as the macropore approached the soil surface), the more rapid the response at the drain outlet in addition to an increased percentage of total drain flow through the macropore (35-40%). Breakthrough with surface-connected macropores was significantly faster than with buried macropores, suggesting that breaking surface connectivity of macropores by tillage may be an important management strategy. For surface-connected macropore experiments, the average ratio of steady-state total (macropore and matrix) to matrix flow rates decreased as the distance from the drain increased: 2.4, 2.1, and 1.6 for distances of 0, 6.25, and 12.5 cm, respectively. Extrapolating this data to distances beyond 12.5 cm suggested that macropores located within 20 to 25 cm of the drain act as though directly connected in this sandy loam soil. This research verifies the "contributing area concept hypothesized in previous field and numerical modeling studies.