EVALUATING FLOW AND TRANSPORT THROUGH MACROPOROUS SOILS: AN INNOVATIVE APPROACH TO USING THE TENSION INFILTROMETER

Macropores, such as fractures, animal burrows, and root holes, have a significant influence on flow and transport processes in the vadose zone, particularly in low permeability structured soils. Preferential flow along macropores has implications for recharge rates and the vulnerability of shallow aquifers as a result of enhanced connectivity between meteoric surface water and groundwater. A novel approach to using a tension infiltrometer was employed in this study to investigate macropore flow and transport processes at two field sites in southern Ontario. A series of equal-volume infiltration experiments were conducted at different tensions to enhance (small tensions) or limit (large tensions) the degree of macropore flow. Dye and fluorescent microsphere tracers were applied in all tests as surrogates for dissolved and colloidal contaminant species, respectively. Upon completion of infiltration, excavations were completed to examine the dye-stained flow patterns, map soil and macropore features, and collect soil samples for analysis of microspheres. Linear macropores, in the form of earthworm burrows and root holes, were the most prevalent macropore type at both sites. There was a clear relationship between the vertical extent of infiltration and the applied tension as a result of preferential flow along macropores. Under the smallest applied tensions (<1.0 cm), dye staining was observed between 0.7 and 1.0 m depth, although only a fraction of the visible macropores contributed to flow at these depths. On all excavated sections, the dye patterns correlated reasonably well with the presence of microspheres in the soil samples. The tension infiltrometer was also used to infiltrate dye along an exposed vertical soil face, thereby providing a rare opportunity to directly observe transient macropore flow processes. The resulting vertical flow velocities within the macropores were on the order of tens of meters per day, illustrating the rapid connection between surface water and groundwater, even under partially saturated conditions. Overall, the tension infiltrometer performed extremely well as a tool for controlling macropore flow in situ and, together with the dye and microsphere tracers, provided unique and valuable insights into small-scale flow and transport behavior.