Areal Extent of Preferential Flow with Profile Depth in Sand and Clay Monoliths: Fid-Bau Portal

Areal Extent of Preferential Flow with Profile Depth in Sand and Clay Monoliths

Bergström, Lars F. / Shirmohammadi, Adel

During recent years much attention has been focused on the role of large pores in soil as a preferred route for solute movement. However, most studies have failed to relate the macropore areal extent to profile depth, which was done in this study. The areal extent of pores participating in displacement of water and contaminants with soil depth and its hydrological significance was investigated in two texturally different soils, both under ponded and transient flow conditions. Soil monoliths (1 m long, 0.3 m diameter) of a clay soil and a sandy soil were used for this purpose. Water (100 mm) stained with acid red (azophloxine) at a concentration of 3.8 g/L was applied to each monolith under the two hydro-logically different regimes. Subsequently, they were cut in 0.1-m increments and each transect was examined visually and recorded graphically for the areal extent of stained soil. A considerably larger area was stained at each depth in the sand compared with the clay soil, for example, around 83 and 22% at 70-cm depth in the sand and clay, respectively, under ponded flow conditions. In the clay soil, a much smaller area was stained throughout the soil profile when the soil was exposed to transient compared with ponded flow conditions. Below the topsoil (30-cm depth), the mean stained area was less than 10% under transient flow conditions, whereas up to about 50% was stained under ponded flow conditions. This difference did not occur in the sand. Even though the infiltration rate was much higher in the sand (i.e., the time periods required to infiltrate 100 mm of stained solution were, on average, 33 min for the sand monoliths and 4 h 21 min for the clay monoliths), the arrival time of stained solution in leachate was considerably faster in the clay (on average, 1 h 8 min in the clay under ponded flow conditions, whereas only clear water eluted in the sand monoliths throughout the experiment). These results clearly show that preferential flow in clay soils is a major route for the displacement of contaminants through the soil profile.