Influence of macroporosity on preferential solute and colloid transport in unsaturated field soils

Abstract

Transport of solutes and colloids in soils, particularly those subject to preferential flow along macropores, is important for assessing the vulnerability of shallow groundwater to contamination. The objective of this study was to investigate flow and transport phenomena for dissolved and colloid tracers during large infiltration events in partially saturated, macroporous soils. Controlled tracer infiltration tests were completed at two field sites in southern Ontario. A tension infiltrometer (TI) was used to infiltrate water with dissolved Brilliant Blue FCF dye simultaneously with 3.7 µm and 0.53 µm diameter fluorescent microspheres. Infiltration was conducted under maximum infiltration pressure heads ranging from − 5.2 to − 0.4 cm. All infiltration test sites were excavated to examine and photograph dye-stained flow patterns, map soil features, and collect samples for microsphere enumeration. Results indicated that preferential transport of dye and microspheres via macropores occurred when maximum pressure heads were greater than − 3.0 cm, and the corresponding infiltration rates exceeded 2.0 cm h − 1 . Dye and microspheres were detected at depths greater than 70 cm under the highest infiltration rates from both sites. Microsphere concentrations in the top 5–10 cm of soil decreased by more than two orders of magnitude relative to input concentrations, yet remained relatively constant with depth thereafter. There was some evidence for increased retention of the 3.7 µm microspheres relative to the 0.53 µm microspheres, particularly at lower infiltration pressures where straining and attachment mechanisms are most prevalent. Microspheres were observed within dye stained soil matrix surrounding individual macropores, illustrating the significance of capillary pressures in controlling the vertical migration of both tracers in the vicinity of the macropores. Overall, microsphere distributions closely followed the dye patterns, with microsphere concentrations at all depths directly related to the intensity (or concentration) of dye staining. It is concluded that the flow system influenced transport to a much greater degree than differences between dissolved and colloidal species, and hence a dye tracer could serve as a reasonable surrogate for colloid distributions in the vadose zone following individual infiltration events.