Using image analysis of tracer staining to examine the infiltration patterns in a water repellent contaminated sandy soil

Water repellency enhances preferential flow because it prevents water from entering hydrophobic areas. To investigate the influence of water repellency on flow patterns, tracer experiments were conducted on a sandy soil contaminated with hydrophobic tar oils. On two separate occasions, the soil was irrigated with different amounts of Brilliant Blue FCF (C.I. 42090) solution. The following day, soil profiles were exposed, images captured and the soil sampled for water repellency, dye concentration, water content and organic matter content. To describe the flow patterns, dye coverage maps were produced using digital image processing. In most profiles, preferential flow prevailed with the dye coverage ranging between 8 and 40%. The water repellency, heterogeneously distributed throughout the upper 10 cm, prevented infiltration into large parts of the soil profile. Areas of high water repellency coincided with very low water contents and were not dye stained. Bypassing of contaminated areas in the topsoil could reduce the leaching of toxic solutes and the subsequent risk for the groundwater. The occurrence of fingering, most distinct at high irrigation rates, was presumably not due to water repellency alone but also soil layering. Preferential flow in the subsoil could decrease the travel time of contaminants to the groundwater.