Preferential transport of a bromide tracer applied in a pulse of ponded water

The objective of this study was to quantify relations between preferential transport of a solute and initial water content, infiltration rate, and porosity in a field soil where preferential transport was mainly due to soil heterogeneity. We measured the horizontal and vertical distribution of a tracer chemical applied with ponded water to study the flow paths of the tracer. The soil at the site is a Bosville fine sandy loam (fine-mixed, thermic Albaquic Paleudalfs). Strontium bromide (SrBr2) tracer was applied with a dye (methylene blue) in a 100 or 50-mm pulse of water to soil within eight double ring infiltrometers. After 48 h the soil in each infiltrometer was sampled to 0.7 m. Twelve horizontally oriented, continuous soil samples 0.1 m long were collected at each depth. There were very few dye stains of root hairs, root channels, and pores to a depth of about 50 to 80 mm. Recoveries of Br to 0.5 m ranged from 36 to 56% applied. Bromide recovery was negatively correlated with initial water content and positively with total porosity. Below 0.35 m in depth resident solute concentration at a sampling position was positively correlated with concentration in the layer above indicating preferential vertical flow paths. It appeared that a large fraction of solute transport was through the highly porous areas of the cross-section of soil bounded by the infiltrometer ring. The preferential transport of Br in this study was influenced largely by the properties of the clay layer at 0.35 m that had the lowest conductivity and lowest porosity in the profile and appeared to have cracks filled with sand