Visualization of transport pathways for organic compounds in undisturbed soil monoliths

Small amounts of organic compounds are often found at much larger depths than expected from their organic carbon normalized distribution coefficient (Koc). In this study the food dye Brilliant Blue (BB) was used to visualize the (preferential) transport pathways in undisturbed soil monoliths taken from an Orthic Luvisol and from a Eutric Cambisol. After spiking the monoliths with 1.0 MBq of either 14C-labeled benazolin (leacher) or 14C-labeled benzo[a]pyrene (BaP, nonleacher), they were intermittently irrigated under free-draining conditions twice weekly with 4 mm of rain water for 17 weeks. BB (4 g L− 1) was added to the rain water during the last 4 weeks. After irrigation, the monoliths were sliced and photographs were taken from horizontal cross-sections. Soil was dried and incinerated to measure total 14C-activity. The mean travel depth for benazolin (6–7 cm) was larger than for BaP (2 cm), which is in line with their Koc values, although BaP was much more mobile than expected from the Koc value. BB patterns, arbitrarily classified into five intensity classes, showed that large parts of the monolith were bypassed below the completely stained upper two to five centimeter. Furthermore, BB patterns indicated the locations of strongly sorbing compounds in the deeper soil layers and are therefore a helpful tool to selectively sample the soil for these compounds. A continuously stained root channel below the 20-cm depth contained substantial amounts of BaP, indicating that soil structure cannot be neglected to assess the fate of nonleachers in undisturbed soils.

► Benazolin and benzo[a]pyrene transport was studied in undisturbed soil monoliths.
► Fast breakthrough was found due to by-passing unstained parts of the soil matrix.
► Dye tracing is a helpful tool to selectively sample soil for less mobile compounds.
► Root channels are hydrological active transport pathways for benzo[a]pyrene.