Flow and transport processes in a macroporous subsurface-drained glacial till soil: II: Model analysis

The experimental results from a field-scale tracer experiment in a subsurface-drained glacial till soil were analyzed by the application of a single/dual porosity model (MACRO), optionally accounting for concurrent and interacting flow and transport in the bulk soil porosity as well as in the macropores. The model analysis showed that macropore flow is essential in describing the observed transport phenomenon on a short as well as a longer time scale. The diffusive exchange of solute between the matrix and the macropores was very sensitive and critical for the model prediction of the drainage concentration. The exchange was overpredicted and too rapid when the soil aggregate size (distance between macropores) obtained from an image analysis of soil cores was used in the model. On this basis, the model assumption of instant equilibration of the solute across the matrix porosity, disregarding small-scale concentration gradients, is questioned. Decreasing the domain exchange resulted in an improved model correspondence with the drainage chemograph. The drainage flow pattern was altered between drainage seasons owing to the changes in hydraulic efficiency of surface-vented macropores influenced by the physical disturbance and compaction of the soil surface. Hypothetically introducing fully surface-connected macropores into the calibrated model resulted in a 22% increase in the loss of solute to the drain, indicating the significance of the hydraulic conditions at the soil surface and the model specification thereof. (C) 1998 Elsevier Science B.V. All rights reserved.