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 sub surface-drained glacial till soil were analyzed by the application of a single/dual porosity model (MACRO), optionally accounting for concur rent and interacting flow and transport in the bulk soil porosity as w ell as in the macropores. The model analysis showed that macropore flo w is essential in describing the observed transport phenomenon on a sh ort as well as a longer time scale. The diffusive exchange of solute b etween the matrix and the macropores was very sensitive and critical f or the model prediction of the drainage concentration. The exchange wa s overpredicted and too rapid when the soil aggregate size (distance b etween macropores) obtained from an image analysis of soil cores was u sed in the model. On this basis, the model assumption of instant equil ibration of the solute across the matrix porosity, disregarding small- scale concentration gradients, is questioned. Decreasing the domain ex change resulted in an improved model correspondence with the drainage chemograph. The drainage flow pattern was altered between drainage sea sons owing to the changes in hydraulic efficiency of surface-vented ma cropores influenced by the physical disturbance and compaction of the soil surface. Hypothetically introducing fully surface-connected macro pores 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.