FLOW AND TRANSPORT PROCESSES IN A MACROPOROUS SUBSURFACE-DRAINED GLACIAL TILL SOIL - I - FIELD INVESTIGATIONS

The qualitative and quantitative effects of macropore flow and transpo rt in an agricultural subsurface-drained glacial till soil in eastern Denmark have been investigated. Three controlled tracer experiments on individual field plots (each approximately 1000 m(2)) were carried ou t by surface application of the conservative chloride ion under differ ent application conditions. The subsequent continuous long-term monito ring of the rate and chloride concentration of the drainage discharge represented an integrated and large-scale approach to the problem. In addition, point-scale determination of macropore structure and hydraul ic efficiency, using image analysis and tension infiltration, and of s oil water content, level of groundwater table, and chloride content of soil water within the soil profile yielded insights into small-scale processes and their associated variability. Macropore how was evidence d directly by the rapid (within 10 mm of water input) and abrupt chlor ide breakthrough in the drainage water at 1.2 m depth in two of the tr acer experiments. In the third experiment, the effect of macropore tra nsport was obvious from the rapid and relatively deep penetration of t he tracer into the soil profile. Dye infiltration experiments in the f ield as well as in the laboratory supported the recognition of the dom inant contribution of macropores to the infiltration and transport pro cess. The soil matrix significantly influenced the tracer distribution by acting as a source or sink for continuous solute exchange with the macropores. An average field-determined active macroporosity constitu ted 0.2% of the total porosity, or approximately 10% of the total macr oporosity. (C) 1998 Elsevier Science B.V. All rights reserved.