FIELD-SCALE SOLUTE TRANSPORT IN A HEAVY CLAY SOIL

The transport of a bromide tracer was studied in a heavy clay soil. After applying the tracer on day 0, water and bromide content profiles in the unsaturated zone were determined on days 6, 46, 209, 335, and 572. Furthermore, water samples were taken frequently from the groundwater and drain discharge and analyzed for bromide content. In the clay soil studied, solute transport occurred in three domains: large continuous macropores, smaller more tortuous mesopores, and the pores inside the soil aggregates. Bromide transport through the first domain, i.e., preferential flow through large macropores, amounted only to a few percent. This resulted, however, in maximum bromide concentrations in groundwater and drain discharge. Quantitatively, the most important solute transport occurred through the mesopores. This transport was characterized by high spatial variability, significant lateral transport, and low mobile water volumes (6%). At the field scale this resulted in the rapid leaching of solute in comparison with earlier experiments on sand and loam soils. After only 6 days and 34 mm net precipitation, the average bromide concentration peak had reached a depth of 55 cm. In spite of the heterogeneous transport and the presence of preferential flow, field-average solute concentration profiles exhibited regular and smooth shapes. Solute transport inside the soil aggregates played only an indirect role, i.e., via solute retardation as a result of convection and diffusion into the aggregates.