Leaching patterns of heavy metals and nitrogen evaluated with a modified tanks-in-series model

Leaching rates of heavy metals (zinc and copper) and nitrogen were measured in 500-mm deep monolith lysimeters containing a heavy clay soil. The purpose of this study was to investigate the influence of preferential flow on leaching of these chemicals under different precipitation regimes. N was applied at a rate of 100 kg ha(-1) (50 kg NO3-N ha(-1) and 50 kg NH4-N ha(-1)), Zn, at a rate of 100 kg ha(-1), and Cu, at 40 kg ha(-1). Simultaneously, Br was applied at a rate of 100 kg ha(-1) to provide information on water movement through the profiles. A modified version of the 'tanks-in-series model' was used to describe the distribution function of non-reactive tracer (Br) travel time and compare the results with experimental data obtained in the lysimeters. Rapid discharge of Br in the monoliths was taken as a clear evidence for preferential flow. This flow behaviour was well described by the model through the parameters n and f(m), that is, in terms of the degree of mixing of water in soil and the ratio between mobile regions and the maximum amount of water in a monolith when freely drained. When determining parameter values, the required experimental period was made shorter by using a peak coordinate of distribution of non-reactive tracer travel time, instead of the moment analysis. The model could use the cumulative leachate as the only variable in the distribution function of tracer travel time under non-steady flow conditions. N leaching occurred in the form of NO3-N, which was also largely displaced through preferential flow. Zn leaching was also enhanced by preferential flow, whereas Cu leaching was negligible. Concentrations of NO3-N and Zn in leachate in the treatments, which received the element, often exceeded the Swedish drinking water guideline. The results obtained in this study show that preferential flow may have a substantial influence on leaching of heavy metals and nitrogen, and the combination of monolith lysimeters and the modified tanks-in-series model may be used to classify soils in terms of the occurrence of such flow behaviour. (C) 2000 Elsevier Science B.V. All rights reserved.