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 fm, 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.