Modelling Contamination of Field Drainage Water by Ammonium following Slurry Spreading

Simulations were carried out with the dual-porosity soil water and contaminant transport model MACRO to represent leaching of ammonium through field drains following spreading of slurry. These drain flows can contain short-lasting loadings of ammonium, but the concentration of ammonium varies very widely dependent on weather and soil conditions. The model was calibrated on the basis of an earlier study in which ammonium concentrations in drain flows were measured following slurry spreading. Predictive simulations were carried out representing spreading slurry on each day for 10 yr at two sites. These showed high leaching loads where slurry was spread on wet soil. When slurry was spread on relatively dry soil, leaching loads were low, even when heavy rain occurred within a few days following spreading. This differed from the situation with some other contaminants where rainfall following spreading had a significant effect on contaminant loads. Benefits of selecting ‘spreading days’ in relation to soil and weather conditions were investigated. This could reduce (almost to zero) the number of days when high losses would occur, as well as the average overall loss level.