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The dual-porosity soil water and contaminant transport model MACRO was tested for its suitability to represent water leaching of phosphorus (P) through field drains following spreading of slurry. These flows are characterised by very high loadings of P, including a high proportion in particulate form, for about one week following winter spreading of slurry, followed by quite a rapid decline to the low background level. Use was made of the option in MACRO for representing colloid facilitated contaminant transport. The model simulates transport through macropores and soil matrix pores (micropores) of contaminant carrying colloids, as well as trapping of colloids by straining and filtration using an adaptation of standard filtration equations. Calibration involved selecting soil hydraulic parameters, colloid filtration coefficients and phosphorus sorption characteristics for two soils from measured and literature values. Both soluble and particulate (colloidally attached) P were represented in simulated outputs. Reasonable agreement was found between simulated and measured water and leached phosphorus flows. Work with the model suggests that macropore flow through the soil to field drains of colloidally transported P is an important component of water pollution associated with slurry spreading. This leads to the recommendation to avoid slurry spreading on very wet macroporous soils in winter, in order to significantly reduce P leaching to surface waters.