Modelling Through Soil Losses of Phosphorus to Surface Waters

Abstract

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