Abstract :
Simulations were carried out with an adaptation of the dual-porosity soil water and contaminant transport model MACRO to represent water flows and 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 1 week following winter spreading of slurry, followed by quite a rapid decline to the low background level. The option in MACRO was used for representing colloid-facilitated contaminant transport. The model simulates rapid transport through macropores and soil-matrix pores (micropores) of contaminant carrying colloids, as well as trapping of colloids by various mechanisms. Both soluble and particulate (colloidally attached) P are represented in simulated outputs. 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. Parameters of the P transport routines were set to those selected in a previous study to represent experimentally measured P leaching data, but some further testing of hydrological routines and selected parameters was carried out. Predictive simulations were carried out representing spreading slurry on each day for 10 yr at two sites. These showed high leaching loads mainly of colloid-associated P where slurry was spread on wet soil. When slurry was spread on relatively dry soil, leaching loads were low (and mainly of soluble P) unless heavy rain occurred within a few days following spreading. Benefits of selecting ‘spreading days’ in relation to soil and weather conditions were investigated. This reduced (but did not completely eliminate) the number of days when high losses would occur, as well as the average overall loss level.
