A river water pollution model for assessment of best management practices for livestock farming

Deposition of livestock excreta on soil, either during grazing or when landspread, leads to extensive water pollution. Important transport mechanisms for diffuse pollution are surface runoff and through-soil inter-aggregate macropore flow. A two-phase approach has been adopted to represent water pollution by these mechanisms. Firstly, MACRO, a two-domain hydrological model from Sweden, described transport of ammonium and Escherichia coli microorganisms from individual fields; trampled pollution ‘hotspots’ were represented by reducing the soil surface layer conductivity. Excreted contaminant inputs to MACRO reflected the different stocking densities in both trampled and main field areas. Secondly, a channel network model simulated contaminant delivery to rivers, using MACRO output as input, with additional inputs to represent animals excreting directly into rivers, and other losses such as an overflowing lagoon.The combined models have been calibrated for a case-study Scottish dairy farm on which river flow and contaminant monitoring equipment were installed. Farm data on livestock numbers, land-spreading activities, etc. were used to set inputs to MACRO. The simulated loads of ammonium in the river showed a pattern similar to estimates from river concentrations above and below the farm, particularly in peak timings related to heavy rainfall events. The models are being used to assist in assessing engineering solutions for pollution mitigation. Providing water troughs to discourage animals from entering the river to drink has been represented in the model by excluding excreta inputs directly to the river, and is considered to have reduced pollutant loads.