New model to screen nitrogen from farms
23 September 2009, by Sara Coelho
Pig farm neighbours have more on their hands than just nasty smells: the ammonia released by manure can have damaging effects to the environment. Now scientists at the Centre for Ecology & Hydrology are tackling the problem with a new model.
One of the main chemicals released to the atmosphere by animal manure is ammonia, a compound rich in nitrogen. The ammonia is dispersed by prevailing winds and most of it doesn't travel far, a few kilometres at the most. When it falls, the nitrogen is deposited on nearby soils and vegetation.
'Nitrogen is a natural fertiliser, so if it falls on a farm it's not a concern,' says Mark Theobald, who researched the problem at the Centre for Ecology and Hydrology. But too much nitrogen landing on a nature reserve or in a river or lake can cause eutrophication, or undue fertilization, which can lead to loss of biodiversity.
Theobald and colleagues developed a model - dubbed SCAIL for Simple Calculation of Ammonia Impact Limits - 'to screen the local effects of atmospheric ammonia from pig and poultry farms quickly.'
SCAIL calculates the amount of nitrogen deposited on nature reserves downwind from a farm. The model uses an estimate of the ammonia emitted to the atmosphere, which depends on the kind of farm, the type of animals kept and their number. 'We also include the distance between the farm and the nature reserve and weather parameters such as average wind speed and wind direction,' explains Theobald.
The model was tested using measurements made on eight pig, poultry and dairy farms of different sizes and types. The results were published last week in the journal Science of the Total Environment.
'The first version was underestimating the amount of nitrogen deposited because we were considering area emissions,' he explains. In real life, the ammonia is released to the atmosphere by ventilation chimneys, not over a wide area.
'We corrected the problem by calibrating the model with the measurements and now SCAIL is probably more accurate than we need from a quick assessment tool,' says Theobald.
SCAIL is now being used to screen pig and poultry farms located near nature reserves, according to new European Union directives. 'The model does not give a detailed assessment, but it's useful to flag out which farms are harmless and which farms need a closer look,' says Theobald, who is now working in the Technical University of Madrid, Spain.
Scientists at the Centre for Ecology and Hydrology are now working on a version of the SCAIL model adapted to predict the effects of the emission of sulphur dioxide and nitrogen oxides from combustion sources such as small power plants.
Theobald MR, et al, A simple model for screening the local impacts of atmospheric ammonia, Sci Total Environ (2009), doi:10.1016/j.scitotenv.2009.08.025
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