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Magnitude of Impacts
The magnitude of each diffuse pollution impact is less likely to be determined by
overall catchment characteristics and more by individual farming practice and
dependant on the problems and other features specific to the farm that will be
highlighted by a farm scale diffuse pollution audit.
Significance
Once sensitivities and magnitudes have been assessed, the matrix approach shown
in Table 1 allows an appraisal of the overall significance of different pollution risks,
e.g. nitrate.
A farm with predominantly sandy or shallow soils within an NVZ (HIGH sensitivity),
where more than 50% of land is ploughed and cropped in autumn (HIGH magnitude),
would be expected to have a SUBSTANTIAL risk of nitrate diffuse pollution into
groundwater.
A mixed beef and spring cropping farm (LOW magnitude), with predominantly heavy
soils outwith an NVZ (LOW sensitivity), would be expected to have a SLIGHT risk of
nitrate diffuse pollution and would therefore be a lower priority for nitrate-targeted
BMPs.
By carrying out this type of appraisal for each diffuse pollutant, the pollutants can be
prioritised and BMP selection can proceed.
Farm Audits
An audit of potential sources of diffuse pollution on the farm is carried out. This is
done partly by interviewing the farmer and collecting data from him and partly by
walking the farm and farm steading and observing any problems. These may include
problems related to fertiliser and manure usage, problems related to pesticide usage
and problems related to soil management including soil erosion. The aim of the
audit is to identify all potential problems that can then be prioritised. A useful way
of considering diffuse pollution issues is to regard it as a three-stage process.
Without each stage being present, diffuse pollution does not occur. A treatment
train approach to BMP implementation may address the problem at each stage in
the process.
First, there must be a source of the pollutant. This might be excess nitrogen in
soil or the nutrients in slurry spread on the soil surface. A source in itself does not
constitute pollution however.
Second, there must be a pathway. Excess nitrogen in agricultural soil is not
a pollutant although it may be one in the soil of adjacent natural or semi-natural
habitats. It only becomes a pollutant if there is a mechanism or pathway, determined
by landscape characteristics such as slope, soil type and vegetation, for the excess
nutrient to move from the soil, where it may be a desirable component of soil fertility,
to a receptor. The pathway might be leaching of nitrates to a groundwater receptor,
transfer of nitrates in drain flow to a surface water receptor or denitrification and
transfer of oxides of nitrogen to the atmosphere acting as a receptor.
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