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Third, there must be a receptor. This is some component of the environment where
the substance being considered has undesirable consequences and is a pollutant.
Phosphorus in agricultural soil is not usually a pollutant. It is an essential plant
nutrient and a component of soil fertility. Phosphorus in loch water may be a
pollutant as it may cause eutrophication. Phosphorus in the soil in an area of land
containing interesting assemblages of wild plants may also be a pollutant as it may
tend to reduce the biodiversity of the habitat. Diffuse pollution therefore consists
of the transfer of substances from a source via a pathway to a receptor where that
substance has undesirable consequences.
Critical areas
On some farms, most of the transfer of certain pollutants arises from a relatively
small proportion of the total farm area. For example, soil erosion can transfer silt and
adsorbed nutrients and pesticides from land to water. On farms where in-field soil
erosion occurs, it is typically present only in limited areas, usually moderate or steep
slopes with susceptible soils that are in arable cropping. Control measures may be
targeted at such areas leaving the rest of the farm unaffected.
Certain even more limited areas are sometimes termed ‘hot spots’. These may
include such areas as stock feed rings, areas where stock habitually shelter and
areas where sprayers are filled. The pollution arising from such hot spots may be out
of proportion to their small area.
Cost-effectiveness and cost benefit analysis of BMPs
Where Environmental Quality Standards (EQS) for a water body are agreed to have
a sound environmental basis, and where several alternative measures are to be
considered the preferred method of evaluation is cost-effectiveness analysis (CEA).
This attempts to evaluate the least cost way of achieving a desired standard. Cost
Benefit Analysis (CBA) is appropriate for evaluating investment projects where
substantial non-market benefits or costs occur. In the SEERAD-funded project on
‘Evaluation of BMPs for diffuse pollution control’ (Vinten et al., 2006), the efficacy of
specific BMPs with respect to two main pollutants, P and FIO, have been studied.
Details of this work will be reported elsewhere (Vinten et al., 2006), but Table 2 gives
an example of the analysis framework used.
Loch Leven is a lowland Loch prone to eutrophication by soluble and particulate
P derived from field run-off and drainage water. This results in economic loss for
downstream users and from recreational users. Much of the non-farm input of P has
been controlled and attention is now focused on control of inputs from farmland. In
1997, a buffer strip was installed on the Green’s Burn to limit soil erosion losses to the
stream. A simple conceptual framework for evaluating pollution control measures is
set out below:
A. Identify Environmental Quality Standards for receiving waters.
B. Estimate the pollutant loading and concentration from the farm before pollution
control measures are installed.
C. Estimate reduction in pollution loading and concentration after installation of
pollution control measures.
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