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five. These loadings are distributed among the significant sources of the pollutant
of concern, with wasteload allocations containing the allowable loadings from point
sources, and load allocations containing the allowable loadings from diffuse sources.
A margin of safety is identified at this stage to take account of uncertainty in the
analysis, although this is often arbitrarily determined rather than rigorously quantified.
The margin of safety may be applied implicitly, using conservative assumptions, or
explicitly, by setting aside a portion of the total loading capacity. Finally, in step six
an implementation and monitoring plan is devised to oversee changes on the ground
and determine whether the TMDL has resulted in attaining the desired level of water
quality. A conceptual overview of how the TMDL framework could be integrated with
river basin management planning under the WFD is presented in Figure 1.
Figure 1: Conceptual overview of how water quality could be managed under
the Water Framework Directive using the Total Maximum Daily
Loads framework (in grey) as a means of regulation
CASE STUDY: TARLAND BURN
Introduction
The Tarland Burn is a tributary of the River Dee in NE Scotland, and drains an area of
intensive mixed land use. Agriculture in the lower zone of the catchment comprises a
mosaic of arable fields and improved grassland, supporting cattle, sheep, barley and
potato. The headwaters of the catchment feature steeper slopes comprising rough
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