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Figure 5: Time series of worst case uncertainty (± 67 %) in daily phosphorus
loads at Coull Bridge during the period of monitoring. At this level
of uncertainty a 79% reduction in diffuse P loading is required to
achieve the target concentration value of 0.05 mg/L for TP
After recalculating the required reduction in diffuse P loading under the best and worst
case error scenarios, we finally calculated the cost of implementing best management
practices (BMPs) to meet the desired level of water quality. A number of BMPs have
been shown to be effective at reducing P losses from agricultural land, particularly
when used as part of a treatment train (Vinten et al., 2005). However, for simplicity,
we opted to consider only a single control measure, namely the installation of buffer
strips. Removal efficiencies of between 50 - 80 % for TP have been reported in the
literature for properly sized and managed buffer strips (Novotny and Olem, 1994).
Under the best case error scenario, we opted to install 6-m wide buffer strips along
29.3 km of streams lying within the zone of arable fields and improved grassland
using Land Management Contracts Menu Scheme (LMCMS) rates for buffer strip
creation and management (£200 per ha), at a total cost of £35,160 over 5 years.
However, in order to meet the desired level of water quantity under the worst case
error scenario, we opted to install 20 m wide buffer along the same stream length
using Rural Stewardship Scheme (RSS) rates for fencing (£3 per m) and the creation
and management of species-rich grassland (£250 per ha), at a total cost of £322,300
over 5 years.
DISCUSSION
The TMDL programme forms the backbone of the EPA’s current watershed level
approach to water quality regulation, and is in many ways similar to the framework
established for river basin management planning under the WFD. One important
difference arises when considering the establishment of environmental objectives,
and that is that the WFD requires all water bodies to achieve both good ecological
and chemical status, whereas the TMDL programme requires only the latter. However,
providing that indicators of ecological status are measurable using numeric targets,
then there is no reason why the TMDL process could not be implemented under
the WFD. Indeed, we believe that the TMDL process offers a robust framework
for controlling both point source and diffuse pollution, as it integrates the variable
effects of flow and targets both acute and chronic impacts. Moreover, it also shifts
the focus of water quality regulation away from symptoms of impairment and on
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