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Given that the release of P from the WWTP and septic tanks would have been
relatively consistent during the period of monitoring, we concluded that loadings
from diffuse sources were probably responsible for the majority of violations of the
target concentration value for TP. We therefore set wasteload allocations for the
WWTP and septic tanks at the estimated values for the period of monitoring, and
set the load allocation for diffuse sources as the estimated value for the period of
monitoring minus the amount by which the actual flux of TP must be reduced in
order to meet the target concentration value for TP. In order to allow for a margin
of safety in our load allocation budget, we derived mean probable errors for best
and worse case data acquisition scenarios using results published in the literature
(Figures 4 and 5). Sources of error in load calculations may be grouped into four
main categories, namely errors associated with the measurement of flow, errors
associated with sample collection, errors associated with sample storage and errors
associated with laboratory analysis. Further sources of error also arise when using
process-based models and GIS tools to estimate source loadings, but these have
yet to be satisfactorily quantified. Therefore, the error ranges presented should be
viewed as minimum estimates.
Figure 4: Time series of best case uncertainty (± 17 %) in daily phosphorus
loads at Coull Bridge during the period of monitoring. At this level
of uncertainty, a 49% reduction in diffuse P loading is required to
achieve the target concentration value of 0.05 mg/L for TP
50