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grassland and heather moorland, supporting widespread plantation forestry. The
village of Tarland has a population of 520, and is served by a waste water treatment
plant (WWTP) situated on the main stem of the burn. The remainder of the population is
rural, with on-site sewerage in the form of septic tanks. Under the recently completed
characterisation and impacts analysis, the Tarland Burn was classified as being at
significant risk (1a) of not achieving good status by 2015, due to diffuse pollution
and morphological pressures. A significant amount of research on diffuse pollution
impacts and best management practices (BMPs) has been conducted within the
catchment over the past 5 years, making it an ideal study site for an assessment of
the suitability of TMDLs as a means of managing diffuse pollution.
Data Acquisition
During the 353-day period from the 3rd of July 2004 to the 20th of June 2005,
intensive stream monitoring was conducted on the main stem of the burn at Coull
Bridge. It should be noted that this location is several kilometres upstream of the
true catchment outflow, meaning that only 72% (51.6 km 2 ) of the true catchment
area (71.6 km 2 ) was monitored. Stage was measured continuously using an SR50
sonic stage sensor, and logged on a CR10X datalogger. The record of stage was
subsequently converted into a time series of discharge using a rating relationship
derived from discrete measurements of discharge over the full range of flows (n = 10,
r 2 > 0.99). The burn was sampled on a daily basis using an automatic pump sampler.
Additional samples were acquired on a four hourly basis whenever flows exceeded
Q10 (2.756 m 3 s- 1 , 1999-2002). All samples were stored in the sampler until collection,
and returned to the laboratory for analysis on a weekly basis. Total phosphorus
(TP) concentrations were then determined colorimetrically (Skaler SAN++) on the
unfiltered samples after persulphate digestion. For days when only a single sample
was collected, the daily TP flux was calculated by multiplying the total daily water
flux by the sample concentration. On days when flows exceeded Q10, the daily TP
flux was calculated by summing fluxes determined for each four hourly period (in turn
determined by multiplying the four hourly water flux by the corresponding sample
concentration).
TMDL Development
As new physico-chemical standards for use under the WFD have yet to be
published, we opted to use a target concentration value for TP of 0.05 mg/L, which
is recommended by the EPA as an indicative value of impairment in many rivers and
streams in the US (EPA, 1999). Over 24% of samples collected during the period of
monitoring had TP concentrations in excess of this standard; under EPA guidelines
this would necessitate the development of a TMDL (EPA, 1999). The total water flux
recorded in the period of monitoring (2.27 x 107 m 3 ) was then multiplied by the
target concentration value for TP (0.05 mg/L) to give the target loading capacity for
TP (1135 kg). This value was then divided by the number of days in the period of
monitoring (353) to give a TMDL of 3.2 kg (presented against actual daily fluxes of
TP in Figure 2). The target loading capacity for TP was then compared with the actual
flux of TP recorded during the period of monitoring (1428 kg), in order to determine
the amount by which the actual flux of TP must be reduced in order to meet the
target concentration value for TP (= 293 kg).
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