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Figure 2: Time series of total daily phosphorus loads at Coull Bridge during
the period of monitoring and the TMDL for TP based on a target
concentration value of 0.05 mg/L
The major sources of P in the catchment were then identified and their respective
contributions to the actual flux of TP recorded during the period of monitoring
determined using GIS-based tools (Ferrier et al., 2004). These tools enable human
population density data from the 2001 census to be used in conjunction with userentered
per-capita rates for specified pollutants to estimate annual pollutant fluxes
from waste water treatment plants (WWTPs) and septic tanks. The estimated annual
P loadings from these sources in the Tarland catchment were adjusted for the length
of the monitoring period and then subtracted from the actual flux of TP recorded
during the period of monitoring to give an estimate of P loading from diffuse sources
(Figure 3). Relationships between loadings of P from major sources in the catchment
and violations of the target concentration value of TP were then examined in order
to determine acceptable load allocations to meet the desired level of water quality.
Analysis of the field data revealed that the export of P during the period of monitoring
was positively correlated with discharge (r = 0.47, P =