Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 345

Figure 10.7 Watershed-scale transport and risk modeling: primary diffuse sources.
the emission of the whole watershed is estimated ( E source ). The output of the
black box is the measured or calculated water quality in the watershed, the
outflow of the black box ( C measured in the river ). The total emitted diffuse contam-
inant load ( E source ) does not reach entirely the surface water compliance point
within a watershed (it would be an unreal worst case). The relation between the
contaminant emission from the diffusely contaminated area and the measured
concentration in the surface water is influenced not only by the hydrological
parameters (included in the GIS model) but also by the natural risk reducing
and retention capacity characterized by RRR watershed of the area/pathway the con-
taminant is transported through (Figure 10.7). Risk reduction and contaminant
retention ability of the transport area/pathway can be significant. The “black box''
considers the risk mitigation processes (retention, biodegradation, biotransforma-
tion of a certain part of the contaminant) along the transport pathway between the
source and the compliance point. Risk mitigation or contaminant retention and
reduction rate ( RRR watershed ) within the black box can be calculated as the ratio
of the output to input concentration at the initial point in time ( t 1 ) and can
be calibrated to steady-state (magnitude and movement direction of the emit-
ted flow is constant in time) or transient conditions (magnitude and movement
direction of the flow varies in time) (Figure 10.7). Similarly, the predicted con-
centration in the compliance point ( PEC river ) can be estimated to the emission
values of any point in time ( t n ) and to steady-state or transient conditions
taking into account the risk mitigation rate ( RRR watershed ) along the transport
pathway.