Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 344

contaminant passing through decreases. According to the black box approach (detailed
below and shown in Figure 10.7) the decrease is due to the RRR rate incorporating
the risk mitigation processes (retention by infiltration, sorption and degradation or
transformation of a certain part of the contaminant) along the transport pathway
between the source and the compliance point. The retention of the black box results
in a difference between the input and output fluxes to and from the black box (input-
output in Figure 10.7), which can be expressed as a function of the emitted contaminant
amount and runoff flux as (1 - RRR)*E/flux . Taking into account the ( RRR watershed )
along the transport pathway (Step 4) the predicted concentration 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 (Step 5).

4 RRR watershed = C t0measured in the river /C emitted

5 C emitted = E t0 /flux GISt0

6 PEC river = RRR watershed ∗ E tn /flux GIStn ,

where:

- E : total emission from diffuse sources (emitted mass);
- C : concentration (mass/volume);
- flux : water flow rate (volume in unit time);

- PEC : predicted environmental concentration;

- outflow from the watershed in the black box;

- RRR watershed : risk reduction and retention rate of the watershed in the black box;
-

river =

t 0 : initial point in time, when the model is calibrated;

- t n : any point in time;

- GIS: GIS-based hydrological modeling.

4.2.3 GIS transport model types
The GIS contaminant transport models vary according to the model's data require-
ments. We distinguish GIS models based on their data requirement:

- GIS transport models with large data requirement : These models take into account
all possible transport pathways and the interaction between the environment and
the contaminant and are able to handle, interpret and visualize on digitized maps
various GIS data layers containing information about hydrology, hydrogeology,
soil type, texture, chemical properties, land uses, land cover, evaporation, infil-
tration, partition, sorption, desorption, chemical and biological transformations,
biological uptake, bioaccumulation, etc. Even if these pieces of information are
available, the uncertainty may be still great;
- GIS transport models with limited data requirement : These models consider the
environment and its interactions with the contaminants along the transport path-
way as a homogenous “black box,'' characterized by input and output data in
terms of the fate and transport of the contaminant. Input data include the esti-
mated emission based either on statistical data (e.g., produced or used amounts
of pesticides, fertilizers, number of animals, emission factors of the activities),
or on measured data (representative subareas, meso- or microcosms) from where