Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 359

infiltration, degradation and transformation. The emission from the source may
change in time with the progress of dominant processes such as precipitation or
mobilization due to chemical and biological impacts (weathering, leaching, redox
processes, etc.). It is, therefore, desirable to express the emission from the source
as a time-dependent flux in terms of milligram of contaminants per liter of water
or kilogram of soil per unit of time. Some contaminants dissolve easily, others are
mainly present fixed to the matrix. Therefore, the source of the various contami-
nants can be subsequently described in terms of flux of contaminants/emission as
a function of time based on measurements (i.e., field or microcosm leaching data)
and the transport scenario defined.
The exact location of the diffusely distributed contaminants is not known, for
this reason instead of a single point, large areas are considered to be the diffuse
pollution source and together with the contaminant transport pathway may be
handled as a black box. The source of diffuse pollution can be a remote point
source (e.g., a smelter) from which untraceable transport paths lead to the con-
taminated area. The sources and their emissions may also be diffusely dispersed
such as the agricultural chemicals (nutrients, pesticides, soil amendments). Data
to characterize a diffuse source and the contaminant transport pathway in terms
of topography, hydrology and hydrogeology are generally available at watershed
scale. However, the characteristics of the contaminants and the watershed area
influencing the contaminants' fate and behavior (other than hydrological trans-
port) cannot be described fully (taking into account the soil type, partition of
contaminants between physical phases, pH and redox potentials, geochemical
properties, etc.). Therefore, in this case, the hydrological model is a numerical
model. For this reason, the fate and behavior of the contaminant is modeled with
an analytical model using a black box, within which neither the environment nor
the contaminant characteristics are detailed, only the input and the output into
and from the black box is known, based on the results of model experiments or
in situ assessments. Instead of complete numerical transport and fate modeling,
GIS-based hydrological modeling is combined with the “black box'' model simu-
lating all the other transport and fate processes. The flux and concentration of the
contaminant in general decreases along the transport pathway (natural attenua-
tion). The decrease occurs in the black box area and it is considered to be the RRR
rate (see Figure 10.7). To find the relationship between the diffuse source and the
resulting diffuse water pollution, we must monitor the contaminant concentration
in the compliance point and characterize the emission from the source based on
statistical data on the production and usage of the contaminant substance or on
the direct or indirect assessment of the emission. This may serve as a basis for the
management of diffuse pollution in large watersheds such as the River Danube or
similar rivers.

- Fourth step : selection of the most adequate transport model.

To find the best fitting transport model, one should create the conceptual model
of the pollution case: identification of the contaminants, the main transport and
fate processes, the sources or source area, transport routes, characterization of
the impacted area, the land uses and receptors. If the conceptual model has been
established and all the necessary input data (emissions, topographic and climate
maps) are known, the best fitting GIS-based watershed scale transport model will