Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 360

be selected to estimate the water and solid flows and the transport of contaminants
from the source through the unsaturated and saturated soil zones or by the surface
runoff to the point of compliance.

- Fifth step : determination of the risk reduction/retention rate (RRR) of the site.

While the transport from point sources can be monitored and data can be collected
for a numerical model, the transport from diffuse pollution sources is partly mod-
eled with analytical models. Most GIS models are based on hydrological transport
models. Hydrological models can characterize water and solid flows, but can-
not describe the fate of the transported contaminants. Therefore, the quantitative
change of the transported contaminant, i.e., partition between the phases, trans-
formation and degradation cannot be followed step by step along the transport
pathway by only hydrological models, but we have to use integrated models which
consider the fate of the contaminants during the transport. If the input informa-
tion is not available to create a GIS-based integrated transport and fate model, we
use the hydrological model and treat the watershed as a black box from the point
of view of its interactions with water or sediment transported contaminants. The
concentration can be calculated in the compliance point by the formula E /runoff
flux. Having the contaminant mass or concentration emitted from the source and
the actual contaminant concentration in the point of compliance selected in the
first step, the risk reduction and retention rate of the site has been determined
by “forward'' model calculation as shown in Figure 10.12. The transport path-
way from the source to the compliance point is handled as a black box with an
average retention potential characterized by the RRR rate ( RRR watershed ). RRR is
calculated e.g. as the ratio of the actual (measured) contaminant concentration in
the point of compliance to the contaminant mass emitted from the source (Figures
10.7 and 10.12).

- Sixth step: “backward'', also called “inverse'' calculation of the acceptable

maximum (emission) from the source.
The target emission from the source corresponding to the quality criteria for each
contaminant at a particular point of compliance is determined by the inverse appli-
cation of the GIS-based transport model. Having set the site-specific quality criteria
in the compliance point ( PNEC ), we can predict the site-specific target value in
the source by backward mode modeling. To do so the known interactions between
the watershed and the contaminant (fate modeling) are taken into account or the
black box approach with the RRR rate obtained by forward transport and fate
modeling is used (Figures 10.11 and Figure 10.12).

This way the transport modeling approach has established the maximum permis-
sible concentration of the contaminants in the source to satisfy the quality criteria
imposed on the groundwater/surface water, on soil or on any of the food chain
elements.
Such an approach was used by Gruiz et al. (2009), Vaszita et al. (2009), as detailed
in Volume 5 to establish the target risk of point and diffuse pollution sources in an aban-
doned mining area in Hungary. Scharff (2013) reported the use of a similar approach
by Hjelmar et al. (2001) in developing acceptance criteria for landfilling of waste.
The transport modeling approach to establish the maximum permissible concen-
trations of the contaminants in the source which satisfy the quality criteria imposed