Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 342

come from and how much contaminant may enter the system without causing unac-
ceptable concentration and risk in a certain point (e.g., the compliance point) (see also
Chapter 4.3.1). The need for good quality, site-specific environmental quality crite-
ria is always the key point of the assessment whether the forward or inverse mode is
applied.
4.2 GIS-based watershed-scale transport and
risk modeling of diffuse pollution
Because of the large surface area containing infinite number of discrete points, the
modeling of transport and risk of diffuse pollution requires a GIS-based watershed-
scale approach. The pollution is transported by runoff from several discrete points
into various directions and it accumulates in the arrival points, depending on rainfall,
topography, soil conditions, geology, surface cover, etc. The watershed-scale approach
requires that the pollutant transport from any discrete point of an extended area be
predicted and studied within the watershed. The watershed-scale GIS flow accumu-
lation model allows for estimation of water and solid fluxes at any point. Digitized
maps of the watershed and the water balance (Figure 10.6) are the starting points of the
GIS-based transport models. Water balance gives the proportions of runoff, infiltra-
tion, evaporation and transpiration of the plants. Land cover, surface characteristics,
soil type and permeability are necessary pieces of information for GIS-based modeling.
Much of the above information is available in digitized form.
When calculating the diffuse pollution load on soil, contaminant deposition from
air, transport by water and solid should be aggregated. The fluxes are quantified with
real volumes (rainfall, runoff, infiltrated and evaporated water) according to the water
balance (Figure 10.6) of the watershed. Once the distribution of the precipitation from
rainfall is quantified, the GIS-based flow accumulation is calibrated and the actual
fluxes are obtained.
The GIS-based transport model can forecast the effect of any natural and man-
made changes including the results of risk reduction measures, and calculate the
necessary scale of decontamination or reduction in emission. Figure 10.6 shows the
scheme of transport and risk modeling for primary diffuse sources (disperse con-
taminating sources) in a watershed while the steps of GIS-based watershed transport
modeling are presented below.
4.2.1 Emitted amount from diffuse sources and the risk
management concept
Watershed scale transport modeling predicts the emitted contaminant amount from
diffuse sources ( E source ) allowing for the calculation of its concentration ( C runoff ) and
prediction of PEC river (Step 1). If PEC river > EQC river , the risk has to be reduced by
emission mitigation (Steps 2 and 3). Estimation of the emission from diffuse sources
may be based on statistics of produced and used amounts of chemicals and on default
emission factors, as well as on an estimate of diffusely dispersed sources in the envi-
ronment (nutrient level, waste mass, etc.). Contaminant leaching models or pilot