technical guidance documents - Institute for Health and Consumer ...

ENVIRONMENTAL EXPOSURE ASSESSMENT
• diffusive transport, as e.g. gas absorption and volatilisation. Diffusive mass transfer between
two compartments goes both ways, the net flow may be either way, depending on the
concentration in both compartments;
• advective transport, as e.g. deposition, run-off, erosion. In the case of advective transport, a
substance is carried from one compartment into another by a carrier that physically flows
from one compartment into the other. Therefore, advective transport is strictly one-way.
Substance input to the model is regarded as continuous and equivalent to continuous diffuse
emission. The results from the model are steady-state concentrations, which can be regarded as
estimates of long-term average exposure levels. The fact that a steady state between the
compartments is calculated, does not imply that the compartment to which the emission takes
place is of no importance.
In a Mackay-type level III model, the distribution and absolute concentrations may highly
depend upon the compartment of entry.
Advective import and export (defined as inflow from outside the model or outflow from the
model environment) can be very important for the outcome of both regional and local model
calculations. Therefore, the concentration of a substance at the “border” of the region must be
taken into account. This is defined as the background concentration of a substance. The
background concentration in a local model can be obtained from the outcome of the regional
model. For substances with many relatively small point sources, this background concentration
may represent a significant addition to the concentration from a local source. The background
concentration in the regional model has to be calculated using a similar box model of a larger
scale, e.g. with the size of the European continent. In this continental model, however, it is
assumed that no inflow of air and water across the boundaries occurs. Furthermore it is assumed
that all substance releases enter into this continental environment. The resulting steady-state
concentrations are then used as transboundary or background concentrations in the regional
model. The continental and regional computations should thus be done in sequence. Figure 1
visualises the relationship between the concentrations calculated for the different model scales.
For both the regional and continental scale, the total emission amounts (through diffuse and
point sources, summed over all stages of the life-cycle) are used.
For the PECregional calculation, in contrast to PEClocal, an average percentage connection rate
to STPs should be included in the calculation. This leads to a more realistic estimation of the
likely background concentration on a regional scale. For the purposes of the generic regional
model, a STP connection rate of 80% (the EU average according to Appendix XII) will be
assumed.
The results from the regional model should be interpreted with caution. The environmental
concentrations are averages for the entire regional compartments (which were assumed well
mixed). Locally, concentrations may be much higher than these average values. Furthermore,
there is a considerable degree of uncertainty due to the uncertainty in the determination of input
parameters (e.g. degradation rates, partitioning coefficients).
Model parameters for PECregional
When calculating the PECregional it is important which modelling parameters are chosen and
what fraction of the total emissions is used as emission for the region. There are two different
possibilities:
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