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APPENDIX VIII
can make a significant contribution to the overall levels. For many substances, however, local
emissions from point sources will need to be considered as well as the wider contribution to the
regional burden. New substances, for example, must be assessed for their impact following local
emissions. In general their contribution to the larger environmental burden will be small until
high annual tonnages are reached.
Local exposure assessment
As with organic compounds, the precise emissions will need to be identified and quantified for
the whole life-cycle of the substance. Emission factors should initially be based on the substance
being considered. It is important to know whether the substance is soluble in water, or can be
transformed into a soluble form. Thus some knowledge of the chemistry of the particular
substance and its interaction with the receiving media is important. Where the metal compound
is soluble or can be transformed to a soluble form, the prediction of the environmental
concentration, PEClocal, can be based on the relevant soluble metal ion. The behaviour of the
substance in a wastewater treatment plant can be modelled using SimpleTreat, although
measured Kp values will have to be used (Section 2.3.7 of main text). Since the actual
bioavailability of the metal ion will be determined by the properties of the receiving media, such
as the pH and water hardness, the precise physico-chemical characteristics of this receiving
media must be defined. In general, it will be defined in a way which optimises the bioavailability
of the toxic species. Speciation models exist which may be used to determine the soluble
fraction. The partitioning behaviour of the substance to sludge/sediment/soil can be based on the
appropriate Kp values for the soluble ion.
In some cases, the metal compound will be only poorly soluble and sufficiently stable to not
rapidly transform to a water soluble form. In these circumstances, the substance itself should be
assessed taking into account its specific partitioning characteristics. For the aquatic environment,
it can be assumed as a first estimate that the substance will dissolve up to its water solubility
limit, and that this fraction will be the bioavailable form. Refinement of the assessment may take
into account kinetics of the dissolution.
Regional exposure assessment
As for organic substances, all emissions from both point and diffuse sources are assumed to
contribute to the regional concentration, PECregional. Because of the wide range of
transformation processes and longer timescales involved, it is assumed that all the individual
metal compounds are changed to the ionic species. Where possible, information on kinetics of
transformation processes should be taken into account.
As bioavailability is influenced by various physico-chemical characteristics of the environment it
is important to define a 'standard environment', especially for a regional assessment. It is
proposed that a regional assessment is carried out under conditions that optimise the bioavailability
with respect to ranges for pH, water hardness etc that are found in the natural environment. This
environment will probably differ for each metal assessed. Multimedia fate models can be used to
assess exposure of man and ecosystems to metals on a regional scale. In applying multimedia fate
models all emissions, including point sources, are assumed to be diffuse.
Transport of metals between the aqueous phase and soil/sediment/suspended matter should be
described on the basis of measured soil/water, sediment/water and suspended matter/water
equilibrium partition coefficients (Kp), instead of using common mathematical relationships
based on, for example, octanol-water partition coefficients, as is usually done for organic
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