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

ENVIRONMENTAL EXPOSURE ASSESSMENT
Such site-specific assessments should only be used when it is known that all the emissions
emanating from the particular point in the life-cycle e.g. manufacture, arise from a limited
number of specific and identifiable sites. In these circumstances each specific point of release
will need to be assessed individually. If it is not possible to make this judgement, then the default
assumptions should be applied. In site-specific assessments, due account can be taken of the true
dilution available to the given emission as well as the impact of degradation, volatilisation, etc.
in the derivation of the PEC. Normally, only dilution and adsorption to suspended sediment need
to be considered but site-specific conditions may indicate that local distribution models can be
used.
It must be noted that with the assumption of complete mixing of the effluent in the surface water
no account is taken of the fact that in reality in the mixing zone higher concentrations will occur.
For situations with relatively low dilution factors this mixing-zone effect can be accepted. For
situations with very high dilution factors, however, the mixing zones may be very long and the
overall area that is impacted by the effluent before it is completely mixed can be very
substantial. Therefore, in case of site-specific assessments the dilution factor that is applied for
calculation of the local concentration in surface water should not be greater than 1000.
If no measured data are available on the partition coefficient between suspended matter and
water, Kpsusp, it can be estimated from the Koc of the substance, determined for other sorbents
like soil or sediments (Section 2.3.5) by taking into account different organic carbon contents of
the media.
For some substances it may be possible that PECs are calculated in water which are in excess of
the water solubility. These results need to be interpreted carefully on a case-by-case basis. The
concentration in surface water will not be corrected, but the result needs to be flagged. The PEC
has to be interpreted based on the effects found in the aquatic toxicity tests.
In a situation where a substance is released through several point sources into the same river, the
resulting cumulative concentration may in a first approach be estimated by assuming it to be
released from one point source. If this PEC leads to “concern” then refined approaches may be
used, such as river flow models, e.g. OECD (1992a) which address the specific emission pattern
as well as river parameters.
The local concentration in surface water is calculated as follows.
76
Clocal
Explanation of symbols
water
=
( 1+
Kp
susp
Clocaleff
- 6
• SUSPwater
•10
)
•
DILUTION
Clocaleff concentration of the substance in the STP effluent [mg . l -1 ] eq. (33)
Kpsusp solids-water partitioning coefficient of suspended matter [l . kg -1 ] eq. (23)
SUSPwater concentration of suspended matter in the river [mg . l -1 ] 15
DILUTION dilution factor [-] 10
Clocalwater local concentration in surface water during emission episode [mg . l -1 ]
(45)