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ENVIRONMENTAL EXPOSURE ASSESSMENT
When results from biodegradation tests simulating the conditions in surface waters are not
available, the use of results from various screening tests may be considered. Table 7 gives a
proposal for first order rate constants for surface water to be used in local and especially,
regional models, based on the results of screening tests for biodegradability. The proposal is
based on general experience in relation to available data on biodegradation half-lives in surface
waters of readily and not readily biodegradable substances.
The assigned degradation half-lives of an inherently biodegradable substance of 150 days in
surface water (Table 7) and 300 – 30,000 days in soil and sediment (Table 8) will only affect
the predicted regional concentration provided that the residence time of the substance is much
larger than the assigned half-life (i.e. only for substances present in soil compartment and
sediment).
It is noted that the conditions in laboratory screening tests are very different from the conditions
in various environmental compartments. The concentration of the test substance is several orders
of magnitude greater in these screening tests than the concentrations of xenobiotic substances
generally occurring in the environment and thus the kinetic regimes are significantly different.
The temperature is also higher in screening tests than those generally occurring in the
environment. Furthermore the microbial biomass is normally lower under environmental
conditions than those occurring in these screening tests, especially in the tests for inherent
biodegradability. These factors are taken into account in the proposed degradation rates and halflives
in Tables 7 and 8.
Table 7 First order rate constants and half-lives for biodegradation in surface water based on results of screening tests on
biodegradability a)
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Test result Rate constant k (d -1 ) Half-life (d)
Readily biodegradable 4.7 . 10 -2 15
Readily, but failing 10-d window b) 1.4 . 10 -2 50
Inherently biodegradable c) 4.7 . 10 -3 150
Not biodegradable 0 ∞
Notes to Table 7:
a) For use in exposure models these half-lives do not need to be corrected for different environmental temperatures.
b) The 10-day time window concept does not apply to the MITI test. The value obtained in a 14-d window is regarded as acceptable in the
Closed Bottle method, if the number of bottles that would have been required to evaluate the 10-d window would cause the test to
become too unwieldy.
c) Only those inherently degradable substances that fulfil the criteria described in note b) to Table 6 above. The half-life of 150 days
reflects a present "best expert judgement".
The general experience is that a substance passing a test for ready biodegradability may under
most environmental conditions be rapidly degraded and the estimated half-lives for such
substances (cf. Table 7) should therefore be regarded as being in accordance with “the realistic
worst-case concept”. An OECD guidance document for classification of chemicals hazardous for
the aquatic environment (OECD, 2001c) contains a chapter on interpretation of degradation data.
Even though this guidance relates to hazard classification and not risk assessment, many of the
considerations and interpretation principles may also apply in a risk assessment context. One
difference is of course that in the risk assessment context not only a categorisation of the
substance (i.e. a classification) is attempted, but instead an approximate half-life is estimated.