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

ENVIRONMENTAL EXPOSURE ASSESSMENT
Another difference is that for risk assessment, the availability of high quality test data is required
in virtually all cases and further testing may therefore be required in the case of low quality data.
In distribution models, calculations are performed for compartments each consisting of
homogeneous sub-compartments, i.e. surface water containing dissolved organic carbon and
suspended matter, sediment containing porewater and a solid phase, and soil containing air,
porewater and a solid phase. Since it is assumed that no degradation takes place in the sorbed
phase, the rate constant for the surface water, bulk sediment or soil in principle depends on the
suspended matter/water, sediment/water or soil/water partition coefficient of the substance. With
increasing hydrophobicity (sorption) of the substance, the freely dissolved fraction present in the
water phase available for degradation decreases, and therefore the overall rate constant should
also decrease. However, for surface waters the influence of sorption is already comprised in the
degradation rates when they are determined for bulk water in simulation tests employing the
same conditions as in the aquatic environment. Neither is it needed to consider the influence of
sorption processes when rate constants are established from screening test results due to the
well-established practice to conclude on biodegradability in the environment from such data.
Also for assessment of biodegradation in soil or sediment, data from relevant simulation tests are
preferred. Simulation tests such as the OECD 307 “Aerobic and anaerobic transformation in
soil” (OECD, 2000b; EU Annex V draft C.23) and the OECD 308 “Aerobic and anaerobic
transformation in aquatic sediment systems” (OECD, 2000c; EU Annex V draft C.24) are
available. The basis for these methods was initially developed for pesticides, e.g. guidelines of
BBA (BBA, 1986; BBA, 1990a) and US EPA. The draft ISO/DIS 14592-1 standard includes an
option for determination of biodegradability in a surface water/sediment suspension. Of course
this test does not directly simulate the conditions in non-disturbed sediment. The measured halflife
in water/sediment tests may be dependent on the relative volume of water and sediment
employed in the test.
When such simulation test data are available, the applicability of the results from the tests should
be evaluated on a case-by-case basis employing expert judgement when used in a risk
assessment.
When no data from tests simulating the conditions in soil or sediment are available, the use of
screening test data may be considered. The guidance for use of such data is based on the general
recognition that for substances with low Kp values at present not enough empirical data are
available to assume some sort of dependence of the soil biodegradation half-life on the
solids/water partition coefficient. Nevertheless, for substances with high Kp values there is
evidence that some sort of Kp dependence exists. Therefore degradation half-life classes for
(bulk) soil, partly based on Kp are presented in Table 8. If a half-life from a surface water
simulation test is available it may, in a similar manner, form the basis for the establishment of a
half-life in soil. The half-lives indicated in the table are considered conservative.
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