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maximum allowable (or trigger) soil concentration (instead of the door height) X
that would protect a certain chosen (high) fraction of species (Y). The maximum
allowable concentrations would differ between compounds, given a chosen Y-value,
due to differences in ecotoxicity of the particular compound, i.e., the position of
the model distributions of panels A, B and C shifts on the X-axis, depending on a
given compound's ecotoxicity. This can be done on any chosen risk limit, a max-
imum allowable risk level on the Y-axis. Another use is also possible: to estimate
the fraction of species (un)protected, given a certain environmental concentration.
These “inverse” and “forward” uses were both proposed as formats of possible SSD
use in the early stage of SSD-theory development by one of the European inventors
of the approach, Professor Nico van Straalen (Van Straalen and Denneman
1989
).
In that paper, the fifth percentile of the distribution of sensitivities amongst soil
species, based on No-Effect type input data, was chosen (Y-axis) as the maximum
tolerable risk (MTR) level (whereby the choice for NOECs as input data and the
fifth percentile of the chosen model were all policy choices), yielding the so-called
HC5
of a compound, the Hazardous Concentration for 5% of the species (X-axis),
more precisely: 5% of the tested species. A low HC5 implies a high ecotoxicity of
a compound.
Since the invention of SSDs, focus has been historically mostly on the “inverse”
use of SSDs, to set environmental quality standards like standards based on HC5.
Within that use, focus was most on the HC5. The “forward” use - not limited to a
chosen maximum tolerable risk level - gained attention only after finding out that
the use of protective standards alone was insufficient to solve existing (mixture)
contamination problems.
14.3.2 Two Practical Needs and Two Useful SSD Applications
Suter and Cormier (
2008
) distinguished two main types of practices in Ecological
Risk Assessment:
1. Criterion Risk Assessment;
2. Conventional Risk Assessment.
The “forward” and “inverse” use of the SSD model fit well with the different
types of Risk Assessment needed in practice.
Criterion
(or according to the terminology in this topic: Soil quality standard)
Risk Assessment begins with a contaminant and the policy-based selection of a type
and level of effect that is not to be exceeded, and then uses the exposure-response
model to eventually estimate a level of exposure that constitutes the criterion (for
example, the HC5 and the formal soil quality standard). In this assessment, the
choice of an effect metric (Y-value) that is consistent with the selected environ-
mental policy goal (the protection endpoint, and its level of protection) is key. In
soil protection and appraisal, the policy goal usually relates to undisturbed species
diversity or undiminished Ecosystem Services, that is: endpoints at the level of the
whole species assemblage. This fits well with the HC-concept.
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