Environmental Engineering Reference
In-Depth Information
regulators. The discussions have been summarized extensively in Posthuma et al.
(
2002b
). The discussions have focused primarily on the “reverse” use of SSDs, the
derivation of quality standards.
The issues as discussed can be divided into two different types:
•
technical issues;
•
validation issues.
Technical issues include the selection and quality assessment of test data, han-
dling multiple test data for a single species, the choice of the statistical approach
(including non-parametric methods and fitting of a standard parametric model), the
choice of a risk limit (like 5%) to derive standards, the choice of the estimated HCp
or its lower confidence interval to be used in regulation, the choice of test endpoints
(e.g., LC50s or NOECs), the required number of input data and the inclusion of
different taxonomic groups. The main driver for these discussions was to get the
“best and most standardized SSD-approach” which should be optimally representa-
tive for “the” soil system. “The” is placed in quotes, because there is no uniform soil
system. Technical discussions of SSDs have paid surprisingly little attention to the
diversity of soil systems
versus
the wish for standardization. Given the emphasis
on the reverse (standards) use of SSDs, validation discussions mainly focused on
the question whether the standards derived with SSDs were indeed sufficiently pro-
tective, and whether any extrapolation of laboratory test data to appraise ecological
impacts in the field can be valid.
These discussions have been worthwhile in a scientific sense, since they have
led to improvements in the methods. However, since they often address a standard
method (e.g., on the number of required test species data, and on representation of
different taxonomic groups as required for standard setting), one should take into
account that a standardized method relates strictly to the idea of a single problem
definition. An example is the derivation of soil quality standards for a compound
in a country. Such standards often have a legal status, and this implies that they
should be consistent in their derivation. This has led to the regulatory choice of
well-defined uniform methods, and the fixation of those methods in guidance doc-
uments. When the problem definition, however, requires a “forward” use of SSDs
(i.e., Conventional Risk Assessment), the uniform choices made for the derivation
of quality standards need not the best ones. For example, when assessing the net
risk to the local soil biota caused by a local metal mixture, one is able to apply
scientific knowledge
not
used in the standard SSD-method. For example, there is
clear evidence that the effects of metals in acid soils are much higher than in neutral
soils (Janssen et al.
1997
). Hence, this should show up in the site-specific (forward)
Risk Assessment, such that a certain metal concentration in an acid soil is indeed
more risky than one in a neutral soil. This can be solved, for example, by selecting
ecotoxicity input data for forward-SSD use from tests in relatively acid test soils, to
implicitly address the high availability of metals in these tests. Re-use of the stan-
dard choice methods (e.g., taking the average of all available data, irrespective of
pH) would result in risk levels indifferent to soil acidity, which is wrong (see e.g.
Cleven et al. (
1993
) for some tabulated examples of large pH effects; page 60 in
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