Environmental Engineering Reference
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Pressure which is realistic for seriously contaminated sites in the Netherlands
and useful for the Triad approach, EC (Effect Concentration) data from the liter-
ature have been used (EC50 values, concentration of toxicant demonstrating 50%
effect) instead of NOEC (No-Observed Effect Concentrations) data (Rutgers
et al. 2008b ). This is a more realistic and less conservative procedure and com-
patible with scaling procedures in the other lines of evidence of the Triad. A
correction for bioavailability, however, is not recommended at this stage, because
frameworks for implementation of bioavailability are still in development (Brand
et al. 2009 ). Some contaminated sites, however, were assessed on the basis
of testing the pore water concentrations, using basic assumptions for ERA in
surface waters (Jensen and Mesman 2006 ; Rutgers and Den Besten 2005 ).
2. Bioassays: Screening level and standardized bioassays are recommended, but
there is no detailed prescription. Relevant aspects for selecting a bioassay
are sensitivity and validity, which should be generally accepted or carefully
addressed. It is not always necessary to select bioassays with autochthonous
organisms or to invest a lot of effort in this kind of tests, because differences
between autochthonous and exotic organisms are usually much smaller than the
differences between exposure conditions in the field and laboratory and other
lab-to-field extrapolation issues (Rutgers and Den Besten 2005 ). Among the
more popular screening tests are elutriate-based bioassays with small inverte-
brates, algae, plants or bacteria (e.g. Microtox ) and whole-soil bioassay with soil
invertebrates or plants. The response of the bioassays is simply expressed as a
fraction of effect, ranging from 0 (no effect) to 1 (maximum theoretical effect
level). More details can be found in the decision support system by Jensen and
Mesman ( 2006 ).
3. Ecological observations in the field: At this stage of the Triad approach it is rec-
ommended to include plant surveys of the contaminated site and reference site(s)
(with no or low contamination levels). As an alternative, simple determination of
the community composition and abundance of soil invertebrates like nematodes,
enchytraeids (pot-worms), earthworms and springtails may be feasible. Again,
the response to the contamination should be expressed as a fraction of effect
ranging from 0 (no effect) to 1 (maximum theoretical effect level). More details
about the calculation of risk from multivariate observations can be found in, for
example, Jensen and Mesman ( 2006 ) and Dagnino et al. ( 2008 ).
Box 15.1 Chemical characterization of effects
The Toxic Pressure (TP) from the complete mixture of contaminants in soil
samples is obtained from mixture modeling using models for concentration
addition (CA) and response addition (RA) (De Zwart and Posthuma 2005 ;
Rutgers et al. 2008b ). In the first steps the combination toxicity of any group
with 1 or more toxicants with a comparable mode of action is calculated using
the CA model:
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