Environmental Engineering Reference
In-Depth Information
For Criterion Assessments, it may be helpful to set remediation triggers in such
a way that they relate to clear types of impact, such as the loss of species. In such
cases, the remediation trigger may be better estimated from an SSD based on EC50s
or LC50s (Lethal Concentrations) than from NOECs. However, the choice of end-
point is a matter of policy and will vary among governments. In the Netherlands, a
remediation-related criterion is based on the HC50 of the SSD based on NOECs.
The preference for an SSD-EC50 or SSD-LC50 for the appraisal of risks in
highly contaminated soils relates to the fact that the SSD for a contaminant based
on EC50s is plotted to the right of the SSD for that same contaminant that is based
on NOECs (see Fig. 14.11 for a comparison of the two SSDs for an aquatic test sys-
tem, and see also Fig. 14.6 on the comparison of HC5-NOECs with HC5-EC50s).
This logical phenomenon implies that the SSDs differ with regards to the upper-
tail concentration at which the estimated PAF-values do not increase any more at
increasing exposure concentrations. In the figure, the fraction of species exposed
beyond its NOEC is approx. 95% at an ambient concentration of 1mg/L, so that
output from such “chronic” SSDs would become meaningless for ranking expected
impacts at concentrations of 1mg/L or higher. This is not (yet) so for the SSD-EC50,
where the expected acute impact levels of exposures of 1 and 10mg/L still differ
(with estimated Potentially Affected Fractions of approx. 60 and 90%). Moreover,
an estimated fraction of species beyond their EC50 is expected to imply serious
consequences for those species, whereas an estimated fraction of species exposed
beyond their NOEC has no such intuitive interpretation.
The example and intuitive reasoning suggest that one should consider the soil
concentrations of the site of concern when selecting appropriate test parameters to
1
Pentachlorophenol
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.0001
0.001
0.01
0.100
1.0
10
100
1000
Toxicity (mg.l -1 )
Fig. 14.11 Different ecotoxicity data for pentachlorophenol in aquatic test systems illustrate that
the position of the NOECs ( left , 13 species) and EC50s ( right , 136 species) values used as input
data for SSDs differ in position, as do the two resulting SSDs (SSD-NOEC and SSD-EC50) fitted
to those data. The SSD-EC50 is to the right of the SSD-NOEC
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