Environmental Engineering Reference
In-Depth Information
When an HC5 based on NOECs is adopted as a regulatory soil quality stan-
dard, it can be understood that an exposure at- or lower than that HC5-concentration
implies the protection of (at least) 95% of the tested species against adverse effects
from contaminants in soil on those vital characteristics that are included in toxicity
tests (like growth and reproduction). When the tested species sensitivity distribution
resembles the natural sensitivity distribution, the HC5 represents a 95%-protection
criterion , which is an exposure level at which the structural properties of real soil
ecosystems are almost all protected. Note specifically, that exceedance of the HC5-
NOEC does not in any way mean an actual loss of 5% of species, although this
sometimes pops up as implicit but wrong “gut feeling” about the meaning of the
HC5 in discussions among stakeholders.
14.4.1.2 Hazard Potential or Toxic Pressure
When assessing existing soil contamination in Conventional Risk Assessments, both
the probabilities of effects and their magnitudes increase with increasing soil con-
centrations, as in the field gradient in Fig. 14.2 . SSDs closely link to this dual
principle of assessing risks: essentially, SSDs help to quantify probable impacts
(assuming that the community is represented by the set of test species), given a
soil concentration of a contaminant.
In the “forward” use, SSDs can be applied to quantify the Hazard Potential
(HP) of a contaminated soil sample, separately for each of the compounds present.
In addition, by mixture modeling (see Section 14.10.6 ) a net impact of a mixture
exposure can be quantified.
The term HP is conceptually consistent with the earlier introduced concept of
HC and with the entity predicted (the fraction of test species that would potentially
suffer at a given exposure). It is equivalent to the earlier term “toxic pressure” (e.g.,
Klepper et al. 1998 ), which suggests that a local mixture of contaminants would
exert a quantifiable pressure on the tested biota when exposed in the soil, with
higher probable impacts at higher toxic pressures. For the remainder of the chap-
ter we will use toxic pressure instead of Hazard Potential. The extent to which these
model results predict effects on real species assemblages in the field depends on the
relationship between the SSD and the field species sensitivity distribution.
14.4.2 Extrapolation: From Probably to Potentially
Affected Fraction
The outcomes of the approaches shown in Panel B and C of Fig. 14.3 would, in the
case of the body height data, be relevant to the set of people attending the meeting.
Likewise, in the case of SSDs, the predicted risks are relevant for the set of species
tested.
Use of the distribution to predict how many people at other meetings could safely
leave the room would however pose an extra problem, namely: a need to extrapolate
the known distribution to another situation.
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