Ecological Risk Assessment of Diffuse and Local Soil Contamination Using Species Sensitivity Distributions - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 660

100

80

High

60

Moderate

40

20

Low

0

0 20 40 60 80 100

Rank ordered exposure concentrations

Fig. 14.12 Imaginary example of cumulative profile plots (a type of Joint Probability Curve, JPC).
The three lines show the results for three sampling events: at the moment of discovery a highly con-
taminated site (“high”) with space-varying contamination (and thus space-varying risk levels), after
remediation of the hot spots within the site (resulting in “moderate” contamination and risks), and
after waiting for autonomous breakdown of organic contaminants by soil microorganisms (“low”:
exposure concentrations and risks have dropped further due to breakdown of contaminants by
microorganisms). In the “high” case, 50% of the soil samples within the site would cause effects
on more than 45% of the species (high risk profile), while in the “low” case, only a few samples
would affect more than 10% of the species. Graphs produced by ETX

However, this model has appeared to be wrong in a multitude of single-species tox-
icity tests of mixtures. An analysis of those tests, using alternative mixture models,
showed that mixture impacts are most often larger than those of the most toxic com-
pound; moreover, mixture impacts could be well predicted based on mode-of-action
considerations and associated models (Altenburger et al. 2000 ; Backhaus et al. 2004 ;
Hermens and Leeuwangh 1982 ; Hermens et al. 1984 ; Silva et al. 2002 ). Due to
the observations in single-species tests of mixtures, there is no reasonable motive
to expect that multi-species responses (which are the cumulation of single-species
responses, though modified by species interactions) should follow the “no addi-
tion” model. Also for species assemblages, the net effect of mixtures expectedly
will resemble an aggregation of the single-compounds effects rather than an effect
level of the most toxic compound.
This assemblage-level impact expectation is difficult to validate. It is techni-
cally very difficult to execute empirical studies such that one can quantify mixture
effects in multi-species mixture studies and to analyze those quantified responses
on the basis of predicted mixtures responses as generated from (simultaneously
run) single-compound effect studies and appropriate mixture models. However, the
study of Pedersen and Petersen ( 1996 ) is supportive of the rule of thumb that mix-
ture effects at the level of species assemblages can be evaluated using a mixture
modeling approach.