Ecological Risk Assessment of Diffuse and Local Soil Contamination Using Species Sensitivity Distributions - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 661

The simplest approach in Ecological Risk Assessment of mixtures is summation
of Hazard Indices over compounds. Like the use of indices for single compounds,
this method has serious drawbacks. The outcomes of index summations may be
meaningless, e.g., especially when different compounds have quality standards
derived with different uncertainty factors.
A pragmatic approach to address mixtures in Conventional Risk assessments
using SSDs was proposed by De Zwart and Posthuma ( 2006 ). These authors devel-
oped a modeling tool to handle ecological mixture risks for species assemblages,
based on toxic pressure assessment. The approach originates from the models for
addressing mixture impacts in basic pharmacology and toxicology, by applying
Concentration and Response additivity modeling in sequence. The method is there-
fore a mixed-model approach and consists of a series of steps (Fig. 14.13 ). Formulae
and software to run the approach are presented in the aforementioned publication.
First, all contaminants of the actual mixture (measured or predicted concen-
trations) are assigned to groups with similar Toxic Modes of Action, such as
cholinesterase-inhibiting insecticides, photosynthesis inhibitors, and narcotics. This
may not be an easy task, since one contaminant can have multiple modes of action.
For example, an insecticide can be harmful to plants too. Various publications are,
however, available to help assigning Toxic Modes of Action in practice, like Escher
and Hermans ( 2002 ) and Verhaar et al. ( 1992 ). Many contaminants can be sim-
ply assigned to a group exerting a general narcotic Toxic Mode of Action. Despite
the technical problem of assigning contaminants to groups, the outcomes of the
cumulative assessment are likely to be quantitatively robust (see below).
Second, the net actual risk of each of the contaminants separately is derived using
the SSDs for those contaminants as in Fig. 14.4 (X Y use), with site measured (or

predicted) concentrations as input (X). This results in an estimate of the PAF for
each of the contaminants at the site. This step is usually preceded by an assessment

→

Narcosis Uncoupling AChE inhibition

e

b h

Modes of action

d

g

a f

c i

Model of
Concentration Addition

CA CA CA

Model of
Response Addition

RRRA RRA RA RA

Fig. 14.13 The mixed-model approach to derive the multi-substance Potentially Affected Fraction
(msPAF) at sites contaminated with mixtures. For a mixture of nine contaminants ( a - i ), assigned
to three Toxic Modes of Action (abbreviated as: narcosis, uncoupling and AChE inhibition), the
first analysis step is determining the msPAF within each contaminant subgroup using the model
of CA, and then to aggregate these msPAF-CA values to an overall msPAF for the whole mixture
using RA modeling. CA = Concentration Addition, RA = Response Addition