Ecological Risk Assessment of Diffuse and Local Soil Contamination Using Species Sensitivity Distributions - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 684

mammals. Those analyses yield ecological soil screening levels, which are defined
as screening values that can be used routinely to identify those contaminants of
potential concern in soils requiring further evaluation in a baseline Ecological Risk
Assessment. Although these screening levels were developed specifically to be
used in the Superfund Ecological Risk Assessment process, other federal, state,
or private environment assessment or remediation programs can use these values
to screen soil contaminants and sites in order to determine if additional ecological
site study was warranted. The ecological soil screening levels were not designed
to be used as remediation targets. For plants and invertebrates, the derivation
procedure of soil screening levels involves collection of ecotoxicity data for plants
and invertebrates and determination of the geometric mean value of the set of
quality-checked and adopted test data (U.S.EPA 2003 , 2005 ).
If mixtures are of concern, a simple concentration additive model, the Hazard
Index ( HI ), is typically used, as follows:

HI = ( C ei / C bi ) . (14.2)

If HQ or HI is greater than 1 for any contaminant or mixture, the assessment may
proceed to the second stage.
Definitive assessments use site-specific information concerning biological
responses including toxicity tests of site media, modeling of food web transfer of
contaminants, measurements of tissue concentrations, in-situ toxicity tests, and site
surveys of biota (Luftig 1999 ; Suter et al. 2000 ). This can include an estimation of
the fraction of species of different types that may be affected (i.e., PAFs, although
that term is not used). Ideally, multiple types of evidence are generated for each end-
point so that the evidence can be weighed and the risks can be estimated with some
confidence (comparable with the concept presented in Rutgers and Jensen ( Chapter
15 of this topic). If significant risks are identified, remediation targets are developed
and remedial alternatives are assessed.
This two-stage assessment process brackets the SSD-based method described
in the rest of this chapter. That is, the quotient method itself is considerably less
sophisticated than the SSD approach, but the weighing of multiple lines of site-
specific evidence is more likely to accurately characterize the ecological risks. The
cost and effort of the weight of evidence approach is justified at many Superfund
sites, because there are relatively few of them and many are large and have signif-
icant ecological resources. Examples include the Fox River/Green Bay system, the
Rocky Mountain Arsenal, the Oak Ridge and Hanford Reservations, and the Coeur
d'Alene River watershed.

14.15 Reflections and Conclusions

Reflection on and comparison of the Dutch and US approaches as two selected
examples shows that the use of one type of model (the SSD) can vary depend-
ing on the context. In particular, it is surprising to see that a small country
like the Netherlands has identified hundreds of thousands of potentially seriously