Protocol for the Derivation of Environmental and Human ... - CCME

Part A, Section 3
Section 3
Use of Canadian Soil Quality Guidelines
The soil quality guidelines derived using the protocol will replace the Interim Environmental Quality
Criteria for Contaminated Sites (CCME, 1991a) and will be used as described in Section 1.3.2.2. Soil
quality guidelines represent "clean down to levels" at contaminated sites and not "pollute up to levels" for
less contaminated sites. They are not intended to be used to manage pristine sites.
The development of ecological effects-based soil quality guidelines is, in a sense, a scaled down risk
assessment for generic conditions and therefore the following uncertainties apply.
Primary Error in Model Input Parameters
Model error created from the inappropriate aggregation of variables (i.e., multiple species toxicity data
and endpoints) used to determine acceptable threshold effect concentrations (TECs), and the error
associated with the input variables (individual toxicity data) themselves must be considered in guidelines
derivation.
An examination of the toxicological data reported for soil-dwelling organisms and terrestrial animals
revealed that common reference toxicity values (i.e., LOEC, NOEC, LC 50 , EC 50 ) were available for
guidelines derivation. While it is possible to quantify the error associated with predictions of LC 50 and
EC 50 data using the confidence intervals reported, it is difficult to estimate the uncertainty associated
with improper use of a statistical model (e.g., probit or logit) applied to the test data (e.g., when data
display hormesis).
Perhaps the most significant source of uncertainty in soil toxicity data available for guidelines derivation
is attributed to LOEC and NOEC data. No observable effects concentration and LOEC data are
hypothesis driven and are thus subject to Type I and Type II error as well as variation of the test design
itself. Again, these data have been generated from the improper use of statistical models (usually by
ANOVA, paired means comparisons, etc.) A sizable proportion of the soil toxicity data available for
guidelines derivation are LOEC and NOEC, and because these data are still considered useful, a
discussion on the reasons for their uncertainty is warranted.
Traditionally, LOEC and NOEC data were estimated without considering the dose-response curve
(i.e., for LOECs, by using the lowest test concentration that is significantly different from the controls; or
for NOECs, the highest test concentration not significantly different from controls). Some researchers
view this as problematic (Bruce and Versteeg, 1992) and suggest that LOEC or NOEC concentrations
suffer from the fact that:
• they must be one of the test concentrations used in the study and are therefore dependent on the
range of concentrations used,
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