The assessment of soil quality has usually focused on human health
protection as the main objective. Recently, criteria for the
protection of ecosystems have been incorporated and
ecotoxicological analyses are recommended to estimate the risk to
ecological receptors associated with contaminants in soils (Calow,
1993; Stephenson et al., 2002; Loibner et al., 2003; Robidoux et
al., 2004b). The ecotoxicological assessment of soils is mostly
based on the toxicity test with selected organisms. Two
complementary approaches are available. The first approach consists
in the identification of toxicity thresholds for each relevant
pollutant, thresholds that are based on the evaluation of effects
of chemical substances on selected organisms representing relevant
ecological receptors. The results of these assays are used for
setting soil quality standards for each pollutant or pollutant
class. Risk assessment tools can be used for this purpose,
pre-establishing acceptable levels of risk. The contamination level
is based on the comparison of the concentration of contaminants
measured in the soil with the standards established from the
thresholds. Although field and semi-field information can be
incorporated in the higher tier steps, the thresholds are mostly
developed from standardised toxicity assays conducted under
laboratory conditions following international (e.g. OECD, ISO) or
national (e.g. USEPA, ASTM) guidelines. In the second approach,
toxicity assays are performed directly with the contaminated media
(soil, water, sediment). This alternative, performing the assays
with environmental samples, constitutes the method called direct
(eco)toxicity assessment (DTA), and is based on modified bioassays.
Most regulations have developed soil quality standards based on
toxicity assays. However, due to the limitations in the lab to
field extrapolation, trends were directed towards the combination
of chemical analysis and DTA (Peterson et al., 1990; Torstensson,
1993; Torslov et al., 1997). In this book, both alternatives will
be compared. The main difference between both approaches is that in
the first case, a reference "uncontaminated" soil sample is spiked
with one or a few chemicals at different concentrations, while in
the DTA approach real soil samples are collected at the
contaminated site, therefore containing a realistic combination of
the different pollutants present in the area, the field sample can
be then tested and/or "diluted" with "uncontaminated" soil to
create a pollution gradient. The toxicity of the spiked or
collected/diluted samples is measured and concentration/response
relationships obtained in both cases. To understand better this
comparison, in this book the term "toxicity test" will be used for
the first approach: toxicity tests with samples spiked at the lab;
while the term "bioassay" will be used for the DTA approach:
samples collected at the field.
General
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