technical guidance documents - Institute for Health and Consumer ...

EFFECTS ASSESSMENT
A suite of soil tests should therefore ideally be designed to obtain data relevant to:
• primary producers (plants);
• consumers (for example invertebrates that represent an important group in the soil
compartment);
• decomposers (comprising microorganisms that play an important role in foodwebs and
nutrients cycling).
Natural soils used in ecotoxicological tests differ in characteristics such as organic matter and
clay content, soil pH and soil moisture content. The bioavailability of the test compound, and
therefore the toxicity observed, is influenced by these soil properties. This means that results
from different test soils cannot be compared directly. As far as possible, toxicity tests should be
conducted in conditions (as regards the nature of the soil, its organic content and any other
parameter that could influence the bioavailability of the substance) where the test substance is
bioavailable to the tests organism(s). However, if possible data should be normalized using
relationships that describe the bioavailability of chemicals in soils. Results are converted to a
standard soil, which is defined as a soil with an organic matter content of 3.4% (see Section
2.3.4). For non-ionic organic compounds it is assumed that bioavailability is determined by the
organic matter content only. NOECs and L(E)C50s are corrected according to the formula :
Explanation of symbols
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NOEC or L(E)C = NOEC or L(E)C Fom
50(standard)
50(exp) •
Fom
NOEC or NOEC or L(E)C50 in experiment [mg . kg -1 ]
L(E)C50exp
soil(standard)
soil( exp)
Fomsoil(standard) fraction organic matter in standard soil [kg . kg-1 ] Table 5
Fomsoil(exp) fraction organic matter in experimental soil [kg . kg-1 ]
NOEC or NOEC or L(E)C50 in standard soil [mg . kg-1 L(E)C50standard
]
It should be noted that this recommended normalisation is only appropriate when it can be assumed
that the binding behaviour of a non-ionic organic substance in question is predominantly driven by
its logKow, and that organisms are exposed predominantly via pore water.
Three situations can be distinguished for deriving a PNECsoil:
• when no toxicity data are available for soil organisms, the equilibrium partitioning method is
applied to identify a potential risk to soil organisms. This method is regarded as a “screening
approach” and is explained in Section 3.6.2.1 (see also Section 3.5.2 sediment);
• when toxicity data are available for a producer, a consumer and/or a decomposer the
PNECsoil is calculated using assessment factors as presented in Section 3.6.2.2;
• when only one test result with soil dwelling organisms is available the risk assessment is
performed both on the basis of this result using assessment factors and on the basis of the
equilibrium partition method. From both PECsoil/PNECsoil ratios the highest one is chosen
for the risk characterisation.
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