3. Effects assessment
3.6 Effects assessment for the terrestrial compartment
3.6 Effects assessment for the terrestrial compartment
Soils used in ecotoxicological tests differ in characteristics like organic matter and clay content, soil pH and soil moisture content. The bioavailability of the test compound, and therefore the toxicity found, is influenced by these soil properties. This means that results from different test soils cannot be compared as such. Subsequently, data have to be normalised using relationships which 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:
NOEC or L(E) C = NOEC or L(E) C Fom
50(standard) Fom
soil(standard) soil( ) 50(exp)
exp
• (55)
Explanation of symbols
NOEC or NOEC or L(E)C50 in experiment [mg.kg-1] L(E)C50exp
Fomsoil(standard) fraction organic matter in standard soil [kg.kg-1] Table 3
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
Three situations can be distinguished for deriving a PNECsoil:
• if 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);
• if toxicity data are available for a producer, a consumer and/or a decomposer the PNECsoil is calculated using assessment factors. The assessment factors are presented in section 3.6.2.2;
• if only one test result with soil dwelling organisms is available the risk assessment is performed both on the basis of this test 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.
3.6.2.1 Calculation of PNEC using the equilibrium partitioning method
As for sediment the equilibrium partitioning method for soil assumes that the bioavailability and therefore the toxicity of chemicals to soil organisms is only determined by the concentration in the pore water of the soil. Further effects that chemicals adsorbed to soil particles have on soil organisms by ingestion are not considered by this approach. The PNECsoil
is calculated as follows:
PNEC 1000 RHO
= K
PNEC water
soil soil
soil water
•
− • (56)
Explanation of symbols
PNECwater Predicted No Effect Concentration in water [mg.l-1]
RHOsoil bulk density of wet soil [kg.m-3] eq. (4)
Ksoil-water partition coefficient soil water [m3.m-3] eq. (9)
PNECsoil Predicted No Effect Concentration in soil [mg.kg-1]
The applicability of the equilibrium partitioning method has been tested less for soil than for sediment-dwelling organisms. Van Gestel and Ma (1993) have shown the model to be valid for short term toxicity of several chlorophenols, chlorobenzenes and chloroanilines to earthworms.
As for sediment the equilibrium partitioning method may not be suitable for lipophilic compounds and species that are exposed primarily through food (Van Gestel, 1992). Therefore the same approach is used as for the derivation of the PNECsediment: in order to take uptake via ingestion of soil into account the PECsoil is increased by a factor of 10 for compounds with a log Kow > 5.
In principle, toxicity data for aquatic organisms cannot replace data for soil dwelling organisms, because the effects on aquatic species can only be considered as effects on soil organisms which are exposed exclusively to the pore water of the soil (Pedersen and Samsoe-Petersen, 1993). Therefore if the ratio PECsoil/PNECsoil calculated via the equilibrium partitioning method is greater than 1, tests with soil organisms are indispensable for effects assessment for the soil compartment.
3.6.2.2 Calculation of PNEC using assessment factors
The same assessment factors are used for the terrestrial system (see Table 16) as for the aquatic system (see Table 14) depending on the type of investigations (short-term or long-term toxicity test), the number of trophic levels tested and the general uncertainties in predicting ecosystem effects from laboratory data. The suggested assessment factors for the soil compartment are not based on comprehensive experience. As already stated information from tests with soil organisms will only be available for some compounds.
Also, in most cases this will be information from short-term tests with earthworms. This means that a deeper understanding of the difference between short- and long-term toxicity for several taxonomic groups and the difference between laboratory and field tests is needed. Also the choice of taxonomic groups for which toxicity data are necessary (conform the base-set of algae, Daphnia and fish for the aquatic environment), is a point of discussion. A data-set consisting of toxicity data for primary producers, consumers and decomposers is preferred. However, an internationally accepted set of standardised ecotoxicological tests for hazard assessment of chemicals for the soil compartment is not available at the moment.
Reference can be made to section 5.2.3 and an OECD project in which a testing strategy for terrestrial ecosystems is being developed (Léon and Van Gestel, 1994). Summarising, the assessment factors proposed in Table 16 must be regarded as indicative factors. As more information on the sensitivity of soil organisms becomes available these factors may have to be adjusted.
Table 16 Assessment factors to derive a PNEC
Information available Assessment factor
L(E)C50 short-term toxicity tests
(e.g. plants, earthworms, or micro-organisms)
1000
NOEC for one long-term toxicity test (e.g. plants) 100
NOEC for additional long-term toxicity tests of two trophic levels
50
NOEC for additional long-term toxicity tests for three species of three trophic levels
10
Field data/data of model ecosystems case-by-case
The PNECsoil is calculated on the basis of the lowest effect value measured. If short-term tests with a producer, a consumer and/or a decomposer are available, the test result is divided by a factor of 1000 to calculate the PNECsoil. If only one terrestrial test is available (earthworms or plants), the risk assessment should be performed both on the basis of this terrestrial test and on the basis of the aquatic toxicity data as an indication of the risk to soil organisms. As a precaution, the larger PECsoil /PNECsoil ratio determines which further actions should be taken in the framework of the further testing strategy. The other factors listed in Table 16 are applied, if more tests than the short-term toxicity test have been conducted.