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Toxicity to soil microorganisms

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Description of key information

No reliable, standard tests of soil microbial inhibition are available. The terrestrial chemical safety assessment has been conducted using the Equilibrium Partitioning method (EQPM).

Key value for chemical safety assessment

Additional information

The terrestrial chemical safety assessment has been conducted using the Equilibrium Partitioning method (EQPM). It is recognised that the aquatic PNEC used in the EQPM does not take into account any indicator for effects in aquatic microorganisms. However, testing analogous alcohols within the Category shows the substances in the category are very rapidly biodegradable and show no significant inhibitory effects on respiration of activated sludge or specific microbial strains relevant to WWTP, at or above the limit of solubility (based on inhibition tests and lack of toxicity in ready biodegradability test). Therefore it is unlikely that the PNECterrestrial based on aquatic ecotoxicity test results would not be protective for terrestrial microorganisms. The chemical safety assessment using EQPM does not suggest any unacceptable risks for the terrestrial compartment.

Therefore, further toxicity testing with terrestrial microorganisms does not need to be conducted.

Moreover, considerable technical difficulties would be expected in the conduct of such a test, due to the very rapid biotic removal of the substance from the test system. Please refer to discussion of the long-term aquatic invertebrate and fish studies (Section 6.1.2 and 6.1.4), and evidence of rapid removal in non-sterilised soils during method development for the adsorption/desorption study with natural soils (Section 5.4.1 and 5.2.3).

A lack of toxicity to soil microbiota was suggested by a recent experimental finding associated with a recent study of adsorption/desorption (OECD 106, Wildlife International, 2015) using decan-1-ol. Significant technical difficulties were encountered during method development for this study using natural standard soils, in that it was not possible to detect sufficient substance and establish equilibrium in non-sterilised soil samples. During method development in preparation for this study, the laboratory reported that after equilibration with soils for 15-minute to 24-hour periods, decan-1-ol dosed into the test vessels was partly or completely transformed into a more polar product, which was clearly distinguishable from the starting material as clear and well separated peaks in the chromatograms. The rate of transformation depends on the soil type. Only after a very short (5-minute) equilibration the parent material remained intact (personal communication, 8 January 2015 from Wildlife International laboratory). Half-lives were not explicitly derived, but chromatograms presented would indicate the half-life of decan-1-ol in the soil test samples was approximately 30 min - 1 hour (silt loam soil); 1 - 2 hours (loamy sand), and 15 - 30 minutes (clay loam). Following sterilisation of the soil samples by autoclaving, the substance remained adequately stable for a 2-hour period, therefore the instability can be attributed to biodegradation of the substance by soil microbiota. The polar degradation product is most likely the corresponding carboxylic acid, though it was not definitively identified. The chromatograms show that decan-1-ol was effectively fully removed in all four soil types by the 24 h time point (in the case of 2 of the soil types, within 2 hours), suggesting the microbial population was not inhibited by decan-1-ol, though this is evidence is not in itself adequate as key data.

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