User Guide to Thresholds and Classification - Environmental ...

393User Guide for Thresholds and ClassificationsThe approach for classifying hazards to the soil environment is tiered, and depends on the type ofinformation available for the mixture itself and for its components. Elements of the tiered approach includeclassification based on:tested mixtures (see section 20.3.1);bridging principles (see section 20.3.2); anda summation approach using the classifications of components (see section 20.3.3).20.3.1. Tested mixturesWhen a mixture has been directly tested, this result should be used in determining whether the substance asa mixture triggers a soil ecotoxicity classification.It should be noted, however, that degradation time in soil cannot be directly tested for mixtures. Therefore,the degradation time of components of the mixture need to be considered when determining whether asubstance is classified as 9.2C or 9.2D, as set out in Table 20.3. Refer to Table 20.6 for a worked exampleof calculating the weighted sum of components.Table 20.3: Classification using test data on the mixtureAcute EC 50 of the tested mixtureComponents in mixture withDT 50 > 30 days*Classification of mixture1 mg/kg Not applicable 9.2A>1 and 10 mg/kg Not applicable 9.2B>10 and 100 mg/kg Yes (weighted sum ≥ 25%) 9.2C>10 and 100 mg/kg No or weighted sum < 25% 9.2D>100 mg/kgNot hazardous to the soilenvironmentNote:*where no data on degradation, the default applies, ie the substance is considered to have DT 50 >30 days; EC 50 =median effect concentration;DT 50 = time required to reduce the concentration of the original substance by 50%. [where appropriate, ie formicrobial function data, the EC 25 value can be used]If the mixture is used as a biocide and does not trigger classification under subclass 9.2, see also chapter 23below.20.3.2. Bridging principlesGuidance on the bridging principles for the classification of mixtures without test data is in chapter 18 above.20.3.3. Classification of mixture based on classifications of components: summationapproachWhen test data on the mixture are not available and the bridging principles are not applicable, thesummation approach is used to derive a soil hazard classification for the mixture.January 2012 EPA0109