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Sediment toxicity

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Description of key information

NOEC 39 mg/kg dwt (89 mg/kg dwt normalised to 5% OC)

Key value for chemical safety assessment

EC10, LC10 or NOEC for freshwater sediment:
89 mg/kg sediment dw

Additional information

No sediment toxicity data are available for the registration substance M3T.

A category approach is applied to this endpoint and is detailed in the Siloxane Category report (PFA, 2017). The hypothesis for read-across of sediment ecotoxicity evidence within the Siloxanes Category is that no structure-based or property-based pattern is evident from the category dataset of existing studies, although patterns are identifiable associated with extrinsic aspects of test design to which effects may be attributed. The approach will be revisited in the event that reliable new data become available. With this in mind, a single overall interpretation is made across the category. To fulfil the requirements of REACH, a conservative approach is made by reading across on a nearest-neighbour basis the reliable data within the category.

In the context of the RAAF, Scenario 6 is expected to apply to this endpoint. It is considered that effects observed in benthic organisms are associated primarily with extrinsic factors associated with test design and not to structural similarities as such.

 

A total of twenty-four sediment toxicity studies for siloxanes and nineteen results from studies of standard duration in standard test species have been reviewed in detail. There is a general trend for studies using natural sediment, which all have pH <~8, to show no effects, or higher NOECs than corresponding studies with artificial sediment. No significant toxicity (NOEC <100 mg/kg) in any organism is found at pH near 7 with natural sediment. The data suggest that it is possible to read across sediment toxicity data between different siloxane structures, especially where natural sediment data are available, given that the studies which are not suspected to be confounded by extrinsic factors show relatively minimal effects across the dataset.

READ-ACROSS JUSTIFICATION

No data are available for the effects of the registration substance on sediment organisms. Data are read-across from the structural analogues decamethyltetrasiloxane (L4; 141-63-9) and octamethyltrisiloxane (L3; 107-51-7).

The registered substance (M3T) and read-across substances (L3, CAS 107-51-7 and L4, CAS 141-62-8) are members of the Reconsile Siloxanes Category. They are methylated siloxanes containing Si atoms linked by oxygen. M3T and L4 are structural isomers with molecular weight 311; L4 is a linear chain with four silicon atoms linked by three oxygen atoms, whereas M3T is a branched structure of four Si atoms, the longest siloxane chain contains three silicon atoms and two oxygen atoms, with a Si-O branch on the central silicon atom in the chain. L3 is a linear chain of 3 Si-O units with molecular weight 237. The registration and read-across substances have similar physicochemical properties (low water solubility, high log Kow and slow hydrolysis rates), are not readily biodegradable and have high potential for adsorption to sediment. A summary of the relevant physicochemical properties of the registration and read-across substances are reported in Section 6 of the IUCLID dossier.

A result for effects in natural sediment on Lumbriculus variegatus is read across from L4. No effects on survival or biomass have been reported when testing L4 at a loading rate of 17 mg/kg dwt sediment with Lumbriculus variegatus. Therefore 28-day NOEC and LC50 values of ≥17 and >17 mg/kg have been determined respectively in a sediment containing 2.5% organic carbon. The results normalised for a standard sediment of 5% organic carbon are equivalent to LC50 and NOEC values of >34 and ≥34 mg/kg dry weight respectively.

A 28 -day study has been conducted with the surrogate substance L4 and Hyalella azteca. 28-day NOEC and LC50 values of ≥68 and >68 mg/kg have been determined for the effects of L4 on mortality and growth of the freshwater amphipod Hyalella azteca in sediment containing 3.7% organic carbon (OC). The results normalised for a standard sediment of 5% organic carbon are equivalent to LC50 and NOEC values of >92 and ≥92 mg/kg dry weight respectively.

The results of all tests are expressed relative to mean measured exposure concentrations in the treated sediment.

 

A true PNEC cannot be calculated from the test data for Hyalella azteca and Lumbriculus variegatus conducted with the analogous substance L4, because the NOEC values that have been determined are limit values.The NOEC value of 39 mg/kg sediment dry weight (89 mg/kg sediment dry weight, normalised to 5% organic carbon) for effects on Chironomus riparius is used as the key value for the CSA.