Octamethylcyclotetrasiloxane

Sediment toxicity

Description of key information

28 d NOEC 13 mg/kg dry weight, Lumbriculus variegatus, measured (Springborn Smithers Laboratories 2009a) 28 d NOEC 44 mg/kg dry weight, Chironomus riparius, measured (Wildlife International 2008) 28 d NOEC 130 mg/kg dry weight, Hyalella azteca, read-across from structural analogue (Springborn Smithers Laboratories 2009b)

Key value for chemical safety assessment

EC10, LC10 or NOEC for freshwater sediment:

13 mg/kg sediment dw

Additional information

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. Therefore, 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 are available 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 those 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.

Data available for the substance:

Two reliable, prolonged sediment toxicity studies have been selected for the registration substance D4. One for effects on Lumbriculus variegatus using natural sediment (Springborn Smithers Laboratories 2009a), and one for effects on Chironomus riparius using artificial sediment (Wildlife International, 2008).

A 28-day EC50 value of >32 mg/kg has been determined for the effects of the test substance on reproduction and biomass of Lumbriculus variegatus in natural sediment. A 28-day NOEC of 13 mg/kg dry weight and LOEC of 19 mg/kg dry weight have been determined in the same test based on effects on mortality.

A 28-day LC50 of 114 mg/kg sediment dry weight has been determined for the effects of the test substance on survival of Chironomus riparius. A NOEC of 44 mg/kg dry weight has been determined for effects on mortality and emergence ratio in the same test.

A result for effects in natural sediment on the invertebrate amphipod Hyalella azteca is read across from decamethylcyclopentasiloxane (D5; CAS 541-02-6). The result from that test is a 28 d EC50 value of > 130 mg/kg sediment dry weight for growth rate and a NOEC of 130 mg/kg sediment dry weight for growth rate.

A NOEC value of 13 mg/kg dw sediment for effects on mortality of Lumbriculus variegatus is used as the key value for the CSA.

Several other studies are available for D4. In two sub-chronic studies with Chironomus tentans14 d NOEC values of 54 and 65 mg/kg dry weight have been reported for effects on mortality and larval growth respectively (Springborn Smithers 1991c and d).

A 28 day study with L. variegatus in artificial sediment is also available. A NOEC of <0.73 mg/kg dry weight and a LOEC of 0.73 mg/kg dry weight based on survival and reproduction have been determined in the study (Wildlife International 2009b). However, there are concerns that equilibration between the organic phase of the sediment may not have been reached due to insufficient equilibration time. It is also thought that the artificial sediment with peat based carbon source and high pH values interfered with the test system to exhibit toxicity that is mediated by the interaction of the substance with components of artificial sediment with peat based carbon source at high pH. Lastly, non-synchronized worms were used in this study, contrary to OECD 225 guideline requirements. Therefore, the result is disregarded because a result for the same species is available from a test with natural sediment.

In addition, a bioconcentration study with L. variegatus has been conducted and is reported in section 4.3.1 (Wildlife International 2009a). The study reports no effects on mortality or reproduction at the highest concentration tested, therefore a 28-day NOEC value of >4.06 mg/kg dry weight sediment has been determined.