Decamethylcyclopentasiloxane

Sediment toxicity

Description of key information

28 d NOEC 70 mg/kg dwt based on development rate, Chironomus riparius, measured (Wildlife International 2008).

28 d NOEC ≥1272 mg/kg dwt based on survival/reproduction and growth, Lumbriculus variegatus, measured (Wildlife International 2007).

28 d NOEC 130 mg/kg dry weight based on growth rate, Hyalella azteca, measured (Springborn Smithers 2009).

28 d NOEC 62 mg/kg dry weight based on mortality, Hyalella azteca, measured (Environment Canada 2010).

Key value for chemical safety assessment

EC50 or LC50 for freshwater sediment:

257 mg/kg sediment dw

EC10, LC10 or NOEC for freshwater sediment:

70 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-eight sediment toxicity studies for siloxanes are available and twenty-two 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:

Sediment toxicity test results are available for the registered substance with three species. A 28-day LC₅₀ value of 257 mg/kg dry weight has been determined for effects on mortality of Chironomus riparius in artificial sediment. A NOEC of 70 mg/kg dry weight for effects on development rate has been determined in the same test (Wildlife International 2008). A similar NOEC and EC₅₀ (69 and 410 mg/kg dry weight based on development rate and growth rate respectively) have been obtained in a study with Chironomus riparius in artificial sediment (Springborn Smithers 2003). However, there are some uncertainties associated with the latter study and it is an older study. Therefore, Wildlife International 2008 has been selected as key study for this species.

A 28-day EC₅₀ value of >1272 mg/kg dry weight and a NOEC of ≥1272 mg/kg dry weight have been determined for effects on survival/reproduction and growth of Lumbriculus variegatus in artificial sediment (Wildlife International 2007).

A 28-day LC₅₀ of 310 mg/kg dry weight has been determined for effects on mortality of Hyalella azteca in natural sediment. A 28-day NOEC of 130 mg/kg and a LOEC of >130 mg/kg have been determined for effects on growth rate (Springborn Smithers 2009).

A 28-day LC50 of 194 mg/kg dry weight, a 28-day NOEC of 62 mg/kg dry weight and a LOEC of 154.3 mg/kg dry weight have been determined for the effects on mortality of Hyalella azteca when exposed via sediment (natural sediment) (Environment Canada 2010). These results are based on mean measured concentrations.

A bioaccumulation study with Lumbriculus variegatus is also available and is described in detail in section 5.3.1 (Krueger et al. 2010). Based on mortality a 28-day NOEC ≥366 mg/kg dry weight sediment has been determined.

There are concerns with studies being conducted in artificial sediment for silicon substances and overestimation of toxicity. This is more relevant for studies conducted with L. variegatus where the organisms are truly benthic species ingesting the sediment. However, the study with L. variegatus reported no effects at the highest concentration tested and the results are consistent with the bioaccumulation study, therefore the study has been included in the assessment for the derivation of PNECs.