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Sediment toxicity

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Description of key information

NOEC for freshwater sediment: 13 mg/kg dwt (30 mg/kg dwt normalised to 5% OC)

Key value for chemical safety assessment

EC10, LC10 or NOEC for freshwater sediment:
30 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. 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 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:

A 28-day LC50and NOEC values of >100 and ≥100 mg/kg dry weight of sediment respectively, have been determined for the effects of the registration substance on survival and growth of the freshwater amphipodHyalella azteca.The sediment contained 3.7% organic carbon. This is equivalent to a NOEC of ≥126 mg/kg in sediment containing 5% organic carbon.

It is possible to calculate the solubility of the substance in organic carbon (OC) according to the following formula:

OC solubility mg/kg = Koc* water solubility

For this substance this is: 1.26E+04 * 0.008 = 100 mg/kg OC.

The sediment in this long-term toxicity study had an OC content of 3.7%, therefore the calculated solubility of the substance in this sediment is 4 mg/kg. This indicates that the substance is not toxic at its solubility in organic carbon.

A short-term (10-day) LC50value of 124 mg/kg has been determined for the effects of the registration substance on mortality ofCorophium volutatorin a sediment containing 0.62% of organic carbon (OC). This is equivalent to an LC50value of 230 mg/kg in sediment containing 5% organic carbon. It is possible that the results overestimate toxicity due to the treatments greatly exceeding the substance's solubility in sediment containing 0.62% OC being exceeded.

The registered substance (Vi4-D4) and read-across substanceD4 (CAS 556-67-2)are members of the Reconsile Siloxanes Category.Octamethylcyclotetrasiloxane (D4, CAS 556-67-2) is a cyclic siloxane made up of 4 Si with methyl groups, linked by oxygen atoms. Vi4-D4 is a cyclic siloxane made up of 4 Si with methyl and vinyl groups, linked by oxygen atoms. Vi4-D4 and D4 have slow hydrolysis rates (63 h at pH 7 and 20-25°C (predicted) and 69-144 h at pH 7 and 25°C, respectively) and similar physico-chemical properties: high molecular weight (MW 344.7 and 296.6 respectively), low water solubility (0.056 mg/l and 0.0073 – 0.0088 mg/l respectively) high log Kow(both 6.5) and high log Koc(both close to 4).The registration and read-across substances are not readily biodegradable and have high potential for adsorption to sediment.

A result for effects in natural sediment onLumbriculus variegatusis read across from D4. The result from that test is a 28 d NOEC of 13 mg/kg sediment dry weight for growth rate (30 mg/kg sediment dry weight, normalised to 5% organic carbon).

A 28-d NOEC result forChironomus ripariusis also read across from D4. A 28-d NOEC of 44 mg/kg dry weight has been determined for effects on emergence and development rate ofChironomus riparius(54 mg/kg dry weight, normalised to 5% organic carbon).

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

The registration substance has an average purity of >80% Vi4-D4, with <15% 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane Vi5-D5 (CAS 17704-22-2; Impurity 1) and <10%2,4,6-trimethyl-2,4,6-trivinylcyclotrisiloxane Vi3-D3 (CAS 3901-77-7; Impurity 2) present as impurities. After due consideration of the properties, the presence of these impurities is not expected to affect the overall hazard profile of the substance. Vi5-D5 and D5 are members of the Siloxane Category, and therefore data for Vi5-D5 are read-across from decamethylcyclopentasiloxane D5 (CAS 541-02-6).Vi3-D3 and D3 have rapid hydrolysis rates (0.6 h at pH 7 and 20-25°C, predicted and 23 minutes at pH 7 and 25°C, measured respectively), and the log Kowof the hydrolysis products is <3; substances with log Kow­ ­<3 are not likely to be sorbed to sediment and therefore testing for sediment toxicity is not considered necessary. There are therefore no studies available with Vi3-D3 or the surrogate D3.

The lowest sediment NOEC obtained for D5 is 70 mg/kg dry weight for effects of D5 on development rate ofChironomus ripariusin artificial sediment. (Wildlife International 2008).