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Sediment toxicity

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Description of key information

Long-term studies are available for two organisms (Hyalella azteca and Lumbriculus variegatus) with the registration substance. The data show no effects at the limit of solubility in organic carbon. A true PNEC cannot be calculated from the test data because the NOEC values that have been determined are limit values. Data for Chironomus riparius with the closely related substance L3 has been read-across. A 28-d NOEC of 39 mg/kg dry weight has been determined for effects of L3 on emergence and development rate of Chironomus riparius (89 mg/kg dry weight, normalised to 5% organic carbon).

Key value for chemical safety assessment

Additional information

A total of twenty-one sediment toxicity studies for siloxanes and sixteen 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 sound studies show relatively minimal effects across the dataset. In the use of the data set for hazard assessment and derivation of predicted no-effect concentration (PNEC), the following approaches are used:

- Where the hydrolysis half-life is >48 hours, the chemical safety assessment will focus on the parent form.

- Where data are available for a substance with natural sediment and with artificial sediment for the same species, the natural sediment data will be given preference over data obtained with artificial sediment.

- PNEC will be determined on a weight-of-evidence basis for each substance, including use of read-across. Equilibrium partitioning calculations will be used if necessary.

Dataset for L4

 

A 28-day study has been conducted with L4 using Hyalella azteca and Lumbriculus variegatus. Data for Chironomus riparius are read-across from L3. The selection of read-across substance is based on structural similarity and key physico-chemical properties (log Kow, log Koc, degradation).

Read-across justification

Read-across from L3 to L4:

L4 and the surrogate substance L3 are linear siloxanes with 4 and 3 silicon atoms, respectively, with methyl branches and alternated by oxygen. L4 and L3 possess similar physicochemical properties: high molecular weight (311 and 237, respectively), low water solubility (both insoluble, at6.7E-03mg/l and3.4E-02mg/l, respectively), high log Kow(8.21 and 6.60 respectively) and high log Koc(5.16 and 4.34, respectively). Both substances have negligible biodegradability and similar slow hydrolysis rates.

No effects at the limit of solubility have been reported in short-term and long-term studies in other trophic levels conducted with both substances.

Given the similar properties and structural similarities, it is considered valid to read across sediment toxicity data from L3 to L4.

 

A comparison of the key physico-chemical properties is presented in the table below:

 

Table: Key physico-chemical properties of L4 and surrogate substance L3

Property

L3 (107-51-7)

L4 (141-62-8)

Molecular weight

237

311

Log Kow(at 25°C)

6.60

8.21

Log Koc(at 23°C)

4.34

5.16

Water solubility (mg/l)

3.4E-02

6.7E-03

Hydrolysis half- life at pH 7 (h)

329

728

 

Sediment toxicity data

A 28 -day study has been conducted with the registered substance and Hyallela azteca. 28-day NOEC and LC50 values of ≥68 and >68 mg/kg have been determined for the effects of the registered substance on mortality and growth of the freshwater amphipod Hyallela 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.

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

OC solubility mg/kg = Koc* water solubility

Which for L4 is 1.4E+05 * 6.7E-03 mg/l = 970 mg/kg OC.

The sediment in this long-term toxicity study had an OC content of 3.7%, therefore the calculated solubility in this sediment is 36 mg/kg. This indicates that the substance is not toxic to freshwater sediment organisms in excess of the OC solubility of the registered substance.

No effects on survival or biomass have been reported when testing the registered substance 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-d NOEC result for Chironomus riparius is read across from L3. A 28-d NOEC of 39 mg/kg dry weight has been determined for effects on emergence and development rate of Chironomus riparius (89 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.