Hexamethyldisiloxane

Sediment toxicity

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

sediment toxicity: long-term

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

2009-04-29 to 2009-05-27

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: The study was conducted according to an appropriate OECD test guideline and in compliance with GLP.

Qualifier:

according to guideline

Guideline:

OECD Guideline 218 (Sediment-Water Chironomid Toxicity Test Using Spiked Sediment)

GLP compliance:

yes

Analytical monitoring:

yes

Details on sampling:

SEDIMENT

- Sampling interval: Sediment samples were collected from the analytical replicates from each test concentration and control shortly after the introduction of the organisms on Day 0, on Day 7 and at test termination on Day 28.

Vehicle:

no

Details on sediment and application:

SEDIMENT

- Formulated sediment: The sediment used in the study was a formulated sediment based on the recommendations of OECD Guideline 218. The sediment was composed of approximately 10% sphagnum peat moss, 20% silt and clay (kaolin clay) and 70% industrial quartz sand. The sand and clay were mixed in a PK Twinshell mixer for 20 minutes without the peat, since the peat was added later. The targeted organic carbon content of the final mixture was 5.0 ± 1.0%. The dry soil was stored under ambient conditions until used. The final pH of the sediment was 7.0. The percent organic carbon of the sediment was found to be 2.2.

Test organisms (species):

Chironomus riparius

Details on test organisms:

TEST ORGANISM

- Source: Egg masses were obtained from Environmental Consulting and Testing, Superior, Wisconsin. The organisms were held for five days prior to the start of the test at approximately the same temperature and in water from the same source as the water used during the test. At test initiation, the midges were collected from the culture and impartially added one and two at a time to test chambers. All transfers were made below the air/water interface using wide-bore pipettes.

Study type:

laboratory study

Test type:

semi-static

Water media type:

freshwater

Type of sediment:

artificial sediment

Limit test:

no

Duration:

28 d

Exposure phase:

total exposure duration

Hardness:

136-144 mg/L as CaCO3

Test temperature:

20 ± 2 ºC

pH:

pH ranged from 8.1 to 8.6

Dissolved oxygen:

≥7.1 mg/L (79% of saturation)

Salinity:

not applicable

Ammonia:

<0.17 to 6.28 mg/L (Ammonia levels exceeded 4.0 mg/L in the overlying water of some replicates on Days 7 and 14; therefore, the overlying water was partially renewed on those days in each replicate to prevent toxicity caused by high levels of ammonia).

Nominal and measured concentrations:

Nominal concentrations in mg/Kg: 0 (Control), 31, 63, 125, 250, 500 and 1000

Arithmetic mean measured concentrations in mg/Kg in the treated sediments: 7.4, 14, 39, 84, 210 and 435

The mean measured concentrations in the treated sediments are equivalent to 24, 23, 32, 34, 42 and 44% of nominal.

The results are interpreted with reference to the mean measured concentrations.

Details on test conditions:

TEST SYSTEM

- Test container (material, size): Test chambers were 2000-mL glass beakers containing approximately 2 cm of sediment and 8 cm of overlying water

- Aeration: yes

- Overlying water renewal: Overlying water was partially renewed on Days 7 and 14 of the test to prevent the buildup of ammonia concentrations to toxic levels

- Aeration frequency and intensity: Loose plastic covers were placed over each test chamber. Each test chamber was gently aerated through a glass pipette that did not extend to a depth closer than 2 cm from the surface of the sediment. Air was bubbled into the test chamber at a rate greater than 1 bubble per second but not so great as to disturb the sediment. .

EXPOSURE REGIME

- No. of organisms per container (treatment): Four replicates were tested in each treatment group with 20 midges in each replicate for a total of 80 midges per treatment group.

- Type and preparation of food: A 28-day ration of food (280 mg Tetramin flake food) was dry mixed into the sediment prior to the addition of the overlying water and 49 hours before adding the test organisms.

OVERLYING WATER CHARACTERISTCS

- Dilution water source: Well Water

- Dilution water chemistry: hardness 136-144 mg/L as CaCO3 , alkalinity 178-180 mg/L as CaCO3 and conductivity 371-393 mhos/cm

OTHER TEST CONDITIONS

- Lighting (quality, intensity, and periodicity): fluorescent lighting with wavelengths similar to natural lighting, intensity was 414 lux at the surface of the water at test initiation, photoperiod was 16 hours light:8 hours dark with a 30-minute transition period.

The sediment contained 72% sand, 10% silt and 18% clay. Textural class: sandy loam. Organic carbon: 2.2%, Organic matter: 3.9%

Reference substance (positive control):

no

Duration:

28 d

Dose descriptor:

LC50

Effect conc.:

166 mg/kg sediment dw

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

test mat.

Basis for effect:

mortality

Remarks on result:

other: 84-435

Duration:

28 d

Dose descriptor:

LOEC

Effect conc.:

84 mg/kg sediment dw

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

test mat.

Basis for effect:

development rate

Duration:

28 d

Dose descriptor:

NOEC

Effect conc.:

39 mg/kg sediment dw

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

test mat.

Basis for effect:

development rate

Reported statistics and error estimates:

The 28-Day LC50 was calculated using the computer software of C.E. Stephan. The program is designed to calculate the LC50 value and 95% confidence interval by probit analysis, the moving average method, or binomial probability with nonlinear interpolation. In this study, the binomial method was used to calculate the LC50 value. The LC50 value was calculated using the mortality data collected at the end of the test. The no-observed-effect-concentration (NOEC) and the lowest-observed-effect-concentration (LOEC) were determined by visual interpretation of the dose-response pattern and statistical analyses of the mean development times, emergence ratios and development rates.

   Table 1. Results of analysis of sediment exposure concentrations

 

Nominal Test Concentration (mg/Kg)	Mean Measured Concentration (mg/Kg)	Mean Percent of Nominal
Negative Control	--	--
31	7.4	24
63	14	23
125	39	32
250	84	34
500	210	42
1000	435	44

 

 

Table 2. Test results

 

Mean Measured Concentration (mg/Kg)	Number Exposed	Percent Emergence	Percent Mortality	Mean Development Time (Days)	Emergence ratio	Development rate
Negative Control	80	88	13	17.7	0.88	0.0604
7.4	80	90	10	18.0	0.90	0.0588
14	80	94	6.3	17.4	0.94	0.0608
39	80	91	8.8	18.9	0.91	0.0588
84	80	71	29	20.3	0.71	0.0519*
210	80	43*	58	21.8*	0.43*	0.0486*
435	80	4*	96	23.8*	0.04*	0.0431*

^*There was a statistically significant difference (p<0.05) from the negative control using Dunnett’s t-test.

Validity criteria fulfilled:

yes

Conclusions:

A 28-Day LC50 value of 166 mg/kg dry weight has been determined for the effects of the sediment incorporated test substance on mortality of Chironomus riparius. A NOEC of 39 mg/kg dry weight for effects on development rate has been determined in the same test.

Reference 2

Endpoint:

sediment toxicity: long-term

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

from 15-05-2012 to 15-06-2012

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: The study was conducted according to the appropriate national guideline (OPPTS Draft Guideline 850.1735), under GLP and analytical monitoring was carried out.

Qualifier:

according to guideline

Guideline:

EPA OPPTS 850.1735 (Whole Sediment Acute Toxicity of Invertebrates, freshwater)

Deviations:

yes

Remarks:

temperature range

GLP compliance:

yes

Analytical monitoring:

yes

Details on sampling:

SEDIMENT
- Concentrations: all
- Sampling interval: 0, 7 and 28 days
- Sample storage before analysis: not reported

PORE WATER - discarded

OVERLYING WATER - discarded

Vehicle:

yes

Details on sediment and application:

PREPARATION OF SPIKED SEDIMENT

- Details of spiking: a primary stock solution of 10 mg/lml was prepared by placing 0.5 g of test substance in a 25 ml flask and bringing it to volume with acetone. the stock solution was then used to make the remaining concentrations by diluting an adequate volume of the stock solution and diluting in in acetone to form 10 ml secondary stock solutions. For each treatment level, 2.0 kg wet weight (1.5 L) of sediment (totaling 0.8500 kg dry weight) was added to a glass jar, along with 4.5 L of laboratory well water, to give an approximate water to sediment volume ratio of 3:1. The jar contents were mixed by shaking the jar to suspend the sediment immediately prior to dosing. Following dosing, the jars were shaken again by hand, well enough to suspend all sediment particles, and thus distribute the test substance. After settling overnight, the water was decanted and the treated sediments were allocated to the replicate test vessels for each exposure level.

- Equilibration time: overnight settling

- Equilibration conditions: not reported

- Controls: The control sediment was prepared using only untreated sediment (no solvent or test material) and overlying water.

- Chemical name of vehicle (organic solvent, emulsifier or dispersant): acetone

- Concentration of vehicle in test medium (stock solution and final test solution):

- Evaporation of vehicle before use: not reported

Test organisms (species):

Hyalella azteca

Details on test organisms:

TEST ORGANISM

- Source: Environmental Consulting & Testing, Superior, Wisconsin

- Age of parental stock (mean and range, SD): 8 days old when the animals arrived at the lab

- Breeding conditions: not reported

- Age of animals at beginning of exposure: 10 days


ACCLIMATION
- Acclimation period: 2 days
- Acclimation conditions (same as test or not): same water and conditions, no substrate
- Type and amount of food: every other day
- Feeding frequency: 2.5 mL of a combination of yeast, cereal leaves and flaked fish food suspension (YCT)
- Health during acclimation (any mortality observed): no mortality observed

Study type:

laboratory study

Test type:

flow-through

Water media type:

freshwater

Type of sediment:

natural sediment

Limit test:

no

Duration:

28 d

Exposure phase:

total exposure duration

Hardness:

range 64 to 68 mg/L as CaCO3

Test temperature:

range: 21 to 25⁰C

pH:

range: 6.8-7.6

Dissolved oxygen:

range: 40 to 100%

Ammonia:

<0.56 mg/l

Nominal and measured concentrations:

Nominal concentrations: 6.3, 13, 25, 50 and 100 mg/kg dry weight
Measured concentrations: 6.0, 10, 19, 31 and 70 mg/kg

Details on test conditions:

TEST SYSTEM

- Test container (material, size): 300 ml glass vessels

- Sediment volume: 100 ml (4 cm layer), equivalent to 118 g per vessel

- Overlying water volume: 175 ml

- Depth of sediment and overlying water: 275 ml

- Aeration: no


EXPOSURE REGIME
- No. of organisms per container (treatment): 10 (80 per treatment)

- No. of replicates per treatment group: 12, 8 with animals and 4 for chemical analysis

- No. of replicates per control / vehicle control: 12, 8 with animals and 4 for chemical analysis

- Feeding regime: daily

- Type and preparation of food: YCL as described above

- Amount of food: 1.5 ml


RENEWAL OF OVERLYING WATER

- Details on volume additions: intermittent delivery system in combination with a calibrated water-distribution system

- Flow-rate: 350 mL per vessel every 24 hours (i.e., approximately two overlying volume replacements per vessel per day).


OVERLYING WATER CHARACTERISTCS

- Type of water (e.g. deionized, ground water, sea water, Elendt medium acc. to OECD 219): laboratory tap water

- Alkalinity: 21 to 23 mg/L as CaCO3

- Conductivity: 320 to 350 µS/cm



SOURCE OF NATURAL SEDIMENT

- Location and description of sampling site: Glen Charlie Pond, Wareham, Massachusetts (Smithers Viscient Batch No. 042012)

- Contamination history of site: No history of contamination at this site is evident based on analyses dating back to 2002


HANDLING OF NATURAL SEDIMENT

- Time of collection: during the day

The sediment was obtained by removing the top 10 to 15 cm of sediment beneath a water depth of approximately 60 cm. The sediment was transferred from the collection site to Smithers Viscient in multiple 20-liter, sealed, polyethylene buckets where it was stored in a refrigerator at approximately 4 °C for four weeks prior to testing.


CHARACTERIZATION OF SEDIMENT

- Particle size distribution
- % sand: 93%
- % silt: 4%
- % clay: 3%
- % solids: 42.5%
- % organic carbon: 3.7%
- sediment pH: 5.6

- Colour/texture: not reported

- Moisture: 21.5%

- Presence of macrophytes/animals: not reported

- Further constituents

- Metals: no

- Organic compounds: no

- Petroleum hydrocarbons: no

- Sediment sieved: yes, wet-pressed through a 2.0 mm sieve

- Ammonia content of pore water: 7.9 mg/l as nitrogen


OTHER TEST CONDITIONS

- Photoperiod: 16 h daylight, 8 h dark

- Light intensity: 500 to 890 lux


EFFECT PARAMETERS MEASURED (with observation intervals if applicable) : All vessels were examined at test initiation and at 24-hour intervals thereafter, until test termination (day 28). Observations of mortality and abnormal behavior were made and the physical characteristics of the test samples were recorded. At test termination (day 28), the total number of surviving amphipods and dry weight of surviving amphipods was determined in each test vessel.


VEHICLE CONTROL PERFORMED: yes


TEST CONCENTRATIONS

- Spacing factor for test concentrations: 2

- Justification for using less concentrations than requested by guideline: n/a

- Range finding study
- Test concentrations: 0.01, 0.1, 1, 10 and 100 mg/kg
- Results used to determine the conditions for the definitive study: no effects at the highest concentration tested

Duration:

28 d

Dose descriptor:

LC50

Effect conc.:

> 70 mg/kg sediment dw

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

test mat.

Basis for effect:

other: survival and weight

Duration:

28 d

Dose descriptor:

NOEC

Effect conc.:

>= 70 mg/kg sediment dw

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

test mat.

Basis for effect:

other: survival and weight

Details on results:

- Mortality of test animals at end of exposure period: 0%

Reported statistics and error estimates:

Shapiro-Wilk’s test and Bartlett's test,Wilcoxon's test with Bonferroni's adjustment.CETISTM Version 1.8.4.20 (Ives, 2011) was used to perform the computations.

Measured concentrations:

Measured sediment concentrations at day 0 in the 6.3, 13, 25, 50 and 100 mg/kg nominal treatment levels were 8.5, 12, 24, 35 and 80 mg/kg, respectively. Measured sediment concentrations at day 7 in the 6.3, 13, 25, 50 and 100 mg/kg nominal treatment levels were 4.7, 12, 18, 31 and 63 mg/kg, respectively, and 4.9, 7.2, 15, 27 and 67 mg/kg, respectively, on day 28. Based on mean measured concentrations, treatment levels were defined as 6.0, 10, 19, 31 and 70 mg/kg.

Survival:

Following 28 days of exposure, amphipod survival in the control and solvent control averaged 90 and 98%, respectively .

At test termination (test day 28), survival observed among amphipods in the 6.0, 10, 19, 31 and 70 mg/kg treatment levels averaged 100, 100, 95, 86 and 99%, respectively. 

Weight:

Following 28 days of exposure, amphipod dry weight in the control and solvent control averaged 0.42 and 0.56 mg per amphipod.  

At test termination (test day 28), dry weight observed among amphipods in the 6.0, 10, 19, 31 and 70 mg/kg treatment levels averaged 0.64, 0.63, 0.58, 0.57 and 0.74 mg per amphipod, respectively. 

Validity criteria fulfilled:

yes

Conclusions:

28 day LC50 and NOEC values of >70 and ≥70 mg/kg have been reported for the effects of the test substance on the mortality and growth rate of the freshwater amphipod Hyalella azteca. The results are based on mean measured concentrations.

Reference 3

Endpoint:

sediment toxicity: long-term

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

2015-11-13 to 2016-12-15

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Qualifier:

according to guideline

Guideline:

OECD Guideline 225 (Sediment-Water Lumbriculus Toxicity Test Using Spiked Sediment)

GLP compliance:

yes (incl. QA statement)

Analytical monitoring:

yes

Details on sampling:

SEDIMENT
- Sampling interval: day 0, 3, 7 , 14,. 28

PORE WATER
- Sampling interval: day 0, 3, 7 , 14,. 28

OVERLYING WATER
- Sampling interval: day 0, 3, 7 , 14,. 28

Vehicle:

yes

Remarks:

acetone

Details on sediment and application:

PREPARATION OF SPIKED SEDIMENT
- Details of spiking: At day -3, 1.5 kg (equivalent to 1.15 L) wet weight of sediment was weighed in individual 3.5 L glass jars, followed by 0.77 L of fortified well water to give an approximate water volume to sediment to headspace volume ratio of 2:3:4. The jar contents were shaken manually to suspend the sediment immediately prior to dosing. Each jar was manually shaken to suspend the sediment and then dosed with 0.85 mL of the appropriate dosing stock solution. The test substance was applied under the surface of the water using a gas tight syringe. Following dosing, the jars were immediately sealed and manually shaken again to re-suspend the sediment, and thus distribute the test substance. After settling overnight, the overlying water was decanted and 1.66g of both Urtica powder and alpha cellulose (0.25% of the total sediment dry weight each) were weighed out and manually mixed into the sediment for each concentration as a supplemental food source for the test organisms. The addition of the food following dosing was to ensure it was adequately mixed into the sediment and equally incorporated for each control and treatment level. Adding the food prior to dosing could result in food being suspended in the overlying water and never reincorporated into the sediment. The final sediment dry weight following this addition was 0.6653 kg per concentration. The sediment was then allocated to the replicate test vessels.
- Controls: solvent, negative
- Chemical name of vehicle (organic solvent, emulsifier or dispersant): acetone
- Concentration of vehicle in test medium (stock solution and final test solution):
- Stock solution: 10 mg/mL
- Dosing stock solutions 23.5, 11.8, 5.85, 3.00, 1.5 mg/mL
- Evaporation of vehicle before use:

Test organisms (species):

Lumbriculus variegatus

Details on test organisms:

TEST ORGANISM
- Common name: Lumbriculus variegatus
- Source: laboratory cultures maintained at Smither Viscient


ACCLIMATION
- Acclimation period: 12-14 days following synchronisation
- Acclimation conditions (same as test or not): organisms were placed in an aquarium with test sediment and laboratoyr well water under flow-through conditions. Renewal of overlying water in holding aquarium with the fortified well water used in the test at a rate of approximately half a volume replacement per day.
- Type and amount of food: 15 mL of finely ground suspension of flaked fish food (100 mg/mL)
- Feeding frequency: weekly
- Health during acclimation (any mortality observed): none

Study type:

laboratory study

Test type:

static

Water media type:

freshwater

Type of sediment:

natural sediment

Duration:

28 d

Exposure phase:

total exposure duration

Hardness:

180 mg/L as CaCO3

Test temperature:

18 to 22°C

pH:

overlying water: 6.2 to 8.0 with the exception of one sample (day 21 of the control) which had a pH of 5.9. While this one measurement was outside of the acceptability criteria of pH 6 to 9, the biological performance in the negative control indicates that pH did not adversely affect the test organism.

Dissolved oxygen:

Dissolved oxygen was maintained above 30% air saturation meeting protocol and guideline criteria

Conductivity:

810 µS/cm

Nominal and measured concentrations:

nominal: 1.9, 3.8, 7.5, 15 and 30 mg/kg

measured: concentrations on day 0: 0.38, 0.80, 2.1, 2.6, and 2.7 mg/kg

measured:concentrations on day 3: 0.33, 0.55, 1.6, 1.9, and 1.4 mg/kg

measured concentrations on day 14: 0.24, 0.48, 1.2, 1.7 and 1.4 mg/kg

measured concentrations on day 28: 0.16, 0.22, 0.59, 0.86, and 0.88 mg/kg

Details on test conditions:

TEST SYSTEM
- Test container (material, size): 600 mL clear glass beakers
- Sediment volume: 75 mL
- Sediment wet weight: 116g (average)
- Overlying water volume: 300 mL
- Depth of sediment and overlying water: sediment 1.5 cm, water 6 cm (ratio 1:4)
- Aeration: yes
- Aeration frequency and intensity: 1 to 3 bubbles per second
- Replacement of evaporated test water, if any:

EXPOSURE REGIME
- No. of organisms per container (treatment): 10
- No. of replicates per treatment group: 4
- No. of replicates per control / vehicle control: total of 6 control
- Feeding regime: added at start of test
- Type and preparation of food: Urtica powder and alpha cellulose added to sediment during sediment preparation

OVERLYING WATER CHARACTERISTCS
- Type of water (e.g. deionized, ground water, sea water, Elendt medium acc. to OECD 219): fortified well water
- Alkalinity: 180 mg/L as CaCO3
- Conductivity: 810 µS/cm

SOURCE OF NATURAL SEDIMENT
- Location and description of sampling site: Glen Charlie Pond, Wareham, Massachusetts. (SV Batch no. 072015)

HANDLING OF NATURAL SEDIMENT
- Storage conditions: under refrigerated conditions at approximately 4ºC

CHARACTERIZATION OF SEDIMENT
- Particle size distribution
- % sand: 91%
- % silt: 6%
- % clay: 3%
- %solids: 44.13%
- Sediment sieved: yes, wet pressed through a 2.0 mm sieve prior to use and characterisation
- pH dry matter and/or whole sediment: 5.3
- Ammonia content of pore water: 6.7 mg/L as N
- Total organic carbon (%): 3.0%
- Proof of absence of chemical contaminants: representative sample analysed for presence of pesticides, PCBs and toxic metals. None detected at concentrations considered to have an adverse impact on the test.

OTHER TEST CONDITIONS
- Light quality: fluorescent bulbs
- Photoperiod: 16 hours light, 8 hours darkness
- Light intensity: 350 to 490 lux

EFFECT PARAMETERS MEASURED (with observation intervals if applicable) :

VEHICLE CONTROL PERFORMED: yes

TEST CONCENTRATIONS
- Spacing factor for test concentrations: nominal L2 concentrations chosen for the definitive study were 1.9, 3.8, 7.5, 15, and 30 mg/kg based on sediment dry weight

PRELIMINARY TESTING/EXPOSURE
- range-finding concentrations: nominal concentrations of 0.025, 0.17, 1.1, 7.5 and 50 mg/kg
- No. of organisms per container (treatment): 10
- No. of replicates per treatment group: 3
- control / vehicle control: negative and solvent control sediments were maintained
- results: Statistical analysis determined no significant reduction in the mean number of surviving oligochaetes or in the biomass among oligochaetes exposed to any treatment level compared to the solvent control.

Reference substance (positive control):

no

Key result

Duration:

28 d

Dose descriptor:

NOEC

Effect conc.:

>= 2.7 mg/kg sediment dw

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

test mat.

Basis for effect:

total number of worms

Remarks:

oligochaete biomass

Duration:

28 d

Dose descriptor:

EC50

Effect conc.:

> 2.7 mg/kg sediment dw

Nominal / measured:

estimated

Conc. based on:

test mat.

Basis for effect:

total number of worms

Remarks:

oligochaete biomass

Remarks on result:

other:

Remarks:

Since no concentration tested resulted in ≥ 50% reduction in the number of oligochaetes recovered, the 28-day EC50 value for oligochaete reproduction was empirically estimated to be > 2.7 mg/kg-dw, the highest initial measured sediment concentration tested.

Duration:

28 d

Dose descriptor:

LOEC

Effect conc.:

> 2.7 mg/kg sediment dw

Nominal / measured:

meas. (arithm. mean)

Conc. based on:

test mat.

Basis for effect:

total number of worms

Remarks:

oligochaete biomass

Details on results:

- mean number of oligochaetes recovered in the control and solvent control was 26 and 29, respectively
- mean biomass per replicate in the control and solvent control was 31 and 29 mg, respectively
- control and solvent control data met the minimum performance criteria of the guideline (i.e., population increase of ≥ 1.8)

At exposure termination (day 28), the mean number of surviving oligochaetes observed among the oligochaetes exposed to the 0.38, 0.80, 2.1, 2.6, and 2.7 mg/kg treatment levels was 30, 31, 31, 26, and 30, respectively. Due to a statistically significant difference between the control and solvent control reproduction, results of treatment levels were compared to the solvent control. Statistical analysis (Bonferroni’s Adjusted t-Test) determined no significant reduction in the mean number of surviving oligochaetes in any of the treatment levels tested compared to the solvent control (29 oligochaetes).

Mean biomass per replicate in the 0.38, 0.80, 2.1, 2.6, and 2.7 mg/kg treatment levels was 29, 29, 37, 29, and 28 mg, respectively. Statistical analysis (Bonferroni’s Adjusted t-Test) determined no significant difference in mean biomass in any of the treatment levels tested compared to the pooled control (30 mg).

Because measured sediment concentrations were well below 80% of nominal on test day 0 and continued to decline throughout the exposure, the initial measured concentrations (day 0) were utilised to express the biological endpoints for the exposure, as recommended in OECD guideline 225, and defined the treatment levels tested as 0.38, 0.80, 2.1, 2.6, and 2.7 mg/kg.

Table: Concentrations of L2 measured in sediment samples during the study exposing oligochaetes (Lumbriculus variegatus) to L2 applied to sediment.

Nominal Sediment Concentration (mg/kg)		Measured Sediment Concentration(mg/kg)			Mean Measured (mg/kg)	Percent of Nominal (%)
	Day 0	Day 3	Day 14	Day 28
Negative Control	< 0.074	< 0.072	< 0.0051	< 0.020	NA	NA
Solvent Control	< 0.074	< 0.072	< 0.051	< 0.020	NA	NA
1.9	0.38	0.33	0.24	0.16	0.28	15
3.8	0.80	0.55	0.48	0.22	0.51	14
7.5	2.1	1.6	1.2	0.59	1.4	18
15	2.6	1.9	1.7	0.86	1.8	12
30	2.7	1.4	1.4	0.88	1.6	5.3

Validity criteria fulfilled:

yes

Conclusions:

No effects on survival or biomass have been reported when testing the registered substance at a loading rate of 2.7 mg/kg dwt sediment with Lumbriculus variegatus. Therefore 28-day NOEC and LC50 values of ≥2.7 and >2.7 mg/kg have been determined respectively in a sediment containing 3.0% organic carbon. 2.7 mg/kg dwt sediment was the highest initial measured concentration and it is equal to 5.3% of the nominal concentration applied; the reason for the low initial measured concentration is thought to be that the substance volatilised rapidly.

Description of key information

A long-term study is available with HMDS for toxicity to Lumbriculus variagatus. Long-term studies are read-across from the structurally related substance L3 for Chironomus riparius and Hyalella azteca.

The data with Hyalella azteca (L3) and Lumbriculus variegatus (HMDS) show no effects at the highest concentrations tested in the studies. Significant losses from the test system were observed in the Lumbriculus variegatus study with HMDS. A true PNEC cannot be calculated from these test data because the NOEC values that have been determined are limit values.

A 28-day 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

EC10, LC10 or NOEC for freshwater sediment:

39 mg/kg sediment dw

Additional information

One sediment toxicity study is available for the registration substance hexamethyldisiloxane (HMDS, L2, CAS 107-46-0). Data have also been read across from the structurally related substance octamethyltrisiloxane (L3, CAS, 107-51-7).

HMDS and L3 are part of the siloxanes category. In general, the read-across approach for the siloxanes category is to read-across data on a “nearest-neighbour” basis. Both substances share similar properties and structural similarities.

The only sediment data available with HMDS determined a limit value NOEC of ≥2.7 mg/kg dry weight, but demonstrated significant losses of the test material from the sediment. It is therefore not appropriate to base PNECsediment on this value. In the absence of further data with HMDS, and the difficulty of keeping the substance in the sediment, it is deemed appropriate to read-across data from L3, where additional sediment dwelling species have been tested and a definitive NOEC value has been determined.

It is considered conservative to read across data from L3 to HMDS because of the differences in volatility and water solubility of the substances. This means that maintaining levels of HMDS in the sediment, to reach the L3 NOEC determined for Chironomus riparius and used to generate the PNEC, would be unlikely.

Data from Lumbriculus variegatus are available with HMDS, and data from Chironomus riparius and Hyalella azteca are read-across from octamethyltrisiloxane (L3, CAS, 107-51-7). The selection of read-across substances is based on structural similarity and key physico-chemical properties (log K_ow, log K_oc, degradation).

HMDS and L3 are both linear siloxanes with two silicon atoms and one oxygen atom, and three silicon and two oxygen atoms, respectively. Each of the silicon atoms are fully substituted with methyl groups.

The Lumbriculus variegatus study with HMDS and the Hyalella azteca study with L3 were conducted in natural sediment, and the Chironomus riparius study with L3 was conducted in artificial sediment. All are considered to be reliable and not confounded by extrinsic factors.

A category approach is applied to this endpoint and is detailed in the Siloxane Category report (PFA, 2017). 

Given that HMDS and L3 have reasonably similar physicochemical properties and are structurally similar, it is considered valid to read across sediment toxicity data from L3 to HMDS in the context of the Siloxanes Category. For further details and justification of read-across please see Section 7.0 of the CSR and Section 6 of IUCLID.

Sediment toxicity data

A 28-d NOEC result for Lumbriculus variegatus is available for HMDS. A 28-d NOEC of ≥2.7 mg/kg dry weight has been determined for effects on oligochaete reproduction and biomass (4.5 mg/kg dry weight, normalised to 5% organic carbon). However, significant losses of the test material from the sediment were observed during the preparation of the test system and during the study period. Measured sediment concentrations were well below 80% of nominal on test day 0 and continued to decline throughout the exposure. The initial measured concentrations (day 0) were utilised to express the biological endpoints for the exposure, as recommended in OECD guideline 225, and defined the treatment levels tested as 0.38, 0.80, 2.1, 2.6, and 2.7 mg/kg. The data shows no effects at the highest concentrations tested in the studies. A true PNEC cannot be calculated from the test data because the NOEC values that have been determined are limited values. No hazard is identified.

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).

A 28-d NOEC result for Hyalella azteca is read across from L3. A 28-d NOEC of ≥70 mg/kg dry weight has been determined for effects on survival and weight of Hyalella azteca.