Decamethyltetrasiloxane

Bioaccumulation: aquatic / sediment

Description of key information

Bioaccumulation: aquatic: BCFss 3870 l/kg (0.43 µg/l) and 1610 l/kg (5.3 µg/l); BCFk 3830 l/kg (0.43 µg/l) and 1760 l/kg (5.3 µg/l). Lipid normalised (to 5%) BCFss 6910 l/kg (0.43 µg/l) and 2880 l/kg (5.3 µg/l); lipid normalised (to 5%) BCFk 6840 l/kg (0.43 µg/l) and 3140 l/kg (5.3 µg/l). A BCF value of 6910 l/kg is used in the exposure assessment as a worst case. BMF 0.44 (lipid-normalised steady-state); BMF 0.48 (lipid-normalised, kinetic). A BMF value of 0.48 is used in the exposure assessment as a worst case. Depuration rate constants from BCF study: 0.211 d-1 (0.43 µg/l); 0.124 d-1 (5.3 µg/l).

Key value for chemical safety assessment

BCF (aquatic species):

6 910 L/kg ww

BMF in fish (dimensionless):

0.48

Additional information

Steady-state BCF values of 3870 l/kg (0.43 µg/l) and 1610 l/kg (5.3 µg/l) and kinetic BCF values of 3830 l/kg (0.43 µg/l) and 1760 l/kg (5.3 µg/l) were determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

Lipid normalised (to 5%) values are: BCF_ss= 6910 l/kg (0.43 µg/l) and 2880 l/kg (5.3 µg/l) and BCF_k= 6840 l/kg (0.43 µg/l) and 3140 l/kg (5.3 µg/l).

Growth correction was not applied, though based on fish weight data reported the growth of the fish during the study was minimal.

A non-guideline fish feeding study is also available. A lipid-normalised steady-state BMF value of 0.44 and lipid-normalised kinetic BMF value of 0.48 were determined in a reliable study conducted in compliance with GLP.The growth corrected kinetic BMF value and growth corrected, lipid normalised kinetic BMF value were calculated and reported to be 1.3 and 3.8 respectively; however, recent scientific discourse on the methodology to calculate growth corrected BCF and BMF values has revealed that these methods violate the rules of mass balance (Gobas et al., 2019). Therefore, the reported growth corrected values are not considered valid for the determination of bioaccumulation.The food in this study was very highly dosed (500 µg/g¹⁴C-L4 nominal; 534 µg/g mean measured), which may limit the applicability of the values obtained.

Fish bioconcentration (BCF) studies are most validly applied to substances with log K_ow values between 1.5 and 6. Practical experience suggests that if the aqueous solubility of the substance is low (i.e. below ~0.01 to 0.1 mg/l) (REACH Guidance R.11; ECHA, 2014), fish bioconcentration studies might not provide a reliable BCF value because it is very difficult to maintain exposure concentrations. Dietary bioaccumulation (BMF) tests are practically much easier to conduct for poorly water-soluble substances, because a higher and more constant exposure to the substance can be administered via the diet than via water. In addition, potential bioaccumulation for such substances may be expected to be predominantly from uptake via feed, as substances with low water solubility and high K_oc will usually partition from water to organic matter.

However, there are limitations with laboratory studies such as BCF and BMF studies with highly lipophilic and adsorbing substances. Such studies assess the partitioning from water or food to an organism within a certain timescale. The studies aim to achieve steady-state conditions, although for highly lipophilic and adsorbing substances such steady-state conditions are difficult to achieve. In addition, the nature of BCF and BMF values as ratio values, means that they are dependent on the concentration in the exposure media (water, food), which adds to uncertainty in the values obtained.

For highlylipophilic and adsorbing substances, both routes of uptake are likely to be significant in a BCF study, because the substance can be absorbed by food from the water. 

Dual uptake routes can also occur in a BMF study, with exposure occurring via water due to desorption from food, and potential egestion of substance in the faeces and subsequent desorption to the water phase. Although such concentrations in water are likely to be low, they may result in significant uptake via water for highly lipophilic substances.

The OECD 305 advocates for calculating a growth dilution correction for kinetic BCF and BMF values, where the growth rate constant (i.e. k_g) can be subtracted from the overall depuration rate constant (k₂). In short, the uptake rate constant is divided by the growth-corrected depuration rate constant to give the growth corrected kinetic BCF or BMF value. However, recent scientific discourse on this topic has pointed out that correcting for growth in the depuration phase and not likewise accounting for the effects of lack of growth in the uptake phase (i.e.with regards to reduced feeding rate or respiration rate for a non-growing fish), results in an equation where the laws of mass balance are violated (Gobas et al., 2019). Essentially, the uptake parameters of the kinetic BCF or BMF calculation (i.e. k₁) are those of a growing fish, but the depuration parameters are altered to reflect no growth (i.e. k₂- k_g). Based on this criticism of the growth dilution correction, these calculations are not considered best practice for the assessment of bioaccumulation (Gobas et al., 2019).

 

Goss et al. (2013) put forward the use of elimination half-life as a metric for the bioaccumulation potential of chemicals. Using the commonly accepted BMF and TMF threshold of 1, the authors derive a threshold value for k_elimination of >0.01 d^-1(half-life 70d) as indicative of a substance that does not bioaccumulate.

Depuration rates from BCF and BMF studies, being independent of exposure concentration and route of exposure, are considered to be a more reliable metric to assess bioaccumulation potential than the ratio BCF and BMF values obtained from such studies.

The growth-corrected depuration rate constant (i.e. k₂ -k_g) of 0.0057d^-1from the BMF study for L4, may not be valid due to the very high loading of the food in this study potentially overloading metabolic/elimination pathways. This depuration rate is therefore not taken into account in the assessment of bioaccumulation.

The depuration rate constants of 0.211 d^-1(0.43 µg/l)and 0.124 d^-1(5.3 µg/l)obtained from the BCF study are considered to be valid and to carry most weight for bioaccumulation assessment. These rates are indicative of a substance which does not bioaccumulate.

Burkhard L. P.et al., 2012 has described fugacity ratios as a method to compare laboratory and field measured bioaccumulation endpoints. By converting data such as BCF and BSAF (biota-sediment accumulation factor) to dimensionless fugacity ratios, differences in numerical scales and unit are eliminated.

Fugacity is an equilibrium criterion and can be used to assess the relative thermodynamic status (chemical activity or chemical potential) of a system comprised of multiple phases or compartments (Burkhard L. P.et al., 2012). At thermodynamic equilibrium, the chemical fugacities in the different phases are equal. A fugacity ratio between an organism and a reference phase (e.g. water) that is greater than 1, indicates that the chemical in the organism is at a higher fugacity (or chemical activity) than the reference phase.

The fugacity of a chemical in a specific medium can be calculated from the measured chemical concentration by the following equation:

f = C/Z

Where f is the fugacity (Pa), C is concentration (mol/m³) and Z is the fugacity capacity (mol(m³.Pa)).

The relevant equation for calculating the biota-water fugacity ratio (F_biota-water) is:

F_biota-water= BCF_WD/LW/ K_lwx ρ_l/ ρ_B

where BCF_WD/LWis ratio of the steady-state lipid-normalised chemical concentration in biota (µg-chemical/kg-lipid) to freely dissolved chemical concentration in water (µg-dissolved chemical/L-water), K_lw is the lipid-water partition coefficient and ρ_lis the density of lipid and ρ_Bis the density of biota.

A study to determine storage lipid-air partition coefficients of cVMS has been carried out (Dow Corning Corporation, 2015c). The conclusion from that study is that partitioning of cVMS compounds between storage lipids and air or water is reasonably similar, but not identical, to octanol. K_{storage lipid-air}values for cVMS were systematically lower than K_octanol-airby 0.2 to 0.4 log units depending on temperature. K_{octanol-water}values may be expected to be similar,

 

The table below presents fugacity ratios calculated from the BCF data for L4using both log K_owand log K_ow-0.4 as a worst case approximation for log K_lw. BMF values do not require adjustments because these values are already equivalent to fugacity-based values.

Table Calculated biota-water fugacity ratios

Endpoint	Exposure concentration	BCF Value	Fbiota-water using Kstorage lipid-water =Kow(log Kow8.21)	Fbiota-water using log Kstorage lipid-water =log Kow-0.4 (7.81)
BCFss	0.43 µg/l	3870	8.17E-04	2.0E-03
BCFss	5.3 µg/l	1610	3.40E-04	8.2E-04
BCFk	0.43 µg/l	3830	8.08E-04	2.0E-03
BCFk	5.3 µg/l	1760	3.71E-04	9.0E-04

The fugacity-based BCF directly reflect the thermodynamic equilibrium status of the chemical between the two media included in the ratio calculations. The fugacity ratios calculated are all below 1, indicating that the chemical in the organism is at a lower fugacity (or chemical activity) than in the water. It should be noted however, that the BCF study may not have reached true steady-state in the timescale of the laboratory studies. The fugacity ratio indicates that uptake may be less than expected on thermodynamic grounds, suggesting that elimination is faster than might be expected on grounds of lipophilicity alone.

Collection and analysis (for L4) of surface sediments and select biota from aquatic food webs has been carried out as part of a 5-year monitoring programme for cyclic volatile methysiloxane (cVMS) materials in the aquatic ecosystem.

As discussed in Section 5.5, studies are available from the first three years of this programme in two locations: Lake Ontario, Canada, and Lake Pepin, USA, and the first two years of the programme in Oslo Fjord, Norway. In addition, the first year data is available from Tokyo Bay. In all environmental monitoring studies to date, L4 was either not detected or less than the Method Detection Limit (MDL) in both surface sediment and biota samples.

References:

Burkhard, L. P., Arnot, J. A., Embry, M. R., Farley, K. J., Hoke, R. A., Kitano, M., Leslie, H. A., Lotufo, G. R., Parkerton, T. F., Sappington, K. G., Tomy, G. T. and Woodburn, K. B. (2012). Comparing Laboratory and Field Measured Bioaccumulation Endpoints. Integrated Environmental Assessment and Management 8, 17-31.

Goss, K-U., Brown, T. N. and Endo, S. (2013). Elimination half-life as a metric for the bioaccumulation potential of chemicals in aquatic and terrestrial food chains. Environmental Toxicology and Chemistry 32, 1663-1671.