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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Description of key information

Bioaccumulation: aquatic: BCFss 1430 l/kg (4 ng/l); 1240 l/kg (39 ng/l). BCFk 1450 l/kg (4 ng/l); 1240 l/kg (39 ng/l). Lipid normalised (to 5%) values are: BCFss = 4210 l/kg (4 ng/l) and 3650 l/kg (39 ng/l) and BCFk = 4260 l/kg (4 ng/l) and 3650 l/kg (39 ng/l). A BCF value of 4260 l/kg is used in the exposure assessment as a worst case.

 

Depuration rate constants from BCF study: 0.0949 d-1 (4 ng/l); 0.121 d-1 (39 ng/l).

Key value for chemical safety assessment

BCF (aquatic species):
4 260 L/kg ww

Additional information

Steady-state BCF values of 1430 l/kg (4 ng/l) and 1240 (39 ng/l) and kinetic BCF values of 1450 l/kg (4 ng/l) and 1240 l/kg (39 ng/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: BCFss= 4210 l/kg (4 ng/l) and 3650 l/kg (39 ng/l) and BCFk= 4260 l/kg (4 ng/l) and 3650 l/kg (39 ng/l).

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

Fish bioconcentration (BCF) studies are most validly applied to substances with log Kow 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 Koc 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 highly lipophilic 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.

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 kelimination 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 depuration rate constants of 0.0949 d-1(4 ng/l)and 0.121 d-1(39 ng/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 (Fbiota-water) is:

Fbiota-water= BCFWD/LW/ Klwx ρl/ ρB

where BCFWD/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), Klw  is the lipid-water partition coefficient and ρlis the density of lipid and ρBis 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. Kstorage lipid-airvalues for cVMS were systematically lower than Koctanol-air by 0.2 to 0.4 log units depending on temperature. Koctanol-water values may be expected to be similar.

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

Table 4.3.2 Calculated biota-water fugacity ratios

Endpoint

Exposure concentration

BCF Value

Fbiota-water usingKstorage lipid-water=Kow(log Kow9.4)

Fbiota-waterusing logKstorage lipid-water=log Kow-0.4 (9.0)

BCFss

4.0 ng/l

1430

6.0E-05

1.1E-04

BCFss

39 ng/l

1240

5.2E-05

9.2E-05

BCFk

4.0 ng/l

1450

6.1E-05

1.1E-04

BCFk

39 ng/l

1240

5.2E-05

9.2E-05

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 ratios indicate 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 L5) of surface sediments and select biota from aquatic food webs has been carried out as part of a monitoring programme for volatile methysiloxane (cVMS) materials in the aquatic ecosystem.

As discussed in Section 5.5, environmental monitoring data are available for L5 from year one of the programme in Tokyo Bay. Measured concentrations of L5 in surface sediment ranged from <MDL to 1.9 ng/g ww, and were greater than the LOQ in 10 samples (LOQ 0.78 ng, MDL 0.26 ng/g ww). L5 was less than the Method Detection Limit (MDL) in biota samples (LOQ 3.2 ng, MDL 1.1 ng/g ww).

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.