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Bioaccumulation: aquatic / sediment

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Description of key information

Bioaccumulation: aquatic:BCFKL2860 l/kg (0.41 µg/l).

Depuration rate constants from BCF study:0.0233 d-1(0.41 μg/l); 0.0260 d-1(4.4 μg/l).

In exposure modelling the BCFKLvalue of 2860 l/kg (0.41µg/l) will be used as a worst case.

Key value for chemical safety assessment

BCF (aquatic species):
2 860 L/kg ww

Additional information

Steady-state BCF (BCFSS) values with fathead minnows (Pimephales promelas) of 1160 l/kg(0.41 µg/l) and 240 l/kg(4.4 µg/l) and kinetic BCF (BCFK) values of 1660 l/kg(0.41 µg/l) and 319 l/kg(4.4 µg/l) were determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP (Dow Corning Corporation, 2005). Lipid normalised (to 5%) values (using lipid content at end of uptake of 2.9%) are: BCFSSL= 2000 l/kg (0.41 µg/l) and 414 l/kg (4.4 µg/l) and BCFKL= 2860 l/kg (0.41 µg/l) and 550 l/kg (4.4 µg/l).

The highest concentration tested in this study (4.4 μg/l) is very close to the water solubility of the test substance (5.17 μg/l). Although the test concentration was adequately maintained at this level during the test, the analytical methodology used involved collection of the water samples from mid-depth using a pipette and analysis of the water samples directly by scintillation counting. Thus the measured levels represent total levels of D6 present and not necessarily dissolved concentrations. It is therefore possible that, at the highest concentration tested, some of the D6 may not have been present in the dissolved phase (and this may explain why a generally lower level of accumulation was seen at the highest test concentration compared with the lowest test concentration). For this reason, the results obtained for the 0.41 μg/l treatment are considered to be the most representative values for the BCF of D6 from this study.

A further BCF study is available with carp (Cyprinus carpio) (CERI, 2010). The study report is only available in Japanese; further pharmacokinetic analysis has been performed by the registrant by examining the data tables available in the report. It appears that juvenile carp were exposed to D6 at aqueous concentrations (mean measured) of 0.9 µg/L and 0.09 µg/L. Both concentrations are below the measured water solubility of D6, 5.13 µg/L. Steady-state BCF (BCFss) values were calculated to be 2300 l/kg and 4000 l/kg for the 0.9 µg/l and 0.09 µg/l treatment groups, respectively. The depuration half-life was estimated by the reviewer as 25 days for both the treatment groups. The lipid content in the fish appears to be presented as 4.85% – 5.78% (average 5.32%). Normalising the BCFss to 5% lipid content would reduce the BCFSSLto 2161 l/kg and 3759 l/kg for the 0.9 µg/l and 0.09 µg/l treatment groups, respectively. At both treatment levels, the concentration of D6 in fish continued to rise during the initial timepoints in the depuration phase. It would therefore appear that steady state had most likely not been reached during the uptake phase; the steady-state BCF may therefore not be the most reliable metric. In the kinetic analysis performed by the registrant, kinetic BCF (BCFK) values of 2969 l/kg and 5179 l/kg, equivalent to a BCFKLof 4872 l/kg and 2793 l/kg, were determined for the 0.9 µg/l and 0.09 µg/l treatment groups, respectively. The report detailing the pharmacokinetic analysis performed by the registrant is attached to the EPSR.

BAF values for Lumbriculus variegatus of 0.66 (28.1 mg a. i. /kg- sediment) and 0.070 (484 mg a. i. /kg -sediment), and kinetic BAF values of 0.67 (28.1 mg a. i. /kg-sediment) and 0.070 (484 mg a. i. /kg-sediment) were determined (Wildlife International Ltd., 2008).

To reduce variability in test results for organic substances with high lipophilicity, bioaccumulation factors should be expressed additionally in relation to the lipid content of the test organisms and to the organic carbon content (TOC) in the sediment (biota-sediment accumulation factor or BSAF in kg sediment TOC kg-1worm lipid content). The average organic carbon content in the sediment was approximately 3.45%, and the lipid content of the worms was 2.59%. The steady-state BSAF values are therefore 0.88 (28.1 mg a. i. /kg) and 0.09 (484 mg a. i. /kg), and kinetic BSAF values are 0.89 (28.1 mg a. i. /kg) and 0.09 (484 mg a. i. /kg).

The key study is selected as the guideline fish BCF study (Dow Corning Corporation, 2005), because it is subject to the most straight forward interpretation with regard to the REACH criteria for bioaccumulation assessment.

One non-standard lower reliability study is available (Dow Corning Corporation 1985a). A bioaccumulation test in Daphnia magna calculated a steady-state BCF of approximately 2400.

Fish bioconcentration (BCF) studies are most validly applied to substances with log Kowvalues 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, 2017), 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 Kocwill 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. This is unavoidable.

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.

Indeed, guideline updates specifically address the need for dosing via the two routes.

The OECD 305 advocates for calculating a growth dilution correction for kinetic BCF and BMF values, where the growth rate constant (i.e. kg) 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- kg). Based on this criticism of the growth dilution correction, these calculations are not considered best practice for the assessment of bioaccumulation (Gobaset al., 2019).

For the purpose of screening, 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 keliminationof >0.01 d-1(half-life 70 d) 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 of0.0233 d-1(at 0.41 μg/l) and 0.0260 d-1(at 4.4 μg/l) obtained from the key BCF study (Dow Corning Corporation, 2005) are considered to be valid and to carry most weight for bioaccumulation assessment. These rates are indicative of a substance which does not bioaccumulate.

Depuration rate constants of 0.0384d-1(at 0.9 μg/l) and 0.0392 d-1(at 0.09 μg/l) were obtained from the supporting aqueous BCF study (CERI, 2010) and depuration constants of 0.134 d-1(at 28.1 mg a.i./kg) and 0.170 d-1(at 484 mg a.i./kg) were obtained from the sediment bioaccumulation study (WildlifeInternational Ltd., 2008). These rates are also consistent with the conclusion that the substance does not bioaccumulate.

As an additional metric, Burkhard 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 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.

The relevant equation for calculating the biota-sediment fugacity ratio (Fbiota-sediment) is:

·      Fbiota-sediment= BSAFOC/LWx Koc/Klwxρs/ 1000

where BSAFOC/LW (kg-organic carbon/kg-lipid weight) is the ratio of the steady-state lipid normalised chemical concentration in the organism (mg-chemical/kg-lipid) to the organic carbon-normalised chemical concentration in the sediment (mg-chemical/kg-organic carbon), Klw is the lipid-water partition coefficient, ρsis the density of the sediment (2400 kg-sediment/m3-sediment), and 1000 corresponds to a unit conversion (L-water/m3-water).

It can be assumed that n-octanol and lipid are equivalent with respect to their capacity to store organic chemicals, i.e. Klw= Kow. For some substances with specific interactions with the organic phase, this assumption is not sufficiently accurate. Measurement of Klwvalues for siloxane substances is in progress. Initial laboratory work with olive oil as lipid substitute indicates that the assumption that Klw= Kowis appropriate (Reference: Dow Corning Corporation, personal communication). However, the calculated fugacity ratios presented here should be used with caution at this stage.

The table below presents fugacity ratios calculated from the key BCF data for D6, using Kowfor Klw.

Table4.3.2Calculated biota-water fugacity ratiosfor D6

Endpoint

Exposure concentration

BCF Value

Fbiota-water*

BCFss

0.41 µg/l

2000

6.30E-05

BCFss

4.4 µg/l

414

1.30E-05

BCFk

0.41 µg/l

2860

9.01E-05

BCFk

4.4 µg/l

550

1.73E-05

*Using log Kow8.87 and log Koc5.9

 

The fugacity-based BCF directly reflects 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 tends to be at a lower fugacity (or chemical activity) than in the water/sediment. It should be noted however, that the BCF studies 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.

Trophic Magnification Factors (TMF) values for D6 have been evaluated in several aquatic systems (Dow Corning Corporation 2009d (Lake Pepin), Dow Corning Corporation 2010a (Inner and Outer Oslofjord), Powellet al.,2012 (Tokyo Bay), Borgået al., 2012 (Lake Mjosa), McGoldricket al., 2014 (Lake Erie), Borgå K.et al., 2013 (Lake Mjosa and Lake Randsfjorden), and the overall weight of evidence indicates that food web dilution is occurring in freshwater and marine systems, with measured TMF values <1 and a low probability that a given TMF will exceed 1.

A summary of the TMF data now follows:

Dow Corning Corporation 2009d: Trophic magnification factor for D6 in the freshwater benthipelagic food web of Lake Pepin ranged from 0.2 to 0.4. These results indicate that D6 is not undergoing trophic magnification in this food web as concentrations were decreasing with increasing species trophic level.

Dow Corning Corporation 2010a: Trophic magnification factor for D6 in the marine benthipelagic food web of the inner Oslofjord ranged from 0.4 to 0.5. Re-analysis of the data in 2014 using a probabilistic approach generated a TMF value of 0.3 for the demersal food web and 0.8 for the pelagic food web. These results indicate that D6 is not undergoing trophic magnification in this food web as concentrations were decreasing with increasing species trophic level.

Dow Corning Corporation 2010a: Trophic magnification factor for D6 in the marine benthipelagic food web of the outer Oslofjord ranged from 0.2 to 0.3. Re-analysis of the data in 2014 using a probabilistic approach generated a TMF value of 0.3 for the demersal food web and 0.9 for the pelagic food web. These results indicate that D6 is not undergoing trophic magnification in this food web as concentrations were decreasing with increasing species trophic level.

Powellet al.,2012: Trophic magnification factor for D6 in the marine pelagic food web of Tokyo Bay ranged from 0.7 - 0.8. These results indicate that D6 is not likely to be undergoing trophic magnification in this food web. However, the TMF is not significantly different from 1.0. Concentrations in the biota were corrected for concentration gradients observed in surface sediments.

Borgået al., 2012: Trophic magnification factor for D6 in the freshwater pelagic food web of Lake Mjøsa, Norway ranged from 0.6 to 0.8. These results suggest that D6 is not undergoing trophic magnification in this food web. However, the analytical method utilized to determine concentrations of cVMS concentrations in biota has not been validated and experts in cVMS analysis question the validity of the results. Another factor contributing to uncertainty around the results is that that only skinless muscle fillets from the fish were analysed instead of whole-body homogenates. Finally, there remains some questions pertaining to how closely all the sampled organisms were associated with one contiguous food web.

McGoldricket al., 2014: Trophic magnification factor of D6 ranged from 0.71 to 0.97 in Lake Erie. These results indicate that D6 is not likely to be undergoing trophic magnification in Lake Erie, however, the 95% confidence interval does include values greater than 1.0.

Borgå K.et al., 2013: Trophic magnification factor for D6 in the pelagic food webs of freshwater lakes in Norway ranged from 1.5 to 2.7. These results suggest that D6 is undergoing trophic magnification in these food webs. However, the analytical method utilized to determine concentrations of cVMS concentrations in biota have not been validated and experts in cVMS analysis question the validity of the results. Additionally, concentrations of D6 were below the limit of quantification in some of the samples, increasing the uncertainty associated with the reported values. Another factor contributing to uncertainty around the results is that that only skinless muscle fillets from the fish were analysed instead of whole-body homogenates. Finally, there remains some questions pertaining to whether or not concentration gradients and variable exposure are impacting the results.

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.