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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Description of key information

BMF 0.162 (kinetic, growth corrected); BMF 0.440 (lipid-normalised, growth-corrected kinetic). A BMF value of 0.440 is selected as the Key value for CSA as a worst case.

Depuration rate constant from BMF study (growth-corrected): 0.0373 d-1

Key value for chemical safety assessment

BMF in fish (dimensionless):
0.44

Additional information

A growth-corrected kinetic dietary BMF value of 0.162 and a growth-corrected, lipid-normalised kinetic dietary BMF value of 0.440 were determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP. Steady-state conditions were not achieved during the uptake phase of the study, and therefore a steady-state BMF is not reported.

The depuration rate from the BMF studies, being independent of exposure concentration and route of exposure, is considered to be a more reliable metric to assess bioaccumulation potential than the ratio BMF values obtained from such a study. 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 70d) as indicative of a substance that does not bioaccumulate.

The depuration rate constant of 0.0373 d-1(corresponding half-life 18.6 days) obtained in the BMF study is indicative of a substance which does not bioaccumulate.

Annex 8 of the OECD TG 305 summarises some approaches currently available to estimate tentative BCFs from data collected in a BMF study. Since the dietary bioaccumulation study gives a depuration rate constant (k­2), if an uptake rate constant (k1) can be estimated from available data, then a kinetic BCF could be estimated for aquatic exposure (i.e. kinetic BCF = k1/k2). The shortcomings inherent in such an estimation are acknowledged in OECD TG 305, significantly, that the overall rate of depuration is assumed to be independent of the exposure route (i.e. the same elimination processes are occurring), and that there are considerable uncertainties in the available methods for estimation of the uptake rate constant.

A detailed analysis of available methods for estimation of the uptake rate constant is presented by Crookes and Brooke (2011). Methods reported correlate the uptake rate to fish weight, Kow, or a combination of these (and other) factors. Most models are derived using a limited training set, and substances within the training sets are predominantly neutral organic substances and do not represent a broad range of functionalities.

In accordance with REACH R7c, estimates of k1should be derived according to all the models available to give a range of BCFs. Crookes and Brooke (2011) found thirteen methods to be potentially suitable for the estimation of k1.The outputs of these thirteen models using the substance physico-chemical data and inputs obtained from the BMF study are reported in the table below (details of the methods are not reported here). Only those methods that require inputs which are readily available are reported (i.e. log Kow, fish weight, dissolved oxygen concentration). :

Table4.3.2 Tentative estimated k1and BCF values

Method for estimating k1(a)

Estimated k1

(l kg-1day-1)

Estimated BCF (l kg-1)

Not growth corrected

Growth corrected

Sijm et al. (1995)

348

5882

9320

Hayton and Barron (1990)

394

6660

10552

Erickson and McKim (1990a)

552

9339

14797

Barber et al. (1991)

541

9159

14512

Barber (2003) - observed

347

5876

9311

Barber (2003) - calibrated

402

6796

10768

Barber (2001)

566

9575

15171

Streit and Sire (1993)

334

5657

8964

Erickson and McKim (1990b)

428

7234

11462

Hendriks et al. (2001)

526

8906

14111

Tolls and Sijm (1995)

1950

32992

52275

Spacie and Hamelink (1982)

2009

33995

53863

Thomann (1989)a(b)

18

310

492

 

 

(a)  Estimates based on fish weight of 3.52g at end of uptake phase. Where a log Kowwas required, a value of 9.0 was used.

(b)  This method also requires dissolved oxygen concentration. A DOC of 10.1mg l-1was calculated from the BMF study data.

Crookes and Brooke tested these methods over the log Kowrange of 3.5 to 8.2, and the training set used did not include any siloxane substances. Poorly soluble substances with a high log Koware expected to have very limited uptake rates from aqueous exposure, due to the limited transfer across gill membranes. The log Kowof the registration substance is 9.0 (calculated by QSAR), and the substance is very poorly soluble (1.3E-06 mg/l at 20-25°C). Given these properties, there is considerable uncertainty in estimation of the uptake rate constant from water using the various methods, and this is reflected in the wide range of tentative BCF values estimated. Bioaccumulation following aqueous exposure to the ultimate degradation product 3,3,3-trifluoropropylmethysilanediol is unlikely, on the basis of a predicted log Kowvalue of 1.0.