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Why was 21-d lowest observed effect concentration (LOEC) D. magna endpoint of 15 µg/L not used for derivation of FWQG?
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No new toxicity data were published since the screening assessment for D4 and its existing chronic database is insufficient for guideline development as per CCME (2007). Therefore, there was no justification to re-calculate a predicted no effect concentration (PNEC)/FWQG for D4 according to CCME (2007). As a result, the PNEC derived in the screening assessment for D4 was adopted as the FWQG and other chronic endpoints were not considered further for FWQG derivation. As an additional line of evidence in support of the FWQG of 0.2 µg/L, the target lipid model (TLM) (McGrath et al. 2018) predicts a FWQG/PNEC of 0.3 µg/L for D4.
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The validity of the critical study (i.e. 14-d LC50 of 10 µg/L in rainbow trout) used to establish the PNEC/FWQG for D4 is being questioned on the basis of Mackay et al. (2015) model which predicts the critical body burden (CBB) of 3 mmol/kg wet weight (ww) cannot be theoretically achieved within the time frame of the study.
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Mackay et al. (2015) model assumptions did not reflect the study conditions in Sousa et al. (1995) for rainbow trout. Notably, the Mackay model assumed a fish weight of 5 g whereas rainbow trout weighed 0.46 g in Sousa et al. (1995). The kinetics occur much faster with a smaller sized fish. Further, it should be emphasized that while 3 mmol/kg ww is often represented as the CBB in fish, it is not an absolute. To that end, Van Wezel et al. (1995) measured lower CBBs (0.3 - 2.4 mmol/kg ww) in rainbow trout for narcotic chemicals. Taking this into consideration, in concert with using appropriate test conditions from Sousa et al. (1995), the Mackay model predicts a CBB can be reached in 6 days at the water solubility limit and a LC50 of 10 µg/L at 14 days.
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It is not appropriate to include other cyclic volatile methylsiloxane (cVMS) concentrations in the tissue concentration sum when comparing to the Federal Aquatic Biota Tissue Guideline (FBTG) since other cVMS assessed by Environment and Climate Change Canada (ECCC)/Health Canada (HC) were not determined to be toxic under the Canadian Environmental Protection Act, 1999 (CEPA).
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Given that concentration addition has been well described for narcotics, theoretically, any narcotic chemical (expressed in µmol/g lipid weight [lw]) can be included in the sum total of tissue concentrations and compared to the tissue guideline. Therefore, the CEPA-toxic status of a chemical bears no relevance on whether it can be included in the sum total. Moreover, this approach aligns with the Government of Canada's priority to manage chemicals in a more holistic way through consideration of the cumulative hazard of chemical mixtures as opposed to managing risk on a chemical by chemical basis.
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Literature published since the screening assessment for D4 indicates it undergoes significant metabolism in fish and other ecological receptors.
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According to Arnot and Gobas (2006) biotransformation rates in fish greater than 0.1 to 0.2/day do not appear to biomagnify in food webs and, by extension, signify metabolism as a significant pathway contributing to chemical elimination. ECCC reviewed the body of literature for which estimates of metabolism rate constants for D4 in fish have been based (Fackler et al. 1995; Domoradzki et al. 2007; Domoradzki and Woodburn 2008; Springer 2008; Domoradzki et al. 2017; Compton 2019). The metabolism rate constants, when normalized to a 10 g fish at 15°C according to procedures outlined by Arnot et al. (2008) (which permits comparison of metabolic rates across studies), ranged from 0.001 to 0.087/day. Therefore, the weight of evidence indicates D4 metabolism in fish is anticipated to be slow.
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The validity of 28-d LOEC in L. variegatus (Krueger et al. 2009) is being questioned on the basis it was determined using artificial sediment and is more conservative than the endpoint reported by Picard (2009).
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Krueger et al. (2009) followed OECD guidelines 225 and 228 for test procedure and sediment preparation, respectively; therefore concerns about the study’s test conditions and/or substrate leading to toxicity are negated. Further, the study was evaluated by ECCC and determined to meet the rigorous quality standards for use in guideline derivation. The comment that the endpoint is invalid because the results were not replicated when the study was repeated with natural sediments (Picard 2009) is unsound. Given the variable composition of natural sediments, standardized sediments have been developed with the express purpose to reduce the variability in sediment toxicity tests due to test substrate differences. Hence, given the difference in substrate types between Krueger et al. (2009) and Picard (2009) (i.e., artificial vs. natural), variability in toxicity results is expected. Further, the rejection of Krueger et al. (2009) by Woodburn et al. (2018) on the basis of the endpoint being a statistical outlier is unsound. It is unacceptable to exclude the lowest, acceptable endpoint in a toxicity dataset on the basis of it being an outlier alone; biological-based arguments need to accompany such a decision. Finally, as a line of evidence to support the use of Krueger et al. (2009) in Federal Sediment Quality Guideline (FSeQG) derivation, it is consistent with the FWQG and equilibrium partitioning theory (Di Toro et al. 1991). When the FWQG is converted to a bulk sediment concentration via equilibrium partitioning, the FSeQG= 0.03 mg/kg dry weight (dw), which is the same value calculated with LOEC from Krueger et al. (2009).
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Break down the safety factor (SF) of 10 applied to the critical sediment toxicity value used to derive the FSeQG.
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Factsheet revised to support the SF used in FSeQG derivation.
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Reliability of Falany and Li (2005) is being questioned in establishing the tolerable daily intake (TDI) for D4.
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ECCC based its TDI on the critical effect level for repeated, oral dose exposure of 100 mg/kg bw/d determined in the screening assessment for D4 (EC, HC 2008). The critical effect level was based on a weight of evidence approach and included evidence of adverse effects in both a 7-day rat study (Falany and Li 2005) as well as a 7-day mouse study (He et al. 2003) which were both observed at 100 mg/kg bw/d. Further, an increase in liver weights at 100 and 25 mg/kg bw in male and female rats, respectively, was noted in a 14-day oral study in rat (Dow Corning 1990). However, given the uncertainty of whether increases in liver weights due to D4 treatment was adaptive or adverse, this effect was considered collectively with effects seen in other organ systems at similar doses when establishing the critical effect level for D4 from repeated-oral exposure. Therefore, the critical effect level for repeated, oral dose exposure is well supported by the screening assessment for D4 (EC, HC 2008).
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Explain individual contributors for the uncertainty factor (UF) used to derive the TDI.
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Revised factsheet provides details on how the UFs were assigned (10 each for subchronic to chronic extrapolation and interspecies variation).
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