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Sediment toxicity

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Description of key information

It is proposed to waive the requirement for toxicity studies on sediment dwelling organisms based on the EU RAR which proposes a PNEC based on anequilibrium partitioning approach.

Key value for chemical safety assessment

Additional information

Extracted from EU RAR (2005), Section 3.2.1.7.2

There is insufficient data available to derive a PNEC from studies on sediment dwelling organisms. According to the Technical Guidance Document and more recent REACH guidance documents, an equilibrium partitioning approach can be used in the absence of experimental data.

For chromium (III) a worst-case aquatic compartment PNEC of 4.7μg/l was derived. According to the Technical Guidance Document, the PNECsediment can be estimated from: 1000 water suso susp-water sediment = × PNEC × RHO K PNEC where RHOsusp = density of suspended matter = 1,150 kg/m3.

The following values for Ksusp-water were derived: Chromium (III) Ksusp-water = 7,500 m3/m3 (acid conditions); Ksusp-water = 75,000 m3/m3 (neutral/alkaline conditions).

Using these values, the PNECsediment can be estimated as follows: For chromium (III), PNECsediment = 31 mg/kg wet weight for acid conditions and 307 mg/kg wet weight for other conditions.

A recent report (Environment Canada, 1997) has derived draft guideline values for chromium based on the results of numerous field surveys. In the approach taken, the data on sediment characteristics and the presence or absence of benthic species was investigated to look for associations between total chromium concentrations and any adverse effect seen. Such an approach cannot prove that a given effect was caused by a given chromium concentration, since it relies on field data where exposures are likely to be to a wide range of substances. Using this approach, a draft threshold effect level (level below which adverse effects are expected to occur rarely) of 37.3 mg/kg dry weight for freshwater sediments and 52.3 mg/kg dry weight for marine sediments was derived for total chromium. The corresponding draft probable effect level (the level above which adverse effects are expected to occur frequently) was estimated to be 90 mg/kg dry weight for freshwater sediments and 160.4 mg/kg dry weight for marine sediments (again for total chromium). Given that the vast majority of chromium (VI) entering into sediment will be converted to chromium (III), the PNECsediment of 31 mg/kg wet weight (which is equivalent to around 80 mg/kg on a dry weight basis) is in reasonable agreement with the draft effect levels derived by Environment Canada (1997).

Cr(VI) data are also retrieved from the EU RAR(2005) and reported in the CSR.