Potassium chromate

Sediment toxicity

Description of key information

Information is based on the EU RAR (2005): European Union Risk Assessment Report: chromium trioxide, sodium chromate, sodium dichromate, ammonium dichromate and potassium dichromate, 3rd. Priority List; Volume 53.

There is insufficient data available to derive a PNEC from studies on sediment dwelling organisms. According to the Technical Guidance Document, an equilibrium partitioning approach can be used in the absence of experimental data. However, such an approach for chromium (VI) should be

considered very tentative in nature as chromium (VI) is likely to be reduced to chromium (III) under the conditions found in most sediments, and

the chromium (III) formed is likely to be of much lower water solubility (and bioavailability).

Additional information

For chromium (VI), a PNECwater of 3.4 µg/L has been derived (statistical approach). For chromium (III) a worst-case PNEC of

4.7 µg/L was derived.

According to the Technical Guidance Document, the PNECsediment can be estimated from:

PNEC_sediment = K_susp-water/RHO_suso x PNEC_water x 1000

where RHO_susp = density of suspended matter = 1,150 kg/m³

The following values for K_susp-water were derived:

Chromium (VI) K_susp-water = 500 m³/m³ (acid conditions); K_susp-water = 50 m³/m³ (neutral/alkaline conditions)

Chromium (III) K_susp-water = 7,500 m³/m³ (acid conditions); K_susp-water = 75,000 m³/m³ (neutral/alkaline conditions)

Using these values, the PNEC_sediment can be estimated as follows:

For chromium (VI), PNEC_sediment = 1.5 mg/kg wet weight for acid conditions, and 0.15 mg/kg wet weight for other conditions.

Similarly, for chromium (III), PNEC_sediment = 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 PNEC_sediment 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).