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A substantial amount of information is available for the toxicity of chromium (VI) to terrestrial organisms. In the environment, it is likely that chromium (VI) will be reduced to chromium (III) in soil, and it is also likely that such conversion would have taken place in many of the toxicity tests.

For chromium (VI), long-term toxicity data are available for three trophic levels (plants, earthworms and soil processes/micro-organisms), with plants generally being the most sensitive species (although a clear NOEC has not been determined for earthworms, the EC50 values are generally higher than those found in the plant experiments). The lowest NOEC from these studies is around 0.35 mg/kg dry weight of soil for plants. According to the Technical Guidance Document, an assessment factor of 10 is appropriate and so the PNECsoil can be estimated as 0.035 mg/kg dry weight. Using the water content of soil from the Technical Guidance Document of 11.8% by weight (20% by volume), this is equivalent to a PNECsoil of around 0.031 mg/kg on a wet weight of soil basis.

Chromium (III) has generally been shown to be less toxic than chromium (VI) to soil organisms. One exception to this may be on the effects seen in some experiments using growth media (no soil) where reduction in root growth was seen at similar concentration as found for chromium (VI). Since chromium (III) adsorbs more strongly onto soil than chromium (VI), it would again be expected that in soils, chromium (III) would be less toxic than chromium (VI). From the available data, the NOEC for chromium (III) to plants is of the order of 100 mg Cr/kg soil, with a NOEC of 32 mg Cr/kg dry soil being reported for earthworms, and a NOEC/LOEC of ~100-330 mg Cr/kg soil also being reported. Applying an assessment factor of 10 to the lowest of these NOECs gives a PNEC for chromium (III) of approximately 3.2 mg Cr/kg dry soil, which is equivalent to a PNEC of around 2.8 mg/kg on a wet weight of soil basis. This value is also lower than the HC5 value for soil processes.

According to the Technical Guidance Document, an equilibrium partitioning approach can also be used in the derivation of the PNECsoil. 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 soils, and the chromium (III) formed is likely to be of much lower water solubility (and bioavailability).

For chromium (VI), a PNECwater of 3.4 µg/l has been derived. For chromium (III) a worst-case PNEC of 4.7 µg/l was derived.

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

PNECsoil = Ksoil-water/RHOsoil x PNECwater x 1000

where RHOsoil = density of soil = 1,700 kg/m3

The following values for Ksoil-water were derived:

Chromium (VI) Ksoil-water = 75 m3/m3 (acid conditions); Ksoil-water = 3.2 m3/m3 (neutral/alkaline conditions)

Chromium (III) Ksoil-water = 1,200 m3/m3 (acid conditions); Ksoil-water = 22,500 m3/m3 (neutral/alkaline conditions).

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

For chromium (VI), PNECsoil = 0.15 mg/kg wet weight for acid conditions, and 0.006 mg/kg wet weight for other conditions. Similarly, for chromium (III), PNECsoil = 3.3 mg/kg wet weight for acid conditions and 62 mg/kg wet weight for other conditions.

The PNECsoil estimated for chromium (III) for acidic conditions using the equilibrium partitioning method is in very good agreement with the values obtained above using the available toxicity data. For the risk assessment the PNECs obtained from experimental data will be used. Thus the PNECsoil for chromium (VI) is taken as 0.031 mg/kg wet weight. The PNECsoil for chromium (III) is taken to be 2.8 mg/kg wet weight.

For the risk characterisation the PNECsoil for chromium (III) is used, as the concentrations of chromium in soil are calculated as chromium (III). It should be noted that the PNEC for chromium (III) is derived from experiments where a highly soluble (and hence bio available) form of chromium (III) has been tested. In the environment, chromium (VI) is likely to be reduced to forms of chromium (III) of limited solubility and bioavailability, where it is unlikely that the concentration of 'dissolved' and hence available chromium (III) will reach the levels where effects might be expected. This is seen in experiments with both soil and aquatic organisms when a form of chromium (III) with low water solubility has been tested. Similarly, it is clear that there are many natural soils where the levels of total chromium are above the PNECs derived here. Again, the main form of the chromium needs to be considered. In natural soils, the majority of chromium will be present as low solubility chromium (III) complexes, where bioavailability is again limited. The PNECs derived are not appropriate for such situations.