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Sediment toxicity

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Description of key information

According to REACH Guidance (RIP 3.2.2: Guidance on information requirements and chemical safety assessment Part B: Hazard Assessment), the Predicted No Effect Level for sediment (PNECsediment) can be derived by using results of tests with sediment living organisms, or by using the equilibrium partitioning method (EPM) when only toxicity data (results of tests or non-test methods) for aquatic (pelagic) organisms are available.


The main conclusions for the freshwater sediment compartment can be formulated as follows:

1) Based on the equilibrium partitioning method (EPM), a PNECsediment of 21.2 g Mo/kg dry wt is derived (input data: PNECaquatic of 11.9 mg/L; KD of 1778 L/kg)

2) Modelled worst-case concentration levels in sediments near Mo-producing industrial plants remain at least two orders of magnitude below the EPM-derived PNECsediment

3) Background or ambient Mo-levels in freshwater sediments remain at least three orders of magnitude below the EPM-derived PNECsediment.


The main conclusions for the marine sediment compartment can be formulated as follows:

1) Based on the equilibrium partitioning method (EPM), a PNECsediment,marine of 2.37 g Mo/kg dry wt is derived (input data: PNECaquatic,marine of 2.28 mg Mo/L; KD of 1,037 L/kg)

2) Ambient Mo-levels in marine sediment remain more than two orders of magnitude below the EPM-derived PNECsediment,marine.



Based on this information it is concluded that there is no need for additional testing, and the EPM-derived values are put forward as the PNEC for the freshwater/marine sediment compartments. Detailed information on the derivation of the freshwater and marine PNEC for sediment can be found in the Background Document “Environmental effects assessment of molybdenum”, which is attached in the technical dossier in IUCLID Section 13.

Key value for chemical safety assessment

Additional information

The estimation of the PNECsediment with equilibrium partitioning method is based on the assumption that the sensitivity of pelagic and sediment living organisms is comparable, but that in sediment the availability of the substance is reduced due to sorption to the (organic matter of the) sediment. This implies the use of partitioning calculations, assuming that equilibrium is obtained. It should be noted that EPM considers only uptake via the water phase. For highly adsorbing chemicals, however, the uptake via other exposure pathways like ingestion or direct contact with sediment becomes more important, depending on the organism used for testing. Uptake via the gut is likely to play an increasingly important role for compounds with a log Kow greater than 5 or with a correspondingly high adsorption or binding behaviour. In such cases, the equilibrium partitioning method can only be used in a modified way. In order to allow for uptake of substances via ingestion of sediment, an additional factor of 10 is applied to the PEC/PNEC ratio for such substances. Based on its physicochemical properties, however, molybdenum cannot be categorized as such a compound, and there is no need to apply this additional safety factor of 10 on a PNEC that is derived with the EPM.

 It should be taken into account that EPM may result both in an overestimation or underestimation of the toxicity to benthic organisms (Di Toro et al. 2005). Therefore, this method can only be used as rough screening for assessing the level of risk to sediment dwelling organisms; based on the outcome of the EPM it is decided whether sediment toxicity tests with benthic organisms are required. If the outcome of the EPM-approach leads to a PEC/PNEC ratio of >1, data improvement is necessary either by refining the exposure assessment or by performing tests with benthic organisms using spiked sediment to support a refined risk assessment for the sediment compartment.