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Diquat C16 -18 has a limited solubility in water and also has a strong tendency to adsorb to negatively charged surfaces such as suspended matter, algae and test vessels or organic material (including dissolved organic matter such as humic acids). Many cationic substances in general but long chain diquats in particular rank among the most difficult substances to test in environmental toxicology. Standard guideline studies are inappropriate to test substances with such properties and the current REACH Guidance Documents do not provide sufficient guidance concerning bioavailability and exposure assessment for cationic surface-active substances like the diquats as these were written with normal hydrophobic chemicals in mind, failing to take into account the lack of bioavailability that occurs in the environment with these substances.

The long-term aquatic ecotoxicity tests with diquat C16 -18 were therefore performed in river water to allow a PECaquatic, bulk/PNECaquatic, bulk approach and is considered to be conservative but more environmentally realistic than the standard method. This approach is based on PEC estimations representing ‘total aquatic concentrations’. To characterize the risk to the aquatic compartment the PECaquatic, bulk is compared with the PNECaquatic, bulk derived from river water ecotoxicity studies (ECETOC, 2001).

In order to class standard laboratory toxicity study valid, it is of particular importance that - besides information on test substance, test method /conditions and test organism used - suitable precautions are taken to prevent the loss of test substance by adsorption and that exposure concentrations are based upon measured levels.

For ecotoxicity tests performed using the “bulk” approach, however, adsorption to suspended matter and DOC is acceptable and only adsorption to glassware should be accounted for. For a valid bulk approach test the concentration-effect relationship should be based on the sum of adsorbed and dissolved substance in the volume of the medium tested. One of the advantages of the bulk approach tests with these difficult substances is that in the presence of suspended matter, humic acids and/or algae, the residual sorption to glassware will be negligible. The results of these bulk approach tests are therefore much easier to interpret, more environmental realistic, and if compared to PECbulk clearly provide a more appropriate assessment of risks for the environment. All effect values given for tests performed in natural river water are therefore based on the nominal test item concentrations.

The PNECaquatic bulk should calculated using the assessment factor proposed by the TGD.