Hexachlorobutadiene (HCBD) is a by-product of chemical industries that produce chlorinated hydrocarbons, such as tetrachloroethylene and carbon tetrachloride. It is mainly used as an industrial solvent.

HCBD is considered a List 1 compound and there have been a number of reviews outlining the fate and behaviour and aquatic toxicity of this compound (e.g. WHO 1984 and CCME 1992).

The presence of HCBD in the environment results from anthropogenic sources and occurs as a tarry by-product. Volatilisation and solublisation from this waste are the primary mechanisms for dispersal into and throughout the environment. Waste holding areas and wastewater from industry are likely to be significant source of contamination.

The main pathways of entry into the environment are emissions from waste.

Monitoring data from the National Rivers Authority and the National Monitoring Programme Survey of the Quality of UK Coastal Waters are presented in Appendix D. However, hexachlorobutadiene was not monitored in either survey.

Insufficient data are available to assess expected hexachlorobutadiene concentrations with toxicity data or relevant standards for the protection of saltwater life.

Intercompartmental transport will chiefly occur by volatilisation, adsorption to particulate matter, and subsequent deposition or sedimentation. Hexachlorobutadiene accumulates in sediment. Photolysis, oxidation and hydrolysis are not expected to be significant removal processes from the aquatic environment. However, sorption to sediments is considered to be an important mechanism for its removal from the water column.

An exhaustive literature review on the toxicity of hexachlorobutadiene to marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents (WHO 1984, CCME 1992). The most sensitive groups of organisms have been identified.

Hexachlorobutadiene is moderately to highly toxic to aquatic organisms.

Fish species and crustaceans have been found to be the most sensitive, 96-h LC50 values ranging from 0.032 to 1.2 and 0.09 to approximately 1.7 mg l ^-1 for crustaceans and fish respectively. In one 28-day early-lifestage test with fathead minnows, reproduction was unaffected at concentrations of up to 0.017 mg l^-1, whereas increased mortality and a decreased body weight were observed at 0.013 and 0.017 mg l^-1.

No data appear to be available on the toxicity of HCBD to sediment-dwelling organisms, but given that HCBD may accumulate in sediments, adverse effects on benthic organisms cannot be excluded.

The substance has a high bioaccumulating potential which has been confirmed by laboratory and field observations. Average steady-state bioconcentration factors of 5,800 and 17,000, based on wet weight, have been determined experimentally in rainbow trout.

Concentrations of hexachlorobutadiene in aquatic organisms, birds and mammals indicate bioaccumulation but not biomagnification. In polluted waters, levels of over 1,000 µg kg^-1 wet weight have been measured in several species and 120 mg kg^-1 (lipid base) in one species. Present levels generally remain below 100 µg kg^-1 wet weight away from industrial outflows.

Potential effects include:

• toxicity of hexabutadiene to invertebrates (particularly crustacea) and fish at concentrations above the EQS of 0.1 µg l^-1 (annual average) in the water column;
• bioaccumulation of hexabutadiene in aquatic organisms, birds and mammals. However, there is little evidence of biomagnification through food chains.

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