Benzene hexachloro- (HCB): Environmental effects
Description
Acute toxic effects may include the death of animals, birds, or fish, and death or low growth rate in plants. Acute effects are seen two to four days after animals or plants come in contact with a toxic chemical substance.
Chronic toxic effects may include shortened lifespan, reproductive problems, lower fertility, and changes in appearance or behaviour.
Entering the environment
Hexachlorobenzene can remain in the environment for a long time, half-life up to 7.5 years. It is only very slightly soluble in water, so most of it will remain in particles on the bottom of lakes and rivers. Hexachlorobenzene adsorbs strongly to soil. High levels can build up in fish, marine mammals, birds, lichens, and animals that eat lichens or fish. It can also build up in wheat, grasses, some vegetables, and other plants.
It has been estimated that about 50% of hexachlorobenzene will eventually end up in terrestrial soil; about 46.75% will end up in aquatic sediments; about 3% will end up in air; and about 0.25% will end up in water.
Where it ends up
Hexachlorobenzene has been detected in environmental samples from around the world, and is recognised as a global pollutant.
Breakdown in soil and groundwater: HCB is a highly persistent compound, with reported field half-lives in the soil environment up to 7.5 years. Evaporation is rapid while it is on soil surfaces, but considerably less so when it is mixed into the soil. HCB is moderately to strongly bound by most soils. Data from testing on hydro-soils indicate that it may be degraded both aerobically and anaerobically. It has low water solubility, and thus is likely to show low mobility in the soil environment. Due to its lengthy persistence, however, even low mobility may result in appreciable travel; therefore, HCB may pose some risk of groundwater contamination. Hexachlorobenzene has been found in well water at low concentrations, up to 5.6 ppb and only in a very small percentage of all of the wells tested.
Breakdown in water: HCB is of low water solubility, so it would most likely reach surface waters via surface run-off by attachment to soil particles. Once in the aquatic environment, it is likely to be short-lived; HCB underwent very rapid, almost complete (that is, less than 5 days) degradation to pentachlorophenol and related compounds in inoculated hydro-soil samples under both aerobic and anaerobic conditions.
Breakdown in vegetation: Breakdown in vegetation appears rapid, with residue levels in grass at approximately 1% of the initial amount after 15 days, and at approximately 0.01% after 19 months.
Breakdown in air: The dominant chemical loss process for gas phase hexachlorobenzene in the troposphere is by reaction with the hydroxyl (OH) radical. Based on this reaction, the atmospheric half-life is calculated to be about 1.4 years. Air to water exchange to (and from) the oceans, and gas-phase hexachlorobenzene and wet and dry deposition of particle-associated hexachlorobenzene, are important.
Environmental guidelines
Australian Water Quality Guidelines for Fresh and Marine Waters: (ANZECC, 1992):
Maximum of 0.007 micrograms/L (i.e. 0.000000007 g/L)
Key
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