Page 183 - Soil and water contamination, 2nd edition

170 Soil and Water Contamination

9.5.1 Aliphatic chlorinated hydrocarbons
The most widely used chemicals from the class of aliphatic chlorinated hydrocarbons consist
of 1 or 2 carbons with one or more chlorine substituents. Examples include dichloromethane
(methylene chloride ), trichloromethane (chloroform), tetrachloromethane (carbon
tetrachloride or tetra), 1,2-dichloroethane (ethylene dichloride), 1,1,1-trichloroethane
(methyl chloroform), 1,1,1,2,2,2-hexachloroethane (perchloroethane), chloroethene (vinyl
chloride), 1,2,2-trichloroethene (TCE or tri), 1,1,2,2-tetrachloroethene (perchloroethylene,
PCE , or per). These compounds are used as organic solvents in the metal and electronic
industries and as dry cleaning liquids in laundries. They are non-flammable, colourless, very
volatile, organic liquids with a sweet odour. In the presence of ultraviolet light or excess heat
they decompose to form the poisonous gas phosgene (COCl ). Their half-lives in air are in
2
the order of a few days to a week. In surface water, the half-lives range from days to weeks.
In soil and groundwater they break down much more slowly because of the much slower
evaporation rate. All of the chemicals listed above are denser than water, and since their
aqueous solubility is limited, they behave like DNAPLs when spilled in substantial amounts.
Large concentrations of the above listed compounds may cause narcosis, lung
irritation, and damage to the liver and kidneys. Tetrachloromethane is one of the most
toxic chlorinated hydrocarbons. It can be absorbed through the skin; exposure to small
amounts can cause severe liver and kidney damage. Chloroform, ethylene dichloride,
perchloroethylene, and trichloroethene have been shown to cause liver cancer in rats
and mice and it is reasonably likely that they are carcinogens to humans as well. Methyl
chloroform is less toxic than other chlorinated hydrocarbons at low concentrations, but may
be lethal when inhaled at high concentrations (ATSDR, 2013).

9.5.2 Hexachlorocyclohexane (HCH )

The most common isomer of hexachlorocyclohexane (HCH ) is γ-HCH (also known as
lindane; see Figure 9.Ib). Lindane is a white solid substance that may volatilise as a colourless
vapour with a slightly musty odour. Lindane used to be used as an insecticide on fruit and
vegetable crops and forest crops. Its use was phased out in the developed countries during
the late 1970s, but it is still a widely used insecticide in Third World countries. The other
isomers of HCH do not have the insecticide property. During the production of lindane,
these isomers are separated and stored. HCH enters the environment due to application
or releases from lindane production facilities. In air, HCH can be present as a vapour or
attached to dust particles. Lindane can persist in air for up to 3 months and can be
transported long distances in the atmosphere. In soil, sediments, and water, microorganisms
break down HCH to less harmful substances. HCH isomers are broken down quickly in
water; lindane does not remain in water longer than 30 days. The HCH degradation in soil
is much slower, especially under aerobic conditions (Herbst and Van Esch, 1991). Under
anaerobic conditions, the degradation may proceed considerably faster. The β-isomer usually
predominates, because it is the most stable of the isomers. The HCH in soil is predominantly
fixed by organic matter in the top 35 cm of the soil.
Plants may take up considerable amounts of HCH from soil; the HCH concentration
in plant tissue is usually positively correlated with the HCH concentration in soil. The
relationships vary with plant species and soil type. In the various plant parts, the HCH
concentrations are higher in the roots and shoots than in stems, leaves, and fruits (Herbst
-1
and Van Esch, 1991). The LD50 value in rats is 90 mg kg and the LC50 value in rainbow
-1
trout is 0.06 mg kg (Alloway and Ayres, 1997).
The effects on human health of high exposures to lindane include muscular weakness,
dizziness, and nausea. Animals that were fed with high levels of HCH showed convulsions

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