Report from the Sub-comittee on the environment and health

Volatilisation <strong>from</strong> soil
Volatilisation <strong>from</strong> plants
Measurements of
volatilisation in <strong>the</strong>
laboratory and <strong>the</strong> field
In <strong>the</strong> case of many pesticides, most of <strong>the</strong> pesticide that lands on <strong>the</strong>
ground can in <strong>the</strong>ory evaporate within a few days, depending on <strong>the</strong>
climatic conditions. Volatilisation increases with rising temperature and
wind velocity. If <strong>the</strong> relative humidity is low, <strong>the</strong> ground surface dries
out, which reduces volatilisation.
Little is known about volatilisation of pesticides <strong>from</strong> plants. High
relative humidity increases <strong>the</strong> volatilisation but can at <strong>the</strong> same time
increase <strong>the</strong> absorption of a pesticide by <strong>the</strong> plant. Once <strong>the</strong> pesticide has
penetrated <strong>the</strong> plant, 95% of it remains <strong>the</strong>re. The volatilisation depends
on <strong>the</strong> species of plant, <strong>the</strong> amount of foliage per unit of area and <strong>the</strong>
nature of <strong>the</strong> surfaces of <strong>the</strong> plant. The spraying technique and <strong>the</strong>
coformulants in <strong>the</strong> pesticide formulation also play a part. These
substances are intended to ensure that pesticides are spread and attach to
and penetrate <strong>the</strong> plant. The ancillary substances can thus help reduce
volatilisation and thus increase <strong>the</strong> substances action time in and on <strong>the</strong>
plant. Volatilisation is greatest <strong>from</strong> small droplets, which have <strong>the</strong>
relatively largest surface, and least <strong>from</strong> large droplets, partly because
absorption by <strong>the</strong> plant is greatest <strong>from</strong> large droplets. The Dutch
authorities have estimated that about 20% of a sprayed pesticide
evaporates <strong>from</strong> plant surfaces and is transported to <strong>the</strong> atmosphere
(Ministerie van L.N.V. 1991).
There have been many foreign studies of <strong>the</strong> volatilisation of pesticides.
Most of <strong>the</strong>m were carried out more than 10 years ago with pesticides
that are no longer used in Denmark. Jansma and Linders (1995) have
reviewed data <strong>from</strong> <strong>the</strong> literature. It appears <strong>from</strong> <strong>the</strong>ir work that both <strong>the</strong>
herbicide DNOC and <strong>the</strong> chlorinated insecticide lindane, which have
been detected in rainwater in Denmark (see section 4.1.5), evaporate very
quickly <strong>from</strong> plant surfaces. In German field tests with different crops, it
was found, for example, that 45% of lindane evaporated <strong>from</strong> green
beans within one hour of spraying, 50% <strong>from</strong> lettuce, 30% <strong>from</strong> kohlrabi
and 25% <strong>from</strong> spring wheat (Boehncke et al.1990). After 3.1 days, 88%
had evaporated <strong>from</strong> <strong>the</strong> spring wheat and more than 90% <strong>from</strong> <strong>the</strong> o<strong>the</strong>r
crops. In several studies, measurements of <strong>the</strong> volatilisation of lindane
<strong>from</strong> <strong>the</strong> ground showed a lower rate of volatilisation but still so much
that most of <strong>the</strong> sprayed pesticide was transported to <strong>the</strong> atmosphere.
Measurements exist of <strong>the</strong> volatilisation of individual substances
included in measurements of <strong>the</strong> content of pesticides in Danish
rainwater. The substances include atrazine and simazine.
For atrazine, measurements in field studies in <strong>the</strong> USA have shown that
up to 9% can evaporate <strong>from</strong> <strong>the</strong> ground surface in 35 days. In ano<strong>the</strong>r
study, 1.3% of simazine evaporated <strong>from</strong> <strong>the</strong> ground within 21 days.
Measurements of volatilisation <strong>from</strong> <strong>the</strong> ground of substances now in use
in Nor<strong>the</strong>rn Europe, showed up to 49% of chlorpyrifos after 26 days, up
to 52% of deltamethrin after 3.1 days and 90% of trifluralin after 2.5-7
days. Field tests in Germany of volatilisation of <strong>the</strong> insecticide
deltamethrin after 3.1 days showed 72% <strong>from</strong> green beans, 34% <strong>from</strong>
kohlrabi, 70% <strong>from</strong> lettuce and 24% <strong>from</strong> spring wheat (Boehncke et al.
1990). The same authors measured up to 100% volatilisation of
mevinphos <strong>from</strong> plants after 3.1 days.
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