Report from the Sub-comittee on the environment and health

Calculation of <strong>the</strong>
volatilisation <strong>from</strong> <strong>the</strong>
surface of <strong>the</strong> ground
48
The simplest model for calculating volatilisation is a so-called 1st order
model. In <strong>the</strong> “Dow model” it is assumed that <strong>the</strong> so-called rate constant
for volatilisation is directly proportional to <strong>the</strong> vapour pressure and
inversely proportional to <strong>the</strong> water solubility and <strong>the</strong> soil-adsorption
constant (Jansma, Linders 1995). This model is used in <strong>the</strong> EU´s PCbased
expert system for assessment of chemical substances, EUSES (EC
1996). The volatilisation according to this model is shown in table 4.18
for substances with high volatilisation that are also widely used.
However, <strong>the</strong> model overestimates <strong>the</strong> volatilisation. Ano<strong>the</strong>r model,
developed recently in <strong>the</strong> Ne<strong>the</strong>rlands, includes <strong>the</strong> substances’ fate in
<strong>the</strong> atmosphere, deposition and effects on plants. The results indicate
that, with <strong>the</strong> pattern of use in <strong>the</strong> Ne<strong>the</strong>rlands in <strong>the</strong> period 1985-1995,
in all 5.5% of <strong>the</strong> amount of herbicides applied evaporated. The
atmospheric deposition corresponded to an average treatment frequency
index on nature areas of 0.02 per year (Klepper et al. 1998).
Table 4.18
Calculated volatilisation of pesticides with potential volatilisation <strong>from</strong>
ground surfaces (Jansma, Linders 1995; Løkke 1998).
Pesticide Quantity sold Percentage
volatilisation after:
in 1997 (1000 kg) 1 day 4 days
Chlorothalonil 39 1 6
Bentazone 79 7 24
Cypermethrin m
2 1 5
Dimethoate 35 1 6
Diuron 23 3 11
Fenpropimorph 278 11 38
Flamprop-M-isopropyl 13 1 6
Pendimethalin 358 99< 99<
Permethrin 2 31 77
Propyzamide 22 57 96
Prosulfocarb 75 26 69
Terbuthylazine 63 8 27
However, <strong>the</strong>se calculations also showed that <strong>the</strong>re is very little
volatilisation <strong>from</strong> ground surfaces of most of <strong>the</strong> pesticides used in
Denmark today. There is apparently considerable volatilisation of
fenpropimorph, pendimethalin, permethrin, propyzamide and
prosulfocarb. There is thus a risk of <strong>the</strong>se substances being spread in <strong>the</strong>
atmosphere and detected in rainwater and surface water. The fate and
occurrence of <strong>the</strong>se substances in <strong>the</strong> atmosphere over Denmark have not
been investigated and are not covered by DMU’s analytical programme
for rainwater (Løkke 1998).
4.6.6 Conclusions
Pesticides evaporate both during and, especially, after spraying. Toge<strong>the</strong>r
with drift, see section 4.6.2, and occasionally wind erosion of soil treated
with pesticides, volatilisation transports relatively large quantities of
pesticides to <strong>the</strong> atmosphere and is thus, quantitatively, <strong>the</strong> principal
transport path for pesticides away <strong>from</strong> <strong>the</strong> sprayed area. The
volatilisation depends on <strong>the</strong> properties of <strong>the</strong> substance and, particularly,
on its vapour pressure. The temperature, water solubility and adsorption