Report from the Sub-comittee on the environment and health

52
atrazine and <strong>the</strong> three primary metabolites are shown in table 4.19.
Deethylatrazine and desisopropylatrazine are less adsorbable than
atrazine.
Table 4.19
Water solubility and adsorption constants (Kd) for atrazine and primary atrazine metabolites. Atrazine
is banned in Denmark.
Water sol.
mM (a)
Kd in soil with
14% organic
matter (b)
Kd in soil with 4
% organic matter
(b)
Kd in soil with 2,3
% organic matter
(b)
Kd in soil with 1,0
% organic matter
(b)
Atrazine 0.15 14 2.6 1.5 0.44
Deethylatrazine 2.0 6.5 0.96 0.58 0.24
Desisopropylatrazine 1.2 8.6 1.2 0.73 0.41
Hydroxyatrazine 0.24 82 4.1 3.7 1.7
(a): Erickson, Lee 1989
(b): Brouwer et al. 1990
Relationship between
degradation and
adsorption, transport,
spraying time and locality
DEPA has published a list of pesticides currently used in Denmark and
<strong>the</strong> associated metabolites (DEPA 1997). Selected examples <strong>from</strong> this
list are shown in table 4.20. It is just as important to determine <strong>the</strong>
persistence of metabolites as <strong>the</strong> persistence of <strong>the</strong> pesticides <strong>the</strong>mselves
because, as mentioned, <strong>the</strong> metabolites are in some cases more toxic than
<strong>the</strong> pesticides <strong>from</strong> which <strong>the</strong>y are formed.
Table 4.20
Pesticides with associated metabolites. Selected <strong>from</strong> DEPA (1997).
Active ingredient Metabolites
dichlorprop 2,4-dichlorophenol
3,5-dichlorocatechol
glyphosate aminomethylphosphonic acid (AMPA)
formaldehyde
glycine
methylphosphonic acid
sarcosin (N-methylglycine)
mancozeb ethylene thiouram disulphide
ethylene thiouram monosulphide
ethylene thiourea
terbuthylazine 2-chloro-4-ethylamino-6-amino-1,3,5-triazine
Most pesticides that are strongly bound in soil degrade slowly. This is
because <strong>the</strong> adsorption reduces <strong>the</strong> contact between <strong>the</strong> pesticides and
<strong>the</strong> microorganisms carrying out <strong>the</strong> degradation. The pesticides can be
released again (desorbed). The rates of adsorption and desorption play an
important role in leaching. The greater <strong>the</strong> adsorption and <strong>the</strong> slower <strong>the</strong>
desorption, <strong>the</strong> lower <strong>the</strong> probability that <strong>the</strong> substances will be leached
through <strong>the</strong> soil to <strong>the</strong> groundwater.
The presence of organic matter, on <strong>the</strong> o<strong>the</strong>r hand, can increase <strong>the</strong> rate
of degradation if <strong>the</strong> organic matter supplies nutrition and thus growth
for <strong>the</strong> microorganisms that break down <strong>the</strong> pesticide. This has been
demonstrated by Mueller et al. (1992) for fluometuron, by Veeh et al.
(1996) for 2,4-D and by Walker et al. (1983) for simazine. The amount of
organic matter in <strong>the</strong> soil at <strong>the</strong> individual locality thus affects <strong>the</strong> rate of
degradation, although it is not known exactly how.