Environmental Engineering Reference
In-Depth Information
dissipation of all pesticides within soil (Lalah, Kaigwara, Getenga et al. 2001). The behaviour and
fate of carbofuran in tropical soils is also further outlined in Chapter 3 (Kenya).
Carbofuran is relatively soluble in water and so has the potential to contaminate a variety of
aquatic resources, including groundwater. Surface water may be compromised via improper dis-
posal, accidental spillages and direct contamination. The latter is most likely when sprays are being
applied but will also occur via run-off of surface and drainage water from fi elds where crops or
soil are treated (Helmut 1990). Field fl ooding following adverse weather or as an agricultural prac-
tice has also resulted in carbofuran-related mortality of non-target organisms such as birds. Various
studies conducted by the Canadian Wildlife Service (among others) are extensively reviewed by
P. Mineau and colleagues in Chapter 8. These indicate that this problem is severe in heavy acidic
soils where carbofuran is known to have a much longer half-life (Mineau 2009).
1.4.1 Carbofuran precursors, metabolism and degradation products
In addition to the pesticide applied, it is essential to monitor its active metabolites and degra-
dation products (Fodor-Csorba 1998). Firstly, because degradation products may indicate an
application has occurred, but secondly, because such products may themselves be highly ecotoxico-
logically relevant. In fl ooded and non-fl ooded soils, carbofuran metabolises to carbofuran phenol,
3-hydroxycarbofuran and 3-ketocarbofuran (the three principle metabolites), and to 3-ketocarbofuran
phenol and 3-hydroxycarbofuran phenol (as shown in Figure 1.9). Given that the carbamate
group is involved in the inhibition of cholinesterase, the metabolites which retain this group (i.e.,
3-hydroxycarbofuran and 3-ketocarbofuran) are likely to be just as toxic as carbofuran itself. The vari-
ous phenol derivatives, which have lost the carbamate group, are consequently not as toxic, if toxic at all.
However, identifying the presence of carbofuran (or its metabolites) in a sample must be con-
sidered in light of the presence or absence of other compounds whose degradation products may
include carbofuran (and/or its metabolites). For example, carbosulfan (see Figures 1.2 and 1.10) is
another carbamate insecticide which has the same core structure as carbofuran, namely hydroxy-
benzofuran (the same metabolite can be formed from either of these carbamates). As previously
mentioned, the core structure is generally considered to be non-toxic, but the carbamate group
is reactive. When the nitrogen-sulfur bond of carbosulfan is broken, carbofuran is formed. The
liberated dibutylaminothio group on the carbosulfan (circled in Figure 1.10) is called a
pro-
group,
meaning that it can liberate the parent compound (carbofuran) by oxidation
in vivo
.
Since such reactions can occur, analyses for carbofuran and/or its metabolites could test positive
even if the actual products applied contained carbosulfan or other structurally similar compounds
such as benfuracarb (Figure 1.3) and furathiocarb (Figure 1.4) as the active ingredient. When the use
of all such structurally similar compounds is illegal, and the principle reason for analysis is simply to
ascertain whether or not poisoning was the cause of death, this is obviously less of an issue. However,
when the use of several compounds is permitted, or one or several are known to be used illegally,
specifi c identifi cation/implication of a compound may prove very important from a legal perspective.
Interestingly, despite the similarity in their chemical structures, carbosulfan (widely known under
the FMC trade/brand name Marshal), differs from carbofuran in terms of its physical properties.
Carbosulfan is not as soluble in water and has a lower vapour pressure (approximately 3000 times
less, and 57% lower (both at 25°C) respectively). Consequently, carbosulfan is actually less prone
than carbofuran to wash off or evaporate from foliar surfaces, and as such it is actually more effec-
tive against soil dwelling insects and nematodes (www.pesticidemanual.com/).
One way to differentiate (analytically) between carbofuran and other structurally similar carba-
mates might be to add a nontoxic marker at manufacture. Since this implies an extra manufacturing
cost, this is perhaps unlikely to happen, but it certainly could be considered. When working with
carbamates and their fate, behaviour and effects in the environment, and where information suggests
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