Environmental Engineering Reference
In-Depth Information
conditions than in temperate climates, and for substances released to the
sewer, the nature of the treatment process (e.g., aerobic, anaerobic) may affect
which products are formed. Many of these routes for pesticides are described in
the compilations of Roberts (
1998
) and Roberts and Hutson (
1999
).
Comparison of persistence data from laboratory studies for pesticide degra-
dates and parent compounds in soils indicates that selected degradates from a
range of chemical classes are often more persistent than their corresponding
parent compound (Boxall et al.
2004b
). Biotic transformation processes gene-
rally result in the formation of degradates that are more polar and water
soluble than the parent compound. Hence, the resulting transport behaviour
of degradates may also be different from that of the parent compound.
A comparison of available data on the sorption potential of degradates and
their parents in soils (Boxall et al.
2004b
) indicates that around a third of the
degradates derived from a range of pesticide types have a K
oc
value at least an
order of magnitude lower than the corresponding parent compound. A small
proportion (3%) has a K
oc
value more than two orders of magnitude lower.
These substances might, therefore, be expected to be significantly more mobile
in the soils and sewage treatment plants (STPs) than the parent compound,
i.e., they are more likely to be released in effluents from treatment works, or to
be transported from soils to surface and groundwaters.
The concern is that, if the assessment process focuses only on the environ-
mental compartments of concern to the parent compound, the presence of
persistent degradates in other compartments might go unnoticed. The chal-
lenge is, therefore, to determine which degradates may be significantly more
mobile and which are going to be persistent.
Ecotoxicity of degradates
Data have been generated on the ecotoxicity of pesticide, veterinary medicine and
biocide degradates because of the requirement of regulatory schemes (e.g.,the EU
pesticide directive 91/414/EEC requires that all major degradates formed at
10%
of the applied parent compound are evaluated). Generally, these studies
have determined acute effects on organisms used in standard toxicity tests
(e.g., Daphnia, rainbow trout, earthworms) the reason being that, for the vast
majority of pesticidemetabolites, the acute package demonstrates no risks, so no
further testing is triggered. A few studies have, however, assessed sub-lethal and
longer term effects (e.g., Osano et al.
2002a
,
b
). The impacts (both acute and longer
term) of a few industrial substances have also been investigated, most notably
degradates of the non-ionic surfactants (nonylphenol mono- and diethoxylates,
nonylphenol carboxylates, nonylphenol ethoxycarboxylates and nonylphenol
itself ). These substances are believed to have oestrogenic activity due to their
ability to mimic the endogenous hormone 17
b
-estradiol ( Jobling et al.
1996
).
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