PARAQUAT

pigs have been accidentally or intentionally
poisoned by paraquat (Philbey et al 2001).
• In the UK there have been numerous
poisonings; for example in 2001 there were
6 intentional poisonings of dogs and one of
a cat (Barnett et al 2002); and in 2006 dogs,
hares, a cat, and a fox were affected (Barnett
et al 2007). There were two incidents, in 1976
and 1990, in which 70-80 hares were killed
following the spraying of paraquat on grass
(ERMANZ undated).
5. Environmental Fate
5.1 Soil
Paraquat is very persistent in soil (US EPA 2009).
It binds readily to both clay and organic matter,
with adsorption increasing with clay content. The
soil K oc (sorption coeffi cient) ranges from 8,400
to 40,000,000 (EC 2003).
Paraquat is assumed to be strongly adsorbed to
clay particles, however the US EPA (2009) notes
that “the potential for desorption does exist”. In
Thailand, 5.83% desorption was found in sandy
loam soils (only 0.17% in clay soils) (Amondham
et al 2006). A trial using vineyard soils in Spain
found 70-90% of paraquat was adsorbed, but
11% was desorbed again (Pateiro-Moure et al
2010).
Adsorption increases with increasing pH, and
decreases with increasing acidity (Muhamad et
al 2010).
In highly organic soils, adsorption is weaker
and paraquat remains herbicidally active for
longer, up to 29 days in one trial on soils with
98% organic matter (IPCS 1984). Certain clay
minerals also adsorb paraquat less strongly.
For example in kaolinite clay the paraquat
slowly became available to plant roots and killed
cucumber seedlings, whereas in soil with 1%
montmorillonite it was not available. At the same
time adsorption of paraquat onto clay minerals
affects their capacity for holding water and
nutrients (Weber & Scott 1966).
The strong adherence to soil limits the availability
of paraquat to plants or other organisms; hence
it is very slowly biodegraded. According to
the US EPA (2009), it is resistant to microbial
degradation under both aerobic and anaerobic
conditions: no microbial degradation was seen
after 180 days of aerobic incubation or after 60
days of anaerobic incubation following a 30 day
aerobic incubation. However other authors report
that paraquat can be signifi cantly degraded by
bacteria, fungi, actinomyces and yeast, using
the paraquat as a nitrogen source (Amondham
et al 2006).
Paraquat does not photodegrade, even when
exposed to natural sunlight for 85 weeks,
according to the US EPA (1997). However
Amondham et al (2006) claim that it does, and
Eisler (1990) states that 50% of paraquat in
the surface soils photodecomposes in 3 weeks
whilst that in subsurface soils does not.
Field studies have found a half-life (DT 50 ) of 7-8
years in the UK and 10-20 years in the USA.
DT 90 values (i.e. 90% degradation) were never
reached. Monitoring for residues in the soil in
Europe found residues of between