Environmental Engineering Reference
In-Depth Information
Large amounts of data are available on the environmental properties of CPY.
These data were summarized and key values were selected for modeling fate in the
environment. The vapor pressure of CPY is 1.73 × 10
−5
torr, solubility in water is
<1 mg L
−1
, and its log K
OW
is 5. The mean water-soil adsorption coeffi cient normal-
ized to fraction of organic carbon in the soil (K
OC
) of CPY is 8.2 × 10
3
mL g
−1
.
Negligible amounts enter plants via the roots, and CYP is not translocated in plants.
Chlorpyrifos has short to moderate persistence in the environment as a result of
several dissipation pathways that may proceed concurrently. Primary mechanisms
of dissipation include volatilization, photolysis, abiotic hydrolysis, and microbial
degradation. Under laboratory conditions, estimates of half-lives of CPY in soils
range from 2 to 1,575 d (N = 126), depending on properties of the soil and rate of
application. As with other pesticides in soil, dissipation of CPY is often biphasic
with an initial rapid dissipation followed by slower breakdown. Laboratory and fi eld
dissipation half-lives are often calculated by assuming 1st order kinetics, which
might over-estimate persistence and potential for runoff into surface waters. At rates
of application that were used historically for control of termites, the degradation rate
is slower than at rates used in agriculture. In agricultural soils under fi eld conditions,
half-lives are shorter (2-120 d, N = 58) than those measured in the laboratory. Half-
lives for hydrolysis in water are inversely related to pH, and range from 16 to 73 d.
CPY is an inhibitor of acetylcholinesterase (AChE) and is potentially toxic to
most animals.
In vivo
and in the environment, CPY is converted (activated) to chlor-
pyrifos oxon (CPYO), which is more reactive with AChE. Similar activation reac-
tions occur with other phosphorothioate insecticides. Co-exposure to other chemicals
can induce mixed-function oxidase enzymes responsible for activation. However,
concentrations required to induce this synergism are large and co-occur rarely. Thus,
this phenomenon is not an issue at environmentally relevant concentrations.
Timing of the use of CPY depends on occurrence of the pests it is used to control.
There is no predominant seasonal use of CPY, although there is a somewhat greater
usage in the winter for tree crops in California and greater use in summer for certain
fi eld crops (e.g., corn).
3
Fate of Chlorpyrifos and Its Oxon in the Atmosphere
and Long-Range Transport
The third paper in the series characterized the fate of CPY and CPYO in a number
of environmental compartments with a focus on transport through the atmosphere
(Mackay et al.
2014
). Detectable concentrations of CPY in air, rain, snow and other
environmental media have been measured in North America and other locations at
considerable distances from likely agricultural sources. Thus, there is a potential for
long-range transport (LRT) in the atmosphere. A simple mass balance model was
developed to quantify likely concentrations of CPY and CPYO at locations ranging
from local sites of application to more remote locations up to hundreds of km dis-
tant. The characteristic travel distance (CTD) is defi ned as the distance at which
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