Fate in the Environment and Long-Range Atmospheric Transport of the Organophosphorus Insecticide, Chlorpyrifos and Its Oxon - Reviews of Environmental Contamination and Toxicology - page 43

Reported half-lives of CPY in soils vary considerably, which has been attributed
to differences in soil organic carbon content, moisture, application rate and micro-
bial activity (Racke 1993 ). Less data were available for water and sediments. From
a critical review of the literature, the half-lives in Table 6 were selected. These are
considerably shorter than those predicted by the EPIWIN program and used by
Muir et al. ( 2004 ). Since these half-lives are uncertain, the selected values must be
regarded as tentative, although they are not critical to the determination of potential
for LRT because deposited CPY evaporates slowly. These half-lives are, however,
important for assessing the extent and duration of exposures in distant water, soil,
and sediment ecosystems.
Volatilization . For LRT in the atmosphere, one of the most important parameters is
the rate of volatilization from surfaces of leaves and soils. Drift is also important but
over shorter distances. The quantity of CPY entering the atmosphere following
application is a function of several variables, including the physical-chemical prop-
erties of the formulation, whether it is applied as a liquid or granular formulation,
the quantity applied, the area to which it is applied, the soil properties where applied,
meteorological conditions, spray composition and related parameters and the result-
ing losses by spray drift. The early period after spraying and particularly 24-48 h
after application is critical in determining the fraction of applied CPY that enters
the atmosphere and becomes subject to LRT (Racke 1993 ). Relatively fast initial
volatilization of applied CPY is observed in the first 12 h after application. The
initial loss rate is hypothesized to result directly from volatilization of the “neat”
formulated product. But, as the CPY sorbs to the substratum (e.g., foliage or soil),
it becomes subject to photolysis, and the rate of volatilization decreases as a func-
tion of time. Photolysis of the formulation occurs on the surface of leaves and soils
to form CPYO, which also volatilizes. These assertions are consistent with the
results of the study by Zivan ( 2011 ), who demonstrated substantial rates of photoly-
sis of CPY to CPYO on various surfaces. In the days subsequent to application,
CPY adsorbs more strongly to soil, penetrates more deeply into the soil matrix,
becomes less available for volatilization, and becomes subject to biological trans-
formation processes.
The model developed here uses illustrative numerical values of quantities applied
and characteristics of the environment to which it is applied. To simulate a desired
application, these parameters can be varied to explore the effects of rates and condi-
tions of application on volatilization. Results of pesticide dissipation studies that
immediately followed application have been complied and reviewed by several
authors (Majewski 1999 ; van Jaarsveld and van Pul 1999 ). Results of two experi-
mental field studies are particularly applicable to this LRT study. In the first study,
two techniques for direct flux measurement were applied to CPY and CPYO follow-
ing application of 0.98 kg CPY(a.i.) ha −1 to recently cut alfalfa in the Central Valley
of California (Rotondaro and Havens 2012 ). The Aerodynamic method gave a max-
imum flux of 0.657 μg m −2 s −1 (2,365 μg CPY m −2 h −1 ) which decreased to
0.002 μg CPY m −2 s −1 (7.2 μg CPY m −2 h −1 ) by 24 h. The Integrated Horizontal Flux
method gave a maximum flux of 0.221 μg CPY m −2 s −1 (797 μg CPY m −2 h −1 ), which