Exposures of Aquatic Organisms to the Organophosphorus Insecticide, Chlorpyrifos Resulting from Use in the United States - Reviews of Environmental Contamination and Toxicology - page 92

Fig. 4 Wash-off fraction vs.
time after application

Properties of CPY in water . For the sensitivity analysis of patterns of use of CPY,
degradation in the EXAMS model was regarded to occur through aerobic aquatic
metabolism, aqueous hydrolysis, and photolysis. Aerobic aquatic metabolism was
represented by the upper 90th centile confidence bound on the mean half-life
(t ½ = 50.8 d) calculated from ( 1 ) by using the data in SI Table A5. Aqueous hydro-
lysis of CYP at pH 5, 7 and 9 has been reported with half-life values of 73, 81, and
16 d, respectively. The longer 81-d value was used for the modeling scenarios. The
half-life for aqueous photolysis was taken as 29.6 d in the simulations. Volatilization
was also simulated using the Henry's law constant of 6.2 × 10 −6 atm m 3 mol −1 . The
calibrated dimensionless value discussed above is not applicable for aquatic media.
Degradation in sediment was assumed to occur via anaerobic aquatic metabolism
(63-d half-life), which was calculated from ( 1 ) by using the reported laboratory
half-life values of 39 and 51 d.

3.3 Selection of Exposure Scenarios

Distribution of CPY across the U.S . Intensity of use was a factor in the selection of
watersheds. County sales data (Fig. 1 ) cannot be used to determine the precise loca-
tion of product use, but it does provide a general indication of the spatial distribution
of CPY use. Areas in the U.S. with the greatest intensity of use from 2010 to 2011
(depicted in blue in Fig. 1 , see SI for color map) include Kern, Tulare, Santa Cruz,
Fresno counties in central California; Lancaster County in southeastern Pennsylvania;
and Calhoun, Decatur, and Mitchell counties in southeast Georgia. Other areas of
high use at a lesser density are depicted by the green shading in Fig. 1 (see SI for
color map).

National vulnerability assessment of CPY runoff . A national vulnerability assess-
ment was conducted to characterize the potential for CPY to be transported beyond
a treated field in runoff water and eroded sediment throughout the conterminous
United States. The assessment involved use of the National Pesticide Tool (NPAT)
to simulate the relative runoff potential of CPY for all major agricultural soil types.