Exposures of Aquatic Organisms to the Organophosphorus Insecticide, Chlorpyrifos Resulting from Use in the United States - Reviews of Environmental Contamination and Toxicology - page 107

Table 8 Characteristics of modeled 96-h time-weighted mean chlorpyrifos concentrations in
water draining the focus watersheds

Concentration
in water ( μ g L −1 )

Concentration in
sediment ( μ g kg −1 )

Half-life Half-life

Location of
watershed 28-d 96-d 28-d 96-d

California Minimum 0.003 0.013 4.74 10.8

Median 0.024 0.251 7.91 14.6

90th centile 1.319 1.543 10.8 20.8

Maximum 1.392 2.142 11.0 22.2

Georgia Minimum 0.004 0.004 0.007 0.007

Median 0.010 0.010 0.023 0.027

90th centile 0.020 0.020 0.050 0.060

Maximum 0.023 0.023 0.064 0.067

Michigan Minimum 0.002 0.007 0.027 0.010

Median 0.011 0.018 0.021 0.038

90th centile 0.018 0.034 0.032 0.060

Maximum 0.028 0.043 0.047 0.074

Values reflect the 90th centile of the annual maxima (1 in 10 yr maximum value)

Annual maximum concentrations . A probability analysis was conducted on the
daily time-series predictions of CPY concentrations in the water and sediment for
each watershed using the RADAR program (SI Appendix E). RADAR converts a
daily time series to an annual maximum series for various exposure durations that
are based on a rolling average calculation. The durations of exposure included the
instantaneous maximum, 96-h, 21-d, 60-d, and 90-d average concentrations.
Maximum daily concentrations predicted for the California, Georgia, and
Michigan watersheds were 3.2, 0.041, and 0.073 μg L −1 , respectively, with the 28-d
aerobic soil metabolism half-life and 4.5, 0.042, and 0.122 μg L −1 , respectively, with
the 96-d soil half-life. For sediments, the maximum daily concentrations predicted
for the California, Georgia, and Michigan watersheds were 11.2, 0.077, and 0.058
μg kg −1 , respectively, with the 28-d half-life and 22.8, 0.080, and 0.087 μg kg −1 ,
respectively, with the 96-d soil half-life. The 96-h time weighted mean concentra-
tions (TWMC), used in the risk assessment (Giddings et al. 2014 ) are summarized
in Table 8 . Concentrations associated with the minimum year, median year, 90th
centile year, and maximum year are presented. Annual maxima concentrations for
all years are provided in SI Appendix D.
The greatest annual concentrations of CPY in water and sediment were predicted
in the California watershed, using the 96-d aerobic soil metabolism half-life.
Predicted concentrations in water ranged from 0.003 to 1.39 μg L −1 for the 28-d
half-life simulations, increasing slightly to a range of 0.013-2.14 μg L −1 for the 96-d
half-life simulations. Similarly, CPY concentrations in sediment ranged from 4.74
to 11.0 μg kg −1 in simulations that used 28-d half-life, and 10.8-22.2 μg kg −1 in
simulations that used a half-life of 96-d. Predicted annual concentrations in the
Georgia and Michigan watersheds were less than those for California. The Georgia
watershed had a maximum 96-h TWMC of 0.023 μg L −1 in water for the 28-d and