Exposures of Aquatic Organisms to the Organophosphorus Insecticide, Chlorpyrifos Resulting from Use in the United States - Reviews of Environmental Contamination and Toxicology - page 90

Table 5 Environmental properties used in model simulations
Property Value used in model simulations

Molecular weight 350.6 g mol −1

Water solubility 0.73 mg L −1

Vapor pressure 1.87 × 10 −5 torr

Henry's law constant
PRZM model 0.5 × 10 −5 (dimensionless)
EXAMS model 6.2 × 10 −6 atm m 3 mol −1
Soil adsorption / desorption, K OC 8,216 cm 3 g −1 (mean K OC from 37 data points)
Aerobic soil metabolism (t½) Run 1: 28.3 d a
Run 2: 96.3 d a
Aqueous hydrolysis (t½) pH 5: 73 d
pH 7: 81 d
pH 9: 16 d
Aqueous photolysis (t½) 29.6 d
Aerobic aquatic metabolism (t½) 50.8 d
Anaerobic aquatic metabolism (t½) 63 d
Foliar degradation (t½) 3.28 d
Foliar washoff (cm −1 ) 0.1
Plant uptake 0.0
a In the text, these numbers are rounded to whole numbers; the table values were used in the
modeling

by calibration of PRZM to achieve 10-15% loss of CPY in the first 2 d and 20-25%
maximum after several weeks, values comparable to that reported in Sect. 1.2 and
by Mackay et al. ( 2014 ). This fitted value is approximately two orders of magnitude
smaller than the dimensionless values for Henry's law constants cited by the U.S.
Department of Health and Health Services (USDHHS 1997 ) and by (Fendinger and
Glotfelty 1990 ; Glotfelty et al. 1987 ; Suntio et al. 1987 ). Other parameters required
for simulating volatilization include the diffusion coefficient of pesticide in air
(DAIR) and the enthalpy of vaporization (ENPY). Values of 4,188 cm 2 d −1 and
14.3 kcal mol −1 were obtained for these parameters from Carousel et al. ( 2005 ).

Soil - water partition coefficients . Soil-water partition coefficients (Kd) were calcu-
lated by multiplying the average K OC of 8,216 cm 3 g −1 times the organic carbon
content (percent) of the soil series used in a specific model simulation. The K oc
value was derived as the mean value from the 37 studies presented in SI Table A4.

Soil degradation of CPY . To bracket the expected environmental behavior, model
scenarios were evaluated with two soil degradation rates. As discussed in Sect. 1.2
and in Solomon et al. ( 2014 ), aerobic soil metabolism appears to be biphasic. The
28-d and 96-d half-lives represent conservative estimates for each phase. Rate con-
stants were not adjusted for individual soils (i.e., to account for effects of hydrolysis
on pH or photolysis).

Foliar processes . Degradation on foliar surfaces was represented by a half-life of
3.28 d. This half-life represents the upper 90th centile confidence bound on the