Properties and Uses of Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 17

Fig. 3 Correlation of
half-life of chlorpyrifos with
pH of soil (data from Bidlack
1979 ; de Vette and
Schoonmade 2001 )

in this compartment. As CPY partitions into the adsorbed compartment, less is
available for degradation, and the rate of desorption, described by the rate constant
k des becomes the rate limiting step. This transition from k m to k des creates the biphasic
behavior in the model. Further details on the equations used the model set-up and
typical results are given in SI Appendix C.
The model results fit the data well (SI Appendix C; SI Table C-2) (Bidlack 1979 ).
The resulting rate constant represents the entire data set for each soil, optimized
simultaneously and represents a consistent model across all the soils considered.
This provides a better representation of the half-life than the values in the original
reports. As noted above, it is expected that the rate constants might be correlated
with the physical and chemical properties of the soils such as % organic matter, etc.
No significant correlation could be found among rate constants or half-lives with the
K OC , or water-holding capacity. It has been suggested that there might be a correla-
tion between the rate constant k m for degradation of CPY, and pH (Bidlack 1979 ).
This is expected, given the dependence of the abiotic hydrolysis on pH, which con-
tributes to this process, but the correlation is not simple. A graph of half-life vs. pH
is shown (Fig. 3 ). It is possible to consider the data in two groups; one group of soils
has half-lives >30 d, which were pH dependent; the other group had shorter half-lives
with a much weaker correlation to pH.
The correlations for the two groups in the range from pH 5 to 8 are given in ( 1 )
and ( 2 ).

( )

2

Group halflife 1 - =-´ 935 10 86 . . pH r = 076 . (1)

( )

2

Group 2 half life - =- ´ 267 30 14 . pH r = 092 . (2)

The mean half-life in the Group-1 was 17.6 d with a 90th centile of 25.9 d and
for Group-2 was 77.7 d with a 90th centile of 97.7 d. The greatest half-life among
the U.S. soils in each group was selected as a conservative value to represent the
group in simulations with the PRZM/EXAMS model runs used to characterize con-
centrations in surface waters (Williams et al. 2014 ). These values were 96 d from
the Stockton soil and 28 d from the Catlin soil (Table 3 ).