Fate in the Environment and Long-Range Atmospheric Transport of the Organophosphorus Insecticide, Chlorpyrifos and Its Oxon - Reviews of Environmental Contamination and Toxicology - page 62

concentrations and deposition rates. There is also a need to focus more on the
transformation products such as CPYO, but major uncertainties exist about the for-
mation rates and properties of transformation products which preclude full interpre-
tation of monitoring data and modeling. It is likely that any risks associated with
LRT are attributable more to CPYO than to CPY; however, the concentrations pre-
dicted in air and water are much smaller than toxicity values for either of these
compounds (Giddings et al. 2014 ) and risks are de minimis . The proposed model
can also be applied to gain an understanding of the likely effects of the various
parameters such as wind speed and temperature.

3 Fate in Water

The fate of CPY in water was extensively reviewed by Racke ( 1993 ), and data are
provided in (Solomon et al. 2013a ); key points are summarized here with a focus on
information that has become available since 1993. As discussed above, there are
significant differences between dissipation and degradation of CPY in water, but
earlier studies did not always distinguish between dissipation and degradation. In
the laboratory, and in the absence of modifiers such as methanol, reported half-lives
(DT 50deg ) for hydrolysis in distilled and natural waters ranged from 1.5 to 142 d (SI
Table 1 ) at pH values between 5 and 9 (Racke 1993 ), which are considered to rep-
resent realistic field values. The mean half-life of these values was 46 d and the
geometric mean was 29 d. At pH <5, reported half-lives were generally longer
(16-210 d) and at pH >9, shorter (0.1-10 d). The presence of copper (Cu ++ ) resulted
in shorter half-lives (<1 d), even at pH <5 (Racke 1993 ). In studies published since
2000, similar half-lives have been reported (SI Table 1 ). A DT 50deg of 40 d for CPY
was reported in distilled water but DT 50deg (120 to 40 d) varied in sterile natural
waters from rivers flowing into Chesapeake Bay. Concentration of Cu ++ was a major
driver of rate of hydrolysis, although other factors such as salinity were also identi-
fied (Liu et al. 2001 ). Concentrations of total suspended solids (TSS) greater than
10 mg L −1 resulted in lesser rates of hydrolysis of CPY, but dissolved organic carbon
did not affect the rate. In water, CPY has been shown to bind strongly with variable
strength and reversibility to Ca-saturated reference smectites but strongly and with
poor reversibility to Ca-saturated humic acid (from Aldrich) (Wu and Laird 2004 ).
The binding to suspended clays might explain the effect of TSS on hydrolysis rate
observed by Liu et al. Half-lives from the newer laboratory-studies ranged from 1.3
to 126 d with a mean and geometric mean of 23 and 13 d, respectively (SI Table 1 ).
The overall mean and geometric mean were 37 and 21 d, respectively (SI Table 1 ).
Under field conditions, it is difficult to separate degradation from dissipation and
the half-lives measured are normally based on the latter (DT 50dis ). A number of
reports have noted relatively rapid dissipation of CPY in microcosms. DT 50dis of
9.6-6.1 d in microcosms treated with 0.005-5 μg L −1 were reported in small
laboratory-based studies conducted in mesocosms in the Netherlands (Daam and
Van den Brink 2007 ). However, smaller DT 50dis values (<4 d) were reported for