deposition by rain or snow, there is a scavenging of gas and aerosol phase pesticides
by the rain drops or snowflakes. In terms of primary sources, after pesticide
application on agriculture fields (rice, cotton, wineyard, etc.) by spraying, pyrethroids may reach surface aquatic environments through edge of field runoff, which
is an advective soil to water input of irrigation water or rain water, entraining
dissolved pyrethroids and also pesticides bound to particles or that have
re-partitioned to the run-off water. Storm events after pesticide treatment have
been shown to release high amount of pyrethroids into freshwater streams in the
vicinity of fields [37]. Despite degradation and dilution processes, pyrethroids
sorbed to river suspensions are effectively transported to the lower stretches of rivers
[18, 63]. Particle vertical settling and sediment resuspension are advective processes
transporting pyrethroids between water and sediment, which transport chemicals in
parallel to the water-sediment diffusive partitioning. Nevertheless, the latter may
only be effective for sediment pore water and benthic waters, while settling of
organic carbon-bound pyrethroids is an advective flux affecting all the water
column. Soils may act as transient repositories for pyrethroids that may gradually
be desorbed into irrigation or rain water by leaching. In addition, sorption to soils,
particle, and sediment may lower their degradability and thus increase their persistence in the environment [65]. Similarly to diffusive sediment-water exchange,
particle-water exchange (or partitioning) continuously occurs, with a distribution
of the chemical between organic carbon and the dissolved phase depending on
temperature and quality of the organic matter.
The key condition for pyrethroids to be transported away from their source is that
they persist long enough in the environment before being degraded. Their potential
for being transported is also dictated by their physicochemical properties. The
octanol-water partitioning coefficient, K OW , characterize the potential of compounds
for being absorbed into organic matter, either in sediments or in suspended particles.
Even though, conceptually, it does not take into account surface adsorption, it is a
common practice to use K OW as a surrogate for adsorption/absorption, as experimentally it is very difficult to discern organic pollutants adsorbed or absorbed to
particulate organic carbon. Henry’s law constant (H) or the dimensionless Henry’s
law constant (H
0
¼ K AW ¼H/RT) of a given pollutant characterizes its air-water
diffusive partitioning and thus its potential to accumulate in water or being volatilized to the atmosphere facilitating their long-range transport. Each pyrethroid has
specific values for these physicochemical constants. Figure 2 shows the phase space
for organic chemicals and compares the values of both constants for pyrethroids to
the values of these partitioning constants for other pollutant classes which behavior
in the environment is better studied and understood. The phase space shown in Fig. 2
provides a simplified view of environmental partitioning and transport potential.
Compounds in the upper area of the plot space have a higher potential to partition to
the gas phase relatively to water than compounds on the bottom area of the plot.
Similarly, compounds plotted on the right area of the plot have a greater potential to
partition to organic carbon relatively to water than those plotted on the left side.
Permethrin is plotted very close to PCB 101, thus have the similar partition characteristics than PCB101 and bifenthrin have an even higher K AW . Therefore, both
98
L. Méjanelle et al.
by the rain drops or snowflakes. In terms of primary sources, after pesticide
application on agriculture fields (rice, cotton, wineyard, etc.) by spraying, pyrethroids may reach surface aquatic environments through edge of field runoff, which
is an advective soil to water input of irrigation water or rain water, entraining
dissolved pyrethroids and also pesticides bound to particles or that have
re-partitioned to the run-off water. Storm events after pesticide treatment have
been shown to release high amount of pyrethroids into freshwater streams in the
vicinity of fields [37]. Despite degradation and dilution processes, pyrethroids
sorbed to river suspensions are effectively transported to the lower stretches of rivers
[18, 63]. Particle vertical settling and sediment resuspension are advective processes
transporting pyrethroids between water and sediment, which transport chemicals in
parallel to the water-sediment diffusive partitioning. Nevertheless, the latter may
only be effective for sediment pore water and benthic waters, while settling of
organic carbon-bound pyrethroids is an advective flux affecting all the water
column. Soils may act as transient repositories for pyrethroids that may gradually
be desorbed into irrigation or rain water by leaching. In addition, sorption to soils,
particle, and sediment may lower their degradability and thus increase their persistence in the environment [65]. Similarly to diffusive sediment-water exchange,
particle-water exchange (or partitioning) continuously occurs, with a distribution
of the chemical between organic carbon and the dissolved phase depending on
temperature and quality of the organic matter.
The key condition for pyrethroids to be transported away from their source is that
they persist long enough in the environment before being degraded. Their potential
for being transported is also dictated by their physicochemical properties. The
octanol-water partitioning coefficient, K OW , characterize the potential of compounds
for being absorbed into organic matter, either in sediments or in suspended particles.
Even though, conceptually, it does not take into account surface adsorption, it is a
common practice to use K OW as a surrogate for adsorption/absorption, as experimentally it is very difficult to discern organic pollutants adsorbed or absorbed to
particulate organic carbon. Henry’s law constant (H) or the dimensionless Henry’s
law constant (H
0
¼ K AW ¼H/RT) of a given pollutant characterizes its air-water
diffusive partitioning and thus its potential to accumulate in water or being volatilized to the atmosphere facilitating their long-range transport. Each pyrethroid has
specific values for these physicochemical constants. Figure 2 shows the phase space
for organic chemicals and compares the values of both constants for pyrethroids to
the values of these partitioning constants for other pollutant classes which behavior
in the environment is better studied and understood. The phase space shown in Fig. 2
provides a simplified view of environmental partitioning and transport potential.
Compounds in the upper area of the plot space have a higher potential to partition to
the gas phase relatively to water than compounds on the bottom area of the plot.
Similarly, compounds plotted on the right area of the plot have a greater potential to
partition to organic carbon relatively to water than those plotted on the left side.
Permethrin is plotted very close to PCB 101, thus have the similar partition characteristics than PCB101 and bifenthrin have an even higher K AW . Therefore, both
98
L. Méjanelle et al.