39
Properties of Pesticides and Contaminants
were correlated with measured emission rates (µg/m 2 ·hr) to develop correlations that could be used to estimate emissions for new application situations. The main driving force for volatilization from soil is vapor pressure,
but volatilization will be lessened by the soil adsorption coeffcient (Koc
[solubility in soil organic carbon]), water solubility (Sw), and depth of incorporation (d). For semivolatile pesticides, two relationships were derived
(Woodrow et al., 1997):
2
Ln flux = 28.355 + 1.6158 Ln R surf , r = 0.988
(3.16)
for surface-applied chemical, where R surf = VP/(Koc × Sw), and
2
Ln flux = 19.35 + 1.0533 Ln R , r = 0.93
(3.17)
inc
for soil-incorporated chemical, where R inc = (VP × AR)/(Koc × Sw × d). VP
is vapor pressure (Pa), Koc is soil adsorption coeffcient (mL/g), Sw is water
solubility (mg/L), AR is application rate (kg/ha), and d is depth of incorporation (cm). A similar relationship was derived for a series of soil fumigants
applied by injection and surface chemigation (Woodrow et al., 2011):
2
Ln flux = 3.598 + 0.940 Ln R, r = 0.994
(3.18)
where fux has the units µg/m 2 · sec and R = (VP × AR)/(Koc × Sw × d). In all
cases, feld measurement of fux was accomplished using techniques discussed below.
3.2.4 Volatilization Flux Methods
Since volatilization is a signifcant process in determining the fate of chemical compounds in the environment, several methods have been developed
for determining fux of chemicals, especially for pesticides applied to soil
(see Chapters 6 and 8). One simple method consists of laboratory soil columns treated with the chemicals of interest. The top of the columns can be
sealed with a chamber through which air is passed, transporting volatilized
chemical vapors into a trap for analysis. If the surface area (A, cm 2 ) of the
top of the soil column is known, along with air fow rate (Q, cm 3 /min) over
the surface of the soil and chemical vapor concentration (C, µg/cm 3 ) in the
exhaust air, fux (F, µg/cm 2 /min) can be calculated:
F = QC/A
(3.19)
Soil type, temperature, airfow rate, soil moisture content, chemical application rate, etc., can be varied to model feld environments. A similar feld
method is the use of closed chambers, with chemical vapor sampling ports,
placed over portions of treated felds (Woodrow and Seiber, 1991). Depending
Properties of Pesticides and Contaminants
were correlated with measured emission rates (µg/m 2 ·hr) to develop correlations that could be used to estimate emissions for new application situations. The main driving force for volatilization from soil is vapor pressure,
but volatilization will be lessened by the soil adsorption coeffcient (Koc
[solubility in soil organic carbon]), water solubility (Sw), and depth of incorporation (d). For semivolatile pesticides, two relationships were derived
(Woodrow et al., 1997):
2
Ln flux = 28.355 + 1.6158 Ln R surf , r = 0.988
(3.16)
for surface-applied chemical, where R surf = VP/(Koc × Sw), and
2
Ln flux = 19.35 + 1.0533 Ln R , r = 0.93
(3.17)
inc
for soil-incorporated chemical, where R inc = (VP × AR)/(Koc × Sw × d). VP
is vapor pressure (Pa), Koc is soil adsorption coeffcient (mL/g), Sw is water
solubility (mg/L), AR is application rate (kg/ha), and d is depth of incorporation (cm). A similar relationship was derived for a series of soil fumigants
applied by injection and surface chemigation (Woodrow et al., 2011):
2
Ln flux = 3.598 + 0.940 Ln R, r = 0.994
(3.18)
where fux has the units µg/m 2 · sec and R = (VP × AR)/(Koc × Sw × d). In all
cases, feld measurement of fux was accomplished using techniques discussed below.
3.2.4 Volatilization Flux Methods
Since volatilization is a signifcant process in determining the fate of chemical compounds in the environment, several methods have been developed
for determining fux of chemicals, especially for pesticides applied to soil
(see Chapters 6 and 8). One simple method consists of laboratory soil columns treated with the chemicals of interest. The top of the columns can be
sealed with a chamber through which air is passed, transporting volatilized
chemical vapors into a trap for analysis. If the surface area (A, cm 2 ) of the
top of the soil column is known, along with air fow rate (Q, cm 3 /min) over
the surface of the soil and chemical vapor concentration (C, µg/cm 3 ) in the
exhaust air, fux (F, µg/cm 2 /min) can be calculated:
F = QC/A
(3.19)
Soil type, temperature, airfow rate, soil moisture content, chemical application rate, etc., can be varied to model feld environments. A similar feld
method is the use of closed chambers, with chemical vapor sampling ports,
placed over portions of treated felds (Woodrow and Seiber, 1991). Depending