47
Properties of Pesticides and Contaminants
moderate vapor pressure (13 mPa), but its water solubility is comparatively
high (~6 g/L). Primarily because of this, measurable residues of aldicarb have
been found in groundwater. The soil fumigants ethylene dibromide (EDB)
and MeBr have comparable K oc values (44 and 56, respectively) and similar
water solubilities (4 and 17 g/L, respectively). But, the vapor pressure of MeBr
(~200 kPa) is over 100 times greater than that for EDB (~1.56 kPa). Measurable
residues of EDB have been found in groundwater, but not MeBr.
3.4.1.3 Distribution between Soil/Sediment and Vapor
Sorption of chemical vapor by soil can be described by the Freundlich isotherm equation, which takes the form
x/m = K*P
1/n
(3.30)
or
(
(3.31)
log x/m) = log K* + 1/n logP
where P is the chemical’s partial pressure. This isotherm adequately describes
adsorption at low P, typical of chemical contaminants in an air basin or airborne
pesticide residues near treated felds. Chemicals in air can be transported to
the soil surface by diffusion, by a process following Fick’s frst law, and advection (mass transport). Diffusion is defned as the movement of chemical along
a concentration gradient from higher to lower concentration. Movement to the
soil is measured as fux, where fux is governed by the chemical’s diffusion
coeffcient (D [cm 2 /sec]) and the concentration gradient (dc/dx):
Flux = − D dc/dx
(3.32)
where fux has the units mass/soil area/time. If the chemical in air has a
moderate vapor pressure and a relatively high K oc value, the soil will become
a sink for the chemical creating a steep concentration gradient. Flux can
occur out of the soil as well. Examples include not only gaseous products of
microbial metabolism (e.g., NO x and H 2 S) but also the soil fumigants that are
used in agriculture to control soil pests. Fumigants are characterized by high
vapor pressures, high diffusion coeffcients, and relatively low K oc values.
These properties allow fumigants to permeate soil for pest control, but they
also drive the fumigants along a steep concentration gradient to the soil surface, where they move away by diffusion and advection.
3.4.1.4 Distribution between Soil/Sediment and Fog/Rainwater
This is a special case for adsorption/absorption equilibria involving all of the
above—water, vapor, and suspended particulates (i.e., soil and dust). An example
Properties of Pesticides and Contaminants
moderate vapor pressure (13 mPa), but its water solubility is comparatively
high (~6 g/L). Primarily because of this, measurable residues of aldicarb have
been found in groundwater. The soil fumigants ethylene dibromide (EDB)
and MeBr have comparable K oc values (44 and 56, respectively) and similar
water solubilities (4 and 17 g/L, respectively). But, the vapor pressure of MeBr
(~200 kPa) is over 100 times greater than that for EDB (~1.56 kPa). Measurable
residues of EDB have been found in groundwater, but not MeBr.
3.4.1.3 Distribution between Soil/Sediment and Vapor
Sorption of chemical vapor by soil can be described by the Freundlich isotherm equation, which takes the form
x/m = K*P
1/n
(3.30)
or
(
(3.31)
log x/m) = log K* + 1/n logP
where P is the chemical’s partial pressure. This isotherm adequately describes
adsorption at low P, typical of chemical contaminants in an air basin or airborne
pesticide residues near treated felds. Chemicals in air can be transported to
the soil surface by diffusion, by a process following Fick’s frst law, and advection (mass transport). Diffusion is defned as the movement of chemical along
a concentration gradient from higher to lower concentration. Movement to the
soil is measured as fux, where fux is governed by the chemical’s diffusion
coeffcient (D [cm 2 /sec]) and the concentration gradient (dc/dx):
Flux = − D dc/dx
(3.32)
where fux has the units mass/soil area/time. If the chemical in air has a
moderate vapor pressure and a relatively high K oc value, the soil will become
a sink for the chemical creating a steep concentration gradient. Flux can
occur out of the soil as well. Examples include not only gaseous products of
microbial metabolism (e.g., NO x and H 2 S) but also the soil fumigants that are
used in agriculture to control soil pests. Fumigants are characterized by high
vapor pressures, high diffusion coeffcients, and relatively low K oc values.
These properties allow fumigants to permeate soil for pest control, but they
also drive the fumigants along a steep concentration gradient to the soil surface, where they move away by diffusion and advection.
3.4.1.4 Distribution between Soil/Sediment and Fog/Rainwater
This is a special case for adsorption/absorption equilibria involving all of the
above—water, vapor, and suspended particulates (i.e., soil and dust). An example