|  Physical
properties of pesticides
The physical properties
of a chemical are a measure of how the chemical interacts with the
environment and can help answer questions like, "Will this
pesticide dissolve in water and run off into streams when it rains?"
or "How long will it take for this pesticide to break down
in the environment?" or "Will this pesticide be likely
to drift through the air?" This page defines a number of physical
properties that affect how pesticides move through the environment
and guidance on how the numbers should be interpreted.
 Water
Solubility
Half-life
Soil
Adsorption Coefficient, Koc
Vapor
Pressure
Air
Pollution Potential
Octanol-Water
Partition Coefficient, Kow
Predicting
Water Contamination Potential
Data
Sources for Physical Property Data
Water
Solubility The
water solubility of a pesticide is a measure of how readily the
chemical will dissolve in water and is typically expressed as the
maximum amount of the pesticide that will dissolve in one liter
of water. Typical concentration units are mg per liter (mg/L) which
is approximately equal to parts per million (ppm) or micrograms
per liter (ug/L), which is approximately equal to parts per billion
(ppb). The larger this number is, the more water soluble the pesticide,
and the more readily the pesticide will be transported away from
the application site by stormwater or irrigation water runoff. The
California Department of Pesticide Regulation has determined that
pesticides with a water solubility greater than 3 mg/L have potential
to contaminate groundwater. In reality, pesticides with water
solubilities less than 3 mg/L have been found in groundwater, so
this is no guarantee (1).
For more about how California Department of Pesticide Regulation
uses these physical properties to regulate pesticides as groundwater
contaminants, see the California
Regulations page.
Reference:
- J. E. Barbash and E.A. Resek, Pesticides in Ground Water:
Distribution, Trends, and Governing Factors, Volume 2 in
the series Pesticides in the Hydrologic System, U.S.
Geological Survey, Ann Arbor Press (Chelsea, MI, 1996).
Half-Life
Half-life is defined as
the time (in days, weeks or years) required for half of the pesticide
present after an application to break down into degradation products.
This time is often expressed as a range (for example, 1-3 days,
2-4 years, etc.) because the rate of pesticide breakdown depends
on a variety of factors including temperature, soil pH, soil microbe
content and whether or not the pesticide is exposed to light, water
and oxygen. It is worth noting that many of the breakdown products
themselves are toxic and may have significant half-lives as well.
There are several different types of half-lives:
- Soil half-life: The amount of time required
for half of the pesticide to degrade in soil. This half-life is
governed by the types of soil organisms that are present that
can break down the pesticide, the soil type (e.g., sand, loam,
clay), pH, and temperature. The
California Department of Pesticide Regulation has determined that
pesticides with an aerobic soil half-life greater than 690 days
or an anaerobic soil half-life greater than 9 days have potential
to contaminate groundwater.
- Photolysis half-life: The amount of time required
for half of the pesticide to degrade from exposure to light. The
California Department of Pesticide Regulation has determined that
pesticides with a hydrolysis half-life greater than 14 days have
potential to contaminate groundwater.
- Hydrolysis half-life: The amount of time required
for half of the pesticide to degrade from reaction with water.
The California Department
of Pesticide Regulation has determined that pesticides with a
hydrolysis half-life greater than 14 days have potential to contaminate
groundwater.
For more about how California Department of Pesticide Regulation
uses these physical properties to regulate pesticides as groundwater
contaminants, see the California
Regulations page.
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Adsorption
Coefficient, Koc The
adsorption coefficient, Koc, is a measure of how strongly
a chemical adheres to soil in preference to remaining dissolved
in water. In more general terms, this parameter is often called
Kd, a distribution coefficient that provides a measure
of how a substance is distributed between any two different media--air/water,
water/soil, or two different immiscible solvents.
Koc is formally
defined as the ratio of the mass of pesticide adsorbed per unit
mass of soil to the mass of the pesticide remaining in solution
at equilibrium. Because it is a ratio of masses, the number is unitless.
The value is dependent on the type of soil and the soil pH, so it
is not uncommon to see a range of values reported in the literature.
Pesticides with high Koc
values are typically not very water soluble and will preferentially
adhere to soils rather than be dissolved in water. This means that
pesticides in this class are unlikely to be carried off-site in
runoff as dissolved substances; instead, they are transported on
sediment particles. For some example values consider DDT with a
Koc of 100,000 (adheres strongly to soil). Diazinon has
a Koc of 1,580 and is readily transported as the free
substance dissolved in water. The California Department of Pesticide
Regulation has determined that pesticides with a Koc
less than 1,900 have potential to contaminate groundwater.
For more about how California Department of Pesticide Regulation
uses these physical properties to regulate pesticides as groundwater
contaminants, see the California
Regulations page.
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Vapor
Pressure, Vp The
vapor pressure of a pesticide is a measure of how readily it will
evaporate. In the PesticideInfo database, we report vapor pressures
measured between 20-25 degrees C, an important distinction
since the vapor pressure of a chemical increases with increasing
temperature. The vapor pressure is a good predictor of the volatility
of the chemical and allows determination of which pesticides might
be prone to evaporate from leaf and soil surfaces off-site after
application.
Formally, vapor pressure
is defined as the pressure exerted by the pure substance in a closed
system at equilibrium. Vapor pressure varies with temperature, increasing
as the temperature increases and decreasing as the temperature decreases.
Vapor pressures are expressed
using a variety of units, including pascals (Pa), millimeters of
mercury (mm Hg, equivalent to Torr, named after the Italian chemist
Torricelli who invented the barometer), pounds per square inch (psi),
and atmospheres (atm). If you plan to compare vapor pressures, be
sure they are all in the same units before proceeding. Some conversion
factors are given below:
1 Pa = 1 kg/m-s2
1 Pa = 7.5 x 10-3 mm Hg (Torr)
1 kPa (kiloPascal) = 1000 Pa = 7.5 mm Hg (Torr)
1 mPa (milliPascal) = 0.001 Pa = 7.5 x 10-6 mm Hg (Torr)
1 atm = 101.325 kPa (kiloPascal)
1 atm = 14.70 psi
1 atm = 760 mm Hg (Torr) Typically pesticides with vapor
pressures greater than 10-6 mm Hg can readily volatilize
and drift away from the application site.
Vapor pressure data were
obtained from one of the following sources:
- The
ARS Pesticide Properties Database, US Department of Agriculture,
Agricultural Research Services, Viewed on 7/27/06.
- Physical Properties
Database, California
Department of Pesticide Regulation, available on request from
J. Stutz at DPR.
- Materials Safety Data
Sheets for the pure compounds.
- US
EPA Fact Sheets
and Reregistration
Evaluation Decision Documents, US EPA Office of Pesticide
Programs, Viewed on 7/27/06.
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Air
Pollution Potential PANNA
has defined an air pollution potential rating for a subset of pesticides
in the PesticideInfo Database that gives a measure of the likelihood
that the pesticide will be found in air after an application.This
rating is based on the numeric value of the vapor pressure of the
pesticide and/or its breakdown product. In general, pesticides with
high vapor pressures are most likely to become airborne after an
application and consequently are more of an inhalation exposure
hazard than those with low vapor pressures. However, pesticides
with low vapor pressures can still become airborne through spray
or dust drift during an application or through transport on dust
particles.
The following scheme was
used to classify pesticides for their air pollution potential:
| Air Pollution Potential |
Vapor Pressure (mm Hg) |
| Very high |
> 10-2 |
| High |
10-2- 10-4 |
| Moderate |
10-4- 10-6 |
| Low |
10-6- 10-8 |
| Very Low |
< 10-8 |
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Octanol-Water
Partition Coefficient, Kow The
octanol-water partition coefficient, Kow, is a measure
of how a chemical will distribute between two immiscible solvents--water
(a polar solvent) and octanol (a relatively non-polar solvent).
The Kow value provides information on the polarity of
the pesticide and is often used as a model for how the pesticide
may be distributed in body tissues, e.g. blood vs. fat tissue. Pesticides
with a long half-life and high Kow have been shown to
bioaccumulate in the food chain.
In more general terms, this
parameter is often called Kd, a distribution coefficient
that provides a measure of how a substance is distributed between
any two different media--air/water, water/soil, or two different
immiscible solvents.
Kow is formally
defined as the ratio of the concentration of pesticide in the octanol
layer to the concentration of the pesticide dissolved in the water
layer. Because it is a ratio of concentrations, the number is unitless.
The value is dependent on temperature.
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Predicting
Water Contamination Potential California
Department of Pesticide Regulation has developed a method to estimate
water contamination potential based on the water
solubility, half-life, and Koc
of pesticide chemicals. This method is described in detail on the
California Regulatory
Information page.
Other factors also play
a role in determining whether a pesticide is likely to contaminate
water, in particular the total amount used, the setting in which
it is applied, and the product formulation.
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Data
Sources for Physical Property Data Physical property
information was obtained from the California Dept. of Pesticide
Regulation, which collects this information as part of the registration
process. For pesticides evaluated through DPR's Groundwater Protection
Program, we use the Specific Numerical Values published in the DPR
Well Inventory reports. For more detail on Specific Numerical Values
and links to the Well Inventory Reports, see the California
Regulatory Information page.
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Last
updated
July 27, 2006
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