Pesticide Soil/Water Behavior
PESTICIDES AND THEIR BEHAVIOR IN SOIL AND WATER
P.S.C. Rao, R.S. Mansell, L.B. Baldwin, and M.F. Laurent
Florida Cooperative Extension Service
Institute of Food and Agricultural Sciences
University of Florida
Pesticides stand out as one of the major developments of the twentieth century.
During the past twenty years, however, concern has arisen as to the extent their
presence in the environment poses a threat to wildlife and mankind.
Certainly, pesticides have improved longevity and the quality of life, chiefly
in the area of public health. Insect control programs have saved millions of
lives by combatting diseases such as malaria, yellow fever and typhus. The use
of pesticides also constitutes an important aspect of modern agriculture, for
without chemicals to control various pests like insects, weeds, plant diseases,
worms and rodents, our food supply would decrease and prices would increase.
Florida's temperate to subtropical climate favors growth of many harmful
insects, weeds and diseases, thus making this state particularly dependent on
pesticides for economical crop management.
Unfortunately, pesticides are poisons and can be particularly dangerous when
misused. Fishkills, reproductive failure in birds, and acute illnesses in people
have all been attributed to exposure to or ingestion of pesticides -- usually as
a result of misapplication or careless disposal of unused pesticides and
pesticide containers. Pesticide losses from areas of application and
contamination of non-target sites such as surface and ground water represent a
monetary loss to the farmer as well as a threat to the environment. Thus careful
management of pesticides in order to avoid environmental contamination is
desired by both farmers and the general public.
The purpose of this fact sheet is to explain how pesticides can move from the
area in which they are applied, and to show how this information can be used,
along with other factors, to select the proper pesticide.
PATHWAYS OF PESTICIDE LOSS
There are basically two ways properly-applied pesticides may reach surface and
underground waters -- through runoff and leaching (Two other pathways of
pesticide loss are through removal in the harvested plant and by vaporization
(volatilization) into the atmosphere. Occurrence of pesticide residues in edible
parts of plants is significant in terms of human exposure, while pesticides
released into the atmosphere have an impact on air quality and create problems
when agricultural workers enter the treated areas. While these two pathways are
important, they will not be considered further in this factsheet, which is
devoted to pesticide behavior in soil and water). Runoff is the physical
transport of pollutants over the ground surface by rainwater which does not
penetrate the soil. Leaching is a process whereby pollutants are flushed through
the soil by rain or irrigation water as it moves downward. In many areas of
Florida soils are sandy and permeable and leaching is likely to be a more
serious problem than runoff. We now have technology to help estimate the
potential contamination of water from a given pesticide. To understand this
technology, it is necessary to know how a pesticide behaves in soil and water.
Once applied to cropland, a number of things may happen to a pesticide. It may
be taken up by plants or ingested by animals, insects, worms, or microorganisms
in the soil. It may move downward in the soil and either adhere to particles or
dissolve. The pesticide may vaporize and enter the atmosphere, or break down via
microbial and chemical pathways into other, less toxic compounds. Pesticides may
be leached out of the root zone by rain or irrigation water, or wash off the
surface of land. The fate of a pesticide applied to soil depends largely on two
of its properties: persistence and solubility.
PERSISTENCE
Persistence defines the "lasting-power" of a pesticide. Most pesticides break
down or "degrade" over time as a result of several chemical and micro-biological
reactions in soils. Sunlight breaks down some pesticides. Generally, chemical
pathways result in only partial deactivation of pesticides, whereas soil
microorganisms can completely break down many pesticides to carbon dioxide,
water and other inorganic constituents. Some pesticides produce intermediate
substances, called "metabolites" as they degrade. The biological activity of
these substances may also have environmental significance. Because populations
of microbes decrease rapidly below the root zone, pesticides leached beyond this
depth are less likely to be degraded. However, some pesticides will continue to
degrade by chemical reactions after they have left the root zone.
Degradation time is measured in "half-life." Each half-life unit measures the
amount of time it takes for one-half the original amount of a pesticide in soil
to be deactivated. Half-life is sometimes defined as the time required for half
the amount of applied pesticide to be completely degraded and released as carbon
dioxide. Usually, the half-life of a pesticide measured by the latter basis is
longer than that based on deactivation only. This is especially true if toxic or
nontoxic metabolites accumulate in the soil during the degradation. Table 1
groups some of the pesticides used in Florida by persistency, or length of
half-life, on the basis of their deactivation in soils.
Table 1: Grouping of pesticides based on persistence in soils.
Persistent
Nonpersistent Moderately Persistent (half-life
(half-life less (half-life greater than greater than
than 30 days) 30 days, less than 100) 100 days)
______________ ________________________ _________
Aldicarb Aldrin Heptachlor Bromacil
Captan Atrazine Linuron Chlordane
Dalapon Carbaryl Parathion Lindane
Dicamba Carbofuran Phorate Paraquat
Malathion Diazinon Simazine Picloram
Methyl parathion Endrin Terbacil Trifluralin
Oxamyl Fonofos TCA
2,4-D Glyphosate
2,4,5-T
SOLUBILITY AND SORPTION
Probably the single most important property influencing a pesticide's movement
with water is its solubility. Soil is a complex mixture of solids, liquids and
gases that provides the life support system for roots of growing plants and
microorganisms such as bacteria. When a pesticide enters soil, some of it will
stick to soil particles, particularly organic matter, through a process called
adsorption and some will dissolve and mix with the water between soil particles,
called "soil-water." As more water enters the soil through rain or irrigation,
the adsorbed pesticide molecules may be detached from soil particles through a
process called desorption. The solubility of a pesticide and its sorption on
soil are inversely related; that is, increased solubility results in less
sorption.
One of the most useful indices for quantifying pesticide adsorption on soils is
the "partition coefficient" (PC). The PC value is defined as the ratio of
pesticide concentration in the adsorbed-state (that is, bound to soil particles)
and the solution-phase (that is, dissolved in the soil-water). Thus, for a given
amount of pesticide applied, the smaller the PC value, the greater the
concentration of pesticide in solution. Pesticides with small PC values are more
likely to be leached compared to those with large PC values.
Partition coefficients of several chemicals are shown in Table 2. Note the wide
range of partition coefficients. Values of partition coefficients listed in
Table 2 are independent of soil type and are characteristic of each pesticide.
The partition coefficient is determined by a pesticide's chemical properties
such as solubility and melting point.
Table 2. Partition coefficients (PC) for selected pesticides
(generic name only)
Pesticide PC Pesticide PC
_________ __ _________ __
Aldicarb 10 Carbaryl 229
Chloramben 13 Monolinuron 237
Carbofuran 29 Prometone 300
2,4-D 32 Ametryn 380
Fenuron 34 Diuron 389
Terbacil 46 Prometryn 513
Propham 51 Trietazine 549
Bromacil 72 Chlorpropham 589
Monuron 135 Linuron 841
Simazine 158 Ipazine 1,161
Dichlobenil 164 Malathion 1,778
Atrazine 172 Chloroxuron 4,986
Fluometuron 174 Methyl parathion 7,079
Cynazine 190 Parathion 7,161
Propazine 207 Chloropyrifos 13,490
DDT 243,000
The partition coefficient makes it possible to put a value on a particular
pesticide's chance of being lost via runoff or leaching in a specific soil, via
the formula:
K = (PC)(%OM)(0.0058)
where K is an index for sorption of a given pesticide on a particular soil, % OM
is the percent of organic matter in the soil, as determined by chemical analysis
of the soil and where PC is the partition coefficient of the pesticide, as
listed in Table 2. Note that for pesticides that are not adsorbed on soil, PC is
equal to zero; hence, K = O. In most soils, inorganic ions such as nitrate and
chloride are not adsorbed by soils. Thus, pesticides with PC or K = O will leach
in a manner similar to nitrate or chloride.
ESTIMATING PESTICIDE LOSS
In evaluating the contamination potential of a particular pesticide, it is
essential to consider its partition coefficient and half-life jointly. For
example, a pesticide with a small PC, say less than 100, and a long half-life,
say more than 100 days, poses considerable threat to ground water through
leaching. On the other hand, a nonvolatile pesticide with a large PC, say 1000
or more, and a long half-life (e.g., more than 100 days) is likely to remain on
or near the surface of soil, increasing its chances of being carried to a lake
or stream in runoff. For pesticides with short half-lives, (less than 30 days),
the possibility of surface or ground water pollution depends primarily on
whether heavy rains or irrigations occur soon after application. Without water
to move, pesticides with short half-lives remain in the biologically active root
zone of soil and may degrade rapidly. In terms of water quality, pesticides with
intermediate PCs and short half-lives may be considered "safest." They are not
readily leached and degrade fairly rapidly.
From the foregoing discussion and Table 3, a qualitative assessment of a
pesticide's potential to pollute surface or ground water is possible.
Quantitative prediction of pesticide loss via runoff and leaching requires
complex computer models which utilize suite-specific soil, crop, and
climatological information. This would include the soil type, the date, amount
and method of application, and the amount, frequency and duration of rain or
irrigation following application.
Table 3. Combination of sorption and persistence of a pesticide for
determining its contamination potential
Partition Has potential
coefficient Pathway for
(PC) Half-life of loss contaminating
___________ _________ _______ ____________
small long leaching ground water
small short leaching ground water a
large long runoff surface water
large short runoff surface water a
a Only if heavy rains or irrigations occur soon after pesticide
application.
PESTICIDE SELECTION AND USE
Agricultural use of pesticides should be part of an overall pest management
strategy which includes biological controls, cultural methods, pest monitoring
and other applicable practices, referred to altogether as Integrated Pest
Management or IPM. When a pesticide is needed its selection should be based on
effectiveness, toxicity to non-target species, cost, and site characteristics,
as well as its solubility and persistence.
Half-lives and partition coefficients are particularly important when the
application site of a pesticide is near surface waters or is underlain with
permeable subsoil and a shallow aquifer. Short half-lives and intermediate to
large PC's are best in this situation.
Many areas of Florida have impermeable subsoils which impede deep leaching of
soluble pesticides. On such land, soluble pesticides with low PCs and
moderate-to-long half-lives require cautious application to prevent rapid
transport in drainage water to a nearby lake or stream. Nonerosive soils are
common to much of Florida and pesticides with large PCs remain on the
application site for a long time. However, the user should be cautious of
pesticides with long half-lives as they are likely to build up in the soil.
In addition to the pesticide solubility and soil permeability it is important
that the pesticide's toxicity to nontarget species be considered. Some of the
pesticides listed in Tables 1 and 2 have severely restricted use due to acute
toxicity or long half-life. An important purpose of the pesticide container's
label is to instruct users to apply the pesticide safely and with minimum threat
to nontarget species, both on and off the application site. Pesticide users
assume responsibility to follow label instructions. It is unsafe and unlawful
not to do so.