Environmental Fate of Common Turf Pesticides
Most models are based on results of studies done in relatively
small geographic areas. Consequently, they are most useful in
areas for which they were developed. In other areas, different
factors and combinations affect the accuracy of the results. Most
important, the interpretation of results requires experience.
Despite considerable amounts of available information, the simple
question, "Will compound X, when I use it, contaminate
groundwater?" is difficult to answer. To illustrate the
point, a sophisticated model was chosen to suggest which of the
compounds that had been used in the past on several golf courses
on Cape Cod might have contaminated groundwater. The computer
selected dicamba and suggested that chlordane would remain in the
topsoil. Ironically, chlordane was found in water samples (GC/MS
analysis), and dicamba was not. This is an extreme example and is
not intended to discredit the use of models.
However, the user needs an answer, and there are situations where
quick answers are needed to assess the potential for groundwater
contamination. In these cases, good knowledge of local soil and
weather conditions (environmental factors) in combination with
some basic information about the chemicals involved
(compound-related factors) provides the basis for a quick and
reasonably good assessment of a field situation.
Two major groups of factors affect the leaching of pesticide
residues into the ground. The relatively large number of factors
may appear confusing at first, but when used with common sense
they can be useful and are not as complicated as they appear.
A. Compound-Related Factors
Taken together, these four factors provide an indication of
- Initial amounts
- Solubility in water
B. Environmental Factors
- Interception by leaves and thatch
- Topography (slope)
- Soil texture
- Soil organic matter
- Root density
- Soil moisture
- Groundwater table
- Size of treated area
Larger initial amounts of chemicals (e.g., > 2-3 lb/A active
ingredient) generally take more time to break down than smaller
amounts (e.g., < 2="" lb/a=""
a.i.).="" precipitation="" or=""
sprinkler="" irrigation="" may=""
wash="" more="" material=""
into="" the="" ground=""
after="" the="" application=""
of="" a="" heavy=""
dose="" than="" after=""
a="" light="" dose.=""
the="" smaller="" the=""
initial="" amounts,="" the=""
smaller="" the="" potential=""
Solubility in water.
As a rule of thumb, highly water-soluble chemicals leach faster
than the less-soluble ones. Chemicals with > 30 ppm solubility
may be considered mobile in sandy soil when their persistence is
high and adsorption low. Solubility of > 30 ppm alone does not
imply that the chemical will contaminate groundwater.
Chemicals persist in the ground for varying periods of time.
Persistence is reported as half-life, i.e., the time it takes for
about 50% of a given amount to break down.
In contrast to older pesticides (e.g., DDT, dieldrin, etc.), most
modern chemicals are moderately persistent to non-persistent.
Compounds with a half-life of > 3-4 months are considered
persistent, while compounds with a half-life of <
1="" month="" are=""
high="" or="" moderate=""
persistence="" alone="" does=""
not="" imply="" that=""
the="" chemical="" will=""
Chemicals are usually more persistent in dry, compacted, cold
soil than in moist, warm, well-aerated soil. Dry spells after an
application may extend the persistence of a chemical in the
Once residues have been leached beyond the root zone, their
persistence is greatly prolonged.
Depending on their composition, most modern chemicals are
moderately or strongly adsorbed on soil organic matter and clay.
Most of the commonly used turf pesticides and herbicides are
strongly adsorbed. Adsorbed chemicals do not move with the soil
water but remain adsorbed while the water moves towards the
Adsorbed compounds are gradually released back into the soil
solution where breakdown takes place. Adsorption is reported as
Kd or, more accurately, as Ko c, whereby Ko c <
300-500="" is="" considered=""
low.="" in="" case=""
of="" doubt,="" company=""
provide="" information="" on=""
the="" ko="" c="" of=""
compounds="" they="" sell.="">
Combinations of these four factors give rise to the elusive term
"potential mobility." We all are painfully aware of the
vagueness of the word "potential" and its political
value. The term is used correctly only in connection with
qualifying factors such as soil condition, precipitation,
groundwater table, etc. It states that under certain conditions
(e.g., sand soil, low soil organic matter [< 1%],=""
high="" precipitation,="" high=""
contamination="" is="" possible,=""
but="" by="" no=""
means="" certain.="" without=""
a="" qualifying="" statement,=""
Notice that chemicals listed as mobile do not necessarily
contaminate groundwater. Most of the modern compounds break down
before they have a chance to reach the groundwater table.
The potential mobility of a chemical is on the high side when all
of the following factors apply: solubility in water > 30 ppm,
soil half-life > 3 months, and low adsorption (Ko c <
300-500);="" it="" decreases=""
with="" every="" factor=""
that="" does="" not=""
Interactions between chemical and environmental factors affect a
chemical's mobility, and these interactions determine its
probability to reach groundwater.
Interception by leaves and thatch.
Leaves and thatch of a dense turf area may intercept up to 90% of
a pesticide application. Topdressing also intercepts chemical
residues. Unless the chemical is watered in, the initial amount
reaching the ground is small, which favors breakdown. Subsequent
moderate to heavy rainfalls, or the use of sprinklers several
days after an application, do wash residues off the leaves;
however, the residues reach the ground in small amounts which
break down faster than full dosages.
Sunlight may break down a chemical deposited on leaf surfaces
(e.g., triclopyr). This factor generally is of minor importance.
Precipitation up to several days after an application washes
residues off the leaves and moves them into the ground. This can
be significant for soluble chemicals (> 30 ppm) in sandy soil
containing small amounts of organic matter. The farther apart the
rainfall events and the less precipitation, the less the
potential for leaching.
The topography of an area may affect the distribution of a
chemical through surface runoff, provided the conditions are
favorable. Dry formulations as well as residues adsorbed on soil
particles are affected. Residues may accumulate in low spots,
thus increasing the residue load of an area. This can be
significant where the groundwater table is high (1-2 ft.).
Soil texture affects the movement of water as the carrier of the
chemical, and indirectly the adsorption of the chemical on soil
particles. Sandy soils retain less water and chemicals than clay
soils or organic soils. The heavier the soil, the lower the
potential for leaching.
Channels resulting from earthworm activity may facilitate the
vertical flow of water, which may favor leaching. However, the
significance of the amounts of residues leached in this fashion
under field conditions is questionable because the channels are
filled with organic material that intercepts residues. To
increase leaching significantly, the top 10-12 inches would have
to be occupied by a large volume of channels, which might render
an area unfit for recreational use.
Soil organic matter.
Soil organic matter adsorbs larger amounts of pesticide residues
than clay. Even 1-1.5% soil organic matter may retain
considerable amounts of residues. In addition, organic matter
serves as nutrient substrate for microorganisms active in the
breakdown of residues. The more organic matter there is, the more
adsorption and breakdown occur, and the likelihood of leaching is
The root zone is the most active part of the topsoil for the
breakdown of chemical residues due to aeration and activity of
microorganisms. Breakdown of residues beyond the root zone is
insignificant. The healthier and the denser the root system, the
more breakdown takes place and the lower is the potential for
Residues of herbicides may move inside treated plants into the
roots, where they may be released into the soil after the plants
have died. These residues do bypass thatch and most of the
organic matter in the topsoil, but the amounts are too small to
Soil moisture is essential for soil microorganisms active in the
breakdown of pesticide residues. Obviously, residues are more
persistent in dry than in moist soils.
All other factors being equal, the closer the groundwater table
to the soil surface, the greater the chance of contamination.
Size of treated area.
Spot treatment contributes less to potential contamination than
treatment of large areas.
Water is necessary to leach chemicals; however, chemicals very
rarely move at the same rate water does. Most chemicals move more
slowly and break down before they are leached past the root zone.
To estimate the potential for groundwater contamination, compare
potential mobility with the environmental factors mentioned above
and use common sense.
- The lower the potential mobility, the less leaching can be
- All factors favoring microbial activity favor breakdown of
These two basic rules give reasonably good indications for the
potential a chemical may have to contaminate groundwater in a
To help apply these two rules to actual field situations,
numerical values were assigned to selected compound-related and
environmental factors affecting leaching. The values applied to a
particular situation can be totaled to help evaluate the leaching
potential. In case of doubt, select a value between two values.
The closer the sum to eight, the less groundwater contamination
is probable. The closer the sum approaches 17, groundwater
contamination is more probable. These figures are suggested as an
aid in the first assessment of a situation; they are not to
replace predictive models. For several compound-related factors,
||> 30 ppm
||> 3 months
||strong (Ko c >
weak (Ko c <>
low (> 3-5 ft.)
||high (> 10
low (< 10="" in./="">
||8 = groundwater
17 = groundwater contamination probable
If the sum approaches 15 or 16, a more accurate assessment of the
situation is warranted.
Groundwater contamination with pesticide residues tends to occur
more often where chemicals are used year after year in relatively
large quantities, and where root systems are not coherent
(primarily agricultural uses).
Misinterpretation of information about pesticides gives rise to
preconceived ideas only time will correct. On the other hand,
interpretation of information is difficult because there are many
different shades of gray. Those who venture into that twilight
zone take the risk of being made responsible for failures. The
pesticide user, especially the one who depends on pesticides to
remain competitive, is caught in the middle. Therefore, this
article was written to help pesticide users decide whether the
use of a particular compound might result in groundwater
contamination or not. If groundwater contamination is probable,
the use of a model is recommended for documentation.
Unfortunately, neither estimates nor models protect from
liability if contamination does occur: The label is the law.
If the data compiled and the estimates in the column
"Potential Mobility" do not concur with similar data
published elsewhere, ask your chemical company representative.
The author thanks Dr. Richard Cooper, University of
Massachusetts, Department of Plant and Soil Sciences, and Mr. Ed
Nash, Superintendent, Bass River Golf Course, South Yarmouth,
Mass.. for their helpful comments.
Pesticide users make a precarious decision every time they apply
a chemical: Will residues of the chemical contaminate the
groundwater or not? To complicate the situation, there is little
information available to make such a decision and to guarantee
its accuracy. Manufacturers are reluctant to elaborate on this.
Scientists use predictive models to make educated guesses about
potential residue movement. Though models are based on defined
environmental factors rarely found in the same combination in the
field, they remain the best available tool to assess the
potential of a chemical for contamination of groundwater.