Environmental Fate of Common Turf Pesticides

Factors Leading to Leaching

By Dr. Karl H. Deubert
University of Massachusetts, Cranberry Experiment Station, East Wareham, Massachusetts
Reprinted from the USGA Green Section Record
1990 July/August Vol 28(4): 5-8

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.

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.



Factors Affecting Leaching

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
  1. Initial amounts
  2. Solubility in water
  3. Persistence
  4. Adsorption
Taken together, these four factors provide an indication of "Potential Mobility."

B. Environmental Factors
  1. Interception by leaves and thatch
  2. Photodegradation
  3. Precipitation
  4. Topography (slope)
  5. Soil texture
  6. Soil organic matter
  7. Root density
  8. Soil moisture
  9. Groundwater table
  10. Size of treated area



Compound-Related Factors

Initial amounts. 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="" for="" groundwater="" contamination.="">

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.

Persistence. 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="" considered="" non-persistent.="" high="" or="" moderate="" persistence="" alone="" does="" not="" imply="" that="" the="" chemical="" will="" contaminate="" groundwater.="">

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 ground.

Once residues have been leached beyond the root zone, their persistence is greatly prolonged.

Adsorption. 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 groundwater table.

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="" representatives="" will="" 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="" groundwater="" table),="" contamination="" is="" possible,="" but="" by="" no="" means="" certain.="" without="" a="" qualifying="" statement,="" the="" term="" "potential"="" is="" misleading.="">

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="" apply.="">

Interactions between chemical and environmental factors affect a chemical's mobility, and these interactions determine its probability to reach groundwater.



Environmental Factors

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.

Photodegradation. Sunlight may break down a chemical deposited on leaf surfaces (e.g., triclopyr). This factor generally is of minor importance.

Precipitation. 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.

Topography (slope). 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. 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 greatly reduced.

Root density. 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 leaching.

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 be significant.

Soil moisture. 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.

Groundwater table. 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.



Discussion

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.

In general:
  • The lower the potential mobility, the less leaching can be expected.
  • All factors favoring microbial activity favor breakdown of chemical residues.

These two basic rules give reasonably good indications for the potential a chemical may have to contaminate groundwater in a particular area.

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, consult.


Solubility > 30 ppm
< 30="">		 2
1
Soil half-life > 3 months
1-2 months
< i="">		 3
2
1
Adsorption strong (Ko c > 300-500)
weak (Ko c <>		 1
2
Soil sand
clay		 2
1
Soil sand
clay		 2
1
Topography hilly
level		 2
1
Water Table high (< 2-3="">
low (> 3-5 ft.)		 2
1
Application blanket coverage
spot treatment		 2
1
Groundwater Recharge high (> 10 in./year)
low (< 10="" in./="">		 2
1
Sum: 8 = groundwater contamination improbable
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.