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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.