Behaviour and Fate of Pesticides in Water
Pesticides are frequently found in surface and ground water and concentrations have been found that could affect small aquatic organisms or animals that feed on fish.
Pesticides may enter surface waters directly through runoff, spills, or various effluents. Contamination also may be indirect, with pesticides first entering the atmosphere or ground water, and then transported to surface waters.
Once in surface water, some pesticides can be deposited in sedimentation areas, which can then act as a long-term source to the water column through resuspension, biotic uptake, and diffusion. Irrespective of the manner in which the water got contaminated, pesticides may undergo a degradation process, which converts the parent chemical to either toxic or non-toxic products.
Behavior and Fate of Pesticides in Waters
The behavior, transport, and fate of an organic chemical in surface waters is controlled by the properties of the chemical and the environmental conditions in the water. The structure of the organic chemical determines its physical, chemical, and biological properties.
The surface water environment that surrounds the organic chemical consists of physical, chemical, and biological components. The interaction of the chemical structure and environmental conditions controls the chemical’s behavior and ultimately its effect on the environment.
The environmental processes that control an organic chemical’s behavior and fate in surface water can be classified into three types:
(1) Transformation processes, which change its chemical structure;
(2) Phase-transfer processes, which control its movement between water, biota, suspended sediments, bed sediments, and the atmosphere; and
(3) Transport processes, which move it away from its initial point of introduction to the environment and throughout the surface water system.
Transformation Processes
The transformation of a pesticide results in changes in its chemical structure. One or more new chemicals are produced, and the original pesticide disappears. These new chemicals can be organic or inorganic molecules and ions.
From an environmental-effects point of view, the ideal fate for a pesticide is ultimate transformation to inorganic species, such as water, carbon dioxide, and chloride ions (termed mineralization).
However, in many instances, the chemicals formed from transformation reactions are long-lived intermediates, which themselves can have a negative impact on the environment.
Often the initial transformation products undergo subsequent transformation reactions before mineralization. By this process, a large number of transformation products can potentially be formed, some of which may retain pesticidal properties.
Some pesticide formulations are applied as inactive agents and gain pesticidal properties only after transformation. The transformation of pesticides in water is brought about by chemical and biological reactions. Another process (photolysis) involves the breakdown of chemical by light.
Chemical transformation
Chemical transformations can be mediated by chemical, biological, or physical means. In surface waters, chemically induced abiotic hydrolysis and oxidation-reduction reactions often occur. Biodegradation is the general term for biologically mediated reactions.
Microorganisms can induce pesticides to undergo both hydrolytic and oxidation-reduction reactions. Photolysis is a chemical reaction induced by the energy from sunlight.
Hydrolysis
Hydrolysis is the chemical (sometimes biologically mediated) reaction of a pesticide with water, usually resulting in the cleavage of the molecule into smaller, more water-soluble portions and in the formation of new C-OH or C-H bonds.
This process is important for many organophosphorus and carbarnate pesticides. The hydrolysis rate of a given organic compound is dependent on the characteristics of the solution.
The strongest factor is pH. Hydrolysis reactions can be a result of direct attack by the water molecule (H20), the hydronium ion (H30+), or the hydroxide ion (OH-). These are termed neutral, acid, and base hydrolysis, respectively.
At low pH, reactions are dominated by acid- catalyzed hydrolysis, whereas at high pH, reactions are dominated by base-catalyzed hydrolysis. At intermediate pH values, both neutral and acid, or neutral and base-catalyzed reactions, can be important to the overall rate of hydrolysis.
It should be noted that acid or base catalysis does not necessarily occur in all hydrolysis reactions, and that neutral catalyzed reactions alone sometimes may govern the overall rate of reaction. In these cases, the rate will not depend on pH.
Temperature also is an important factor. Generally, a temperature rise of 10°C increases the reaction rate twofold to fourfold.
The presence of certain metal ions, humic substances, and particles can catalyze hydrolysis for some compounds. The structure of the pesticide determines which of these processes, if any, are important in its hydrolysis.
Organic chemicals that can undergo hydrolysis on time scales important for consideration of this process in surface water systems (half-lives of days to years) include alkyl halides, aliphatic and aromatic esters, carbamates, phosphoric esters, and phosphoric acid esters.
Some pesticides, such as dichlorvos, undergo hydrolysis at rates too fast (halflives of minutes) to ever be present at significant concentrations in surface waters.
Other pesticides, such as DDT and chlordane, undergo hydrolysis at rates too slow (half-lives of years to decades) to warrant consideration of this transformation process. Others, such as pentachlorophenol (PCP) and benfluralin (benefin), contain no hydrolyzable functional groups.
Oxidation-reduction reactions
Oxidation-reduction reactions are chemically or biologically mediated reactions that involve a transfer of electrons.
The process requires two chemical species to react as a couple: one chemical undergoes oxidation (loses one or more electrons) while another undergoes reduction (gains one or more electrons).
Many oxidation reactions of pesticides in surface waters are biologically or photolytically induced. In reduced environments, such as bed sediments and the hypolimnion of lakes, abiotic reduction reactions can occur when organic or inorganic reducing agents are present, such as certain transition metals (iron, nickel, cobalt, chromium), extracellular enzymes, iron porphyrins, or chlorophylls.
The rates of reduction reactions are dependent on pH and the magnitude of the reduction potential. The reduction half-life of the organophosphorus insecticide (OP) parathion, for example, is on the order of minutes in strongly reducing environments.
Biodegradation
Biodegradation is the transformation of pesticides mediated by living organisms using enzymes. Chemical transformation reactions can cause structural changes in an organic chemical, but biodegradation is the only transformation process able to completely mineralize the pesticide.
Microorganisms degrade (transform) organic chemicals as a source of energy and carbon for growth, although most of their degradative enzymes are not used directly for growth and energy processes, but rather are part of a metabolic sequence that terminates in energy release.
All naturally produced organic compounds can be biodegraded, though this is a slow process for some chemicals.
On the other hand, some synthetically produced organic chemicals, including most pesticides, have structures totally unfamiliar to microorganisms, which may not have the enzymes needed for degradation of these compounds.
This is the primary reason why some pesticides, such as DDE, hexachlorobenzene (HCB), and mirex, are recalcitrant (very long lived) in the environment. However, even these synthetic compounds are observed to slowly biodegrade, probably owing to a process called cometabolism.
In cometabolism, the microorganisms are using other substrates (carbon sources) for growth and energy, and the unfamiliar synthetic compound enters into the process and is transformed. The microorganisms derive no particular benefit from the degradation of this compound.
The rate of biodegradation of a pesticide is dependent on chemical structure, environmental conditions, and the microorganisms present. The structure of the organic chemical determines the types of enzymes needed to cause its transformation.
The concentration of the chemical also can affect its rate of degradation. At high concentrations, a chemical may be toxic to microorganisms; at very low concentrations, it can be overlooked by the organisms as a potential substrate.
The environmental conditions (temperature, pH, moisture, oxygen availability, salinity, and concentration of other substrates) determine the species and viability of the microorganisms present.
Finally, the microorganisms themselves control the rate of biodegradation depending on their species composition, spatial distribution, population density and viability, previous history with the compound of interest, and enzymatic content and activity (Scow, 1990).
Photolysis
Photolytic transformations of pesticides are caused by the addition of energy from sunlight. The earth’s atmosphere filters out light with wavelengths shorter than 290 nm; only wavelengths greater than this reach the earth’s surface.
Pesticides can undergo a direct reaction with sunlight (direct photolysis) or a secondary reaction with a photoactivated, sunlight-induced, short-lived reactive chemical species (indirect photolysis). The type of photoinduced reaction is dependent on the structure of the pesticide and specific environmental conditions.
Directphotolysis
Direct photolysis is the result of absorption of sunlight by a pesticide, causing a chemical transformation, such as cleavage of bonds, dimerization, oxidation, hydrolysis, or rearrangement.
This reaction will occur only if the pesticide absorbs light at wavelengths present in solar radiation. The light absorption spectrum of most pesticides falls outside or near the fringes of the solar spectrum; therefore, direct photolysis is not an important transformation process for many pesticides. Notable exceptions to this are DNOC, fenitrothion, and metoxuron.
Indirect photolysis
Indirect photolysis is usually a photo-induced oxidation reaction. Sunlight excites a photon absorber, such as nitrate or dissolved organic matter, which in turn reacts with dissolved oxygen to form potential photoreactants such as singlet oxygen (1O2), hydroxyl radical ( OH), superoxide anion (027, peroxy radical (ROO ), and hydrogen peroxide (H202).
These highly reactive species randomly attack water, dissolved organic matter, dissolved oxygen, or pesticides, if present. Another type of indirect photolysis, triplet photosensitization, occurs when a photon absorber, such as humic acid, transfers excess energy to a pesticide molecule, which then photodegrades.
The two most important indirect photolysis reactions are singlet oxygen and nitrate-induced photooxidation. Singlet oxygen is a very efficient photoreactant for specific types of chemical structures, including many OPs.
The more general reaction is the nitrate-induced photooxidationthat proceeds through the hydroxy radical intermediate and affects all organic molecules. For any specific surface water, the rate of this reaction is a function of the nitrate concentration.
Phase-transfer processes
Phase-transfer processes involve the movement of a pesticide from one environmental matrix to another. The important processes that can occur in the water environments include water-to-solid transfer (sorption), water-to-biota transfer (bioaccumulation), and water-to-air transfer (volatilization from water).
In addition to these processes, air-to-solid transfer (vapor sorption) is important in soil environments. Although the physical movement of the chemical is involved, these transfer processes should not be confused with transport processes.
Transfer processes are important on the scale of molecular distances (nanometers to micrometers). Once the organic chemical has passed through the physical interface (environmental compartment boundary), it may undergo transport over much larger distances. The phase-transfer processes of sorption and volatilization largely control the overall transport of many pesticides in water.
Pesticides orption in water
Pesticides are distributed between particle surfaces and the water to varying degrees. This process, termed sorption, can play a pivotal role in the environmental behavior, transport, and fate of a pesticide in surface water.
An organic chemical sorbed to a particle surface behaves differently than it does in the dissolved phase. Chemicals associated with particles generally are less available for biodegradation and are not available for volatilization to the atmosphere.
Some particle-associated pesticides, such as atrazine (in soil), undergo sorbent-catalyzed hydrolysis. The extent of sorption of a pesticide is a function of its physicalchemical properties and the properties of the particle and the solution.
Relevant aspects for the solution include pH (especially for organic chemicals having a pKa, from 4 to 8), ionic strength, concentration of dissolved organic carbon (DOC), and, to a lesser extent, temperature.
The ionic strength of the solution affects the activity coefficient of the organic chemical in water. As the ionic strength of an aqueous solution increases, the chemical’s solubility decreases and the extent of sorption slightly increases.
The presence of DOC in the water also can affect the activity coefficient of the pesticide, decreasing the extent of sorption. Because sorption is a surface process, the characteristics of the particles that have the greatest influence on the extent of sorption are surface area and surface coverage by organic films. In most surface waters, the majority of the particulate surface area is contributed by silt, clay, and colloidal size particles.
In addition to having large surface areas, these sizes of particles are generally the most enriched with organic surface films. It has been shown that organic coatings on particles essentially control the extent of sorption for many organic chemicals (Chiou, 1990).
The hydrophobicity of an organic chemical, which can be quantified to some extent by its water solubility or octanol-water partition coefficient, also controls the extent of sorption.
The extent of sorption at equilibrium is commonly defined in terms of a distribution coefficient, Kd, defined as the ratio of the concentrations of the pesticide between the suspended sediments and the water.
An organic carbon-normalized distribution coefficient, Koc, defined as Kddivided by the fractional content of organic carbon in the suspended sediment, also is used widely. For a wide range of pesticides and other organic chemicals, their sorption distribution coefficients have been shown to be correlated strongly with their water solubilities.
This provides a tool for predicting the extent of sorption of a particular chemical in a particular environment where the organic coatings of the particles dominate the sorptive process. Anumber of structure activity relations have been derived for these types of predictions (Lyman, 1990).
Sorption is an extremely complex process. With limited information on sorption and relatively few environmental observations of the process, researchers have often assumed that sorption of organic chemicals is completely reversible, linear (with respect to chemical concentration), and at equilibrium in surface waters.
Studies have shown, however, that sorption and desorption are not completely reversible, at least in the laboratory, and that chemical equilibrium may not be reached for biotic particles in surface waters.
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For most environmental situations, organic contaminants are present at concentrations low enough that a linear Kd value adequately describes its sorptive behavior.
Given the numerous uncertainties in environmental observations of organic chemicals, the quantification of nonequilibrium, nonlinear, and nonreversible sorptive behavior has been difficult.
Bioconcentration and bioaccumulation
Some pesticides concentrate in the living tissues of aquatic organisms, such that the concentration in the organism is greater than in the water. The pesticide can accumulate in tissues by two routes.
One route is through the process called bioconcentration, which is direct water/ tissue partitioning governed by the same mechanisms as sorption of pesticides to organic matter on particles. The second route is through the organism’s diet.
When one organism eats another that has accumulated pesticides in its tissue, some fraction of that pesticide burden is available for accumulation by the consumer. The combination of these two routes, both of which are thought to be important in the environment, is termed bioaccumulation.
It has been observed by many investigators that bioconcentration can be related to hydrophobicity for many persistent chemicals. Thus, numerous structure-activity relations have been developed that relate bioconcentration to a chemical’s water solubility or octanol-water partition coefficient (Bysshe, 1990).
The two most important parameters determining the extent of bioconcentration for a particular compound are the lipid content of the organism and the rate at which the chemical is metabolized in the organism.
Differences of up to two orders of magnitude in bioconcentrationcan be expected for a single compound because of variations in biotic species, sex, life stage, and size (Bysshe, 1990).
The Henry’s Law Constant
Pesticides can be transferred from the dissolved aqueous phase to the vapor phase in the atmosphere as a result of volatilization from water.
This transfer is controlled by the chemical nature of the air-water interface and the mass transfer (advective) rates of the chemical in water (velocity of water flow, etc.), the pesticide’s molecular diffusion coefficients in air and water, and its Henry’s Law constant. Thomas (1990) has suggested that the importance of volatilization for a given chemical can be generalized from its Henry’s Law constant alone.
For pesticides that have a Henry’s Law constant less than 3 x 10-7 atm- m3/mole, volatilization from surface water is unimportant. For pesticides that have a Henry’s Law constant greater than 1 x 10-5 atm-m3/mole, volatilization is significant for all waters.
Many of the volatile pesticides used as fumigants have Henry’s Law constants in the range where volatilization can be a significant process in their environmental behavior. In contrast to this, only a few of the high-use herbicides exhibit any tendency toward volatilization from surface water.
The organochlorine pesticides fall between these two extremes and may or may not volatilize from water, depending on the environmental conditions and the relative concentrations of the compound in the water and in the atmosphere.
Just as pesticides distribute themselves between the water and particle surfaces in water, they also distribute themselves between the air and particle surfaces in soils.
The extent of this vapor sorption (i.e., air-to-solid transfer) is a function of the chemical’s properties, the soil particle’s properties, and the water content of the soil. Sitea (2001) has shown that in dry soils, vapor sorption interactions are stronger between the pesticide and the inorganic surface of the particle (particularly clay surfaces) than with the organic matter on the particle surface.
As the water content of the soil increases, the inorganic surface becomes hydrated and the water outcompetes the pesticides for the inorganic sorption sites. The extent of vapor sorption decreases and the interactions with the organic carbon surface coatings become the dominant mechanism.
