Rates of Pesticides

Another aspect of pesticide-soil interactions that is very important in predicting the effect of pesticides on environmental quality is the degradation rate. It is not the purpose of this discussion to give an in-depth discussion of pesticide degradation in soils. Numerous reviews are available on transformations, metabolic pathways, persistence, and tl/2 values of 

Numerous studies have shown that several factors affect pesticide degradation rates, including soil type, water content, pH, temperature, and clay and organic matter content (Rao and Davidson, 1980). Hamaker (1972) has published an excellent review on the quantitative aspects of pesticide degradation rates in soils. He consider two types of rate models  

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Kinetics of Pesticide Biodegradation. Rates of pesticide biodegradation are important because they dictate the potential for carryover between growing seasons, contamination of surface and groundwaters, bio accumulation in macrobiota, and losses of efficacy. Pesticides are typically considered to be biodegraded via first-order kinetics, where the rate is proportional to the concentration. Figure 2 shows a typical first-order dissipation curve. 

For those pesticides which are utilized as microbial growth substrates, sigmoidal rates of biodegradation are frequentiy observed (see Fig. 2). Sigmoidal data are more difficult to summarize than exponential (first-order) data because of their inherent nonlinearity. Sigmoidal rates of pesticide metabohsm can be described using microbial growth kinetics (Monod) however, four kinetics constants are required. Consequentiy, it is more difficult to predict the persistence of these pesticides in the environment. 

Sorbed pesticides are not available for transport, but if water having lower pesticide concentration moves through the soil layer, pesticide is desorbed from the soil surface until a new equiUbrium is reached. Thus, the kinetics of sorption and desorption relative to the water conductivity rates determine the actual rate of pesticide transport. At high rates of water flow, chances are greater that sorption and desorption reactions may not reach equihbrium (64). NonequiUbrium models may describe sorption and desorption better under these circumstances. The prediction of herbicide concentration in the soil solution is further compHcated by hysteresis in the sorption—desorption isotherms. Both sorption and dispersion contribute to the substantial retention of herbicide found behind the initial front in typical breakthrough curves and to the depth distribution of residues. 

In recent studies, pesticides such as atrazine have been found in precipitation. Therefore volatilization and subsequent transport in the gaseous phase is an important environmental pathway. Vaporization rates of pesticides deposited on surface of soil and plant leaves depend on the physical-chemical properties of the substance. A useful physicochemical criterion is Henry s constant, Ku, which is defined as the equilibrium air-to-water partial pressure ratio of the substance (see Chapter 7). 

The volatilization rate at the surface is further influenced by temperature and by the thickness of the stagnant air layer over the surface. Furthermore, random micrometeorological conditions of the crop stand are also very important (turbulence, thickness of boundary layer, humidity, and wind velocity). Special models are available to calculate the volatilization and vaporization rates of pesticides (Richter, 1999). 

It is also possible to generate microcapsules through interfacial polymerization using only one monomer to form the shell. In this class of encapsulations, polymerization must be performed with a surface-active catalyst, a temperature increase, or some other surface chemistry. Herbert Scher of Zeneca Ag Products (formerly Stauffer Chemical Company) developed an excellent example of the latter class of shell formation (Scher 1981 Scher et al. 1998). He used monomers featuring isocyanate groups, like poly(methylene)-poly(phenylisocyanate) (PMPPI), where the isocyanate reacts with water to reveal a free primary amine. Dissolved in the oil-dispersed phase of an oil-in-water emulsion, this monomer contacts water only at the phase boundary. The primary amine can then react with isocyanates to form a polyurea shell. Scher used this technique to encapsulate pesticides, which in their free state would be too volatile or toxic, and to control the rate of pesticide release. 

A little-known source of biodegradation rates of pesticides is data developed by the California Department of Food and Agriculture (CDFA). They estimated aerobic and anaerobic soil metabolism half-lives from open scientific literature and studies submitted to CDFA from chemical companies in compliance with the data call-in requirements of the Pesticide Contamination Prevention Act. Table 12.15 tabulates these data. 

Nonsingularity of Pesticide-Soil Interactions 136 Degradation Rates of Pesticides 139 Reaction Rates and Mechanisms of Organic Pollutants 143 Supplementary Reading 144... 

Dennison, R. S. J. B. Wedding "Determination of evaporation rates of pesticide droplets." Aerosol Science Laboratory, Ft. Collins, CO, 1982. 

Volatilization of pesticides is an important pathway for their loss from treated agricultural lands. The importance of volatilization in the forest environment has not been established by direct measurement, but can be inferred from volatilization rates of the same pesticides under agricultural conditions and from other data on their behavior in the forest environment. In recent years, several studies of actual volatilization rates of pesticides under field conditions have provided an assessment of the rate of input to the air under typical conditions of use (1). 

Nash, R.G. (1980) Dissipation rate of pesticides from soils. In CREAMS A field scale model for chemical, runoff, and erosion from agricultural management systems. Vol. 3, Knisel, W.G., Editor, pp. 560-594, USDA Conserv. Res. Rep. 26, U.S. Government Printing Office, Washington, DC. 

Vm = maximum rate of pesticide disappearance C = concentration of reactant at time t C0 = concentration of reactant at t 0 Km = an equilibrium constant... 

Insecticides tend to persist longer in soils of high organic matter. In fact, in muck soils (50% or more organic matter), residues have been found bound to soil particles to such an extent that the same amount of toxicant is less effective in muck soil as compared with a sandy type. It has been noted that pesticides are absorbed into crops most readily from sandy soils and least from muck soils. Moisture enhances the release of volatile pesticides from soil particles and also influences the breakdown of other toxicants. Microbial attack has been found to oxidize aldrin to dieldrin, and parathion in the presence of yeast is reduced to the nontoxic aminoparathion in soil. As might be expected, increased soil temperatures can dramatically increase the rate of pesticide loss owing to volatilization and increased breakdown. Cover crops, such as alfalfa, can decrease pesticide volatility from soil whereas cultivation... 

This device causes a fixed rate of pesticide to mix with the water flowing through the hose to which it is attached. The mixture is expelled through a high-volume nozzle. These sprayers usually hold no more than one quart (one liter) of concentrated pesticide, but because the concentrate mixes with the water, they may deliver 20 gallons or more of finished spray solution before refilling. Figure 8.3 depicts a hose-end sprayer used for small pest control projects. 

A residue is the part of a pesticide that remains in the environment for a period of time following application or a spill. Pesticides usually break down into harmless components after they are released into an environment. The breakdown time ranges from less than a day to several years. The rate of pesticide breakdown depends primarily on the chemical structure of the pesticide active ingredient. The rate of pesticide breakdown also may be affected by environmental conditions at the release site, such as ... 

The natural biogeochemical peculiarities (see Chapter 4, Section 4 and Chapter 6, Section 4), which favor the development of endemic diseases were complicated by anthropogenic activity. In the area of the Volga-Ural watershed and the Ustyurt plateau, this activity is connected with oil explorations and oil-chemical industry. The watershed of Amu-Daria and Syr-Daria rivers is the agricultural area with intensive irrigated cotton production with application of heavy rates of pesticides and fertilizers. 

Seiber, J.N. McChesney, M.M. Woodrow, J.E. "Experimental validation of model-predicted volatilization rates of pesticides from water" Paper presented at the 190th National Meeting of the American Chemical Society (AGRO 99), Chicago, IL, Sept. 8-13, 1985. 

A recent field experiment was reported (77) in which DBCP was chisel-injected into fallow plots, in Georgia, at normal and 3X normal rates. Under accelerated irrigation, traces of DBCP had leached to at least 12 m by 8 months, though at normal rates of pesticide and water, leaching below 4 m in 6 months probably had not occurred. The pesticide was present in perched groundwater and there was evidence, due to the existence of restricting clay layers, that lateral movement into control plot subsoil was important. 

The growth rate of pesticide sales over the past five years has been about 14% a year. It is estimated that sales of basic toxicants at the manufacturers level will reach about 700 million in 1968 (I, 2, 3, 4, 5, 6). Based on a conservative growth pattern, say 10% per year, by the mid-Seventies pesticide sales would be over 1 billion (Figure 1). 

Enhanced biodegradation Is the accelerated rate of pesticide degradation, by adapted soli microorganisms, observed In soil following previous application of It or a similar pesticide. Enhanced degradation may result In pest control failure and crop yield reduction."... 

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