Pesticides from soils, loss

FIGURE 4.3 Loss of pesticides from soil, (a) Breakdown of herbicides in soil, (b) Disappearance of persistent organochlorine insecticides from soils (from Walker et al. 2000). 

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... 

Sparks DL (ed) (1986) Soil physical chemistry. CRC Press, Boca Raton, Florida Sparks DL (1989) Kinetics of soil processes. Academic Press, San Diego Sparks DL, Huang PM (1985) Physical chemistry of soil potassium. In Munson RE (ed) Potassium in agriculture, ASA, Madison, Wisconsin, pp 201-276 Sparks DL, Jardine PM (1984) Comparison of kinetic equations to describe K-Ca exchange in pure and mixed systems. Soil Sci 138 115-122 Spencer WF, Cliath MM (1969) Vapor densities of dieldrin. Environ Sd Technol 3 670-674 Spencer WF, Chath MM (1973) Pesticide volatilization as related to water loss from soil. J Environ Qual 2 284-289... 

A simple environmental chamber is quite useful for obtaining volatilization data for model soil and water disposal systems. It was found that volatilization of low solubility pesticides occurred to a greater extent from water than from soil, and could be a major route of loss of some pesticides from evaporation ponds. Henry s law constants in the range studied gave good estimations of relative volatilization rates from water. Absolute volatilization rates from water could be predicted from measured water loss rates or from simple wind speed measurements. The EXAMS computer code was able to estimate volatilization from water, water-soil, and wet soil systems. Because of its ability to calculate volatilization from wind speed measurements, it has the potential of being applied to full-scale evaporation ponds and soil pits. 

Data on exposure and environmental fate are needed, not to determine toxicity, but to provide information that may be useful in the prediction of possible exposure in the event that the chemical is toxic. These tests are primarily useful for chemicals released into the environment such as pesticides, and they include the rate of breakdown under aerobic and anaerobic conditions in soils of various types, the rates of leaching into surface water from soils of various types, and the rate of movement toward groundwater. The effects of physical factors on degradation through photolysis and hydrolysis studies and the identification of the product formed can indicate the rate of loss of the hazardous chemical or the possible formation of hazardous degradation products. Tests for accumulation in plants and animals and movement within the ecosystem are considered in Section 21.7. 

Their volatilization from litter on the forest floor will also be appreciable. With the possible exception of carbaryl, their volatilization after being washed into the soil will be relatively low or insignificant because of their low volatility, low Henry s constants, Kh> and/or their high rates of degradation in the soil environment. The rapid disappearance of the phenoxy herbicides (2, 31) and the insecticide, fenitrothion (28) from vegetation and the forest floor is supporting evidence that volatilization is an important pathway for loss of applied pesticides from the forest canopy and litter on the forest floor. 

Sorption and Desorption Processes. Sorption is a generalized term that refers to surface-induced removal of the pesticide from solution it is the attraction and accumulation of pesticide at the soil—water or soil—air interface, resulting in molecular layers on the surface of soil partides. Experimentally, sorption is characterized by the loss of pesticide from the soil solution, making it almost impossible to distinguish between sorption in which molecular layers form on soil partide surfaces, precipitation in which either a separate solid phase forms on solid surfaces, covalent bonding with the soil particle surface, or absorption into soil partides or organisms. Sorption is generally considered a reversible equilibrium process. 

Pesticide transport by surface runoff and soil erosion is a function of time lag between rainfall and application the chemical nature and persistence of the pesticide the hydrological, soil, and vegetative characteristics of the field and the method and target of application (43). Wauchope (44) found that unless severe rainfall occurred shortly after pesticide application, total losses for the majority of pesticides due to runoff were less than 0.5% of the amount applied in most cases, although single-event losses from small plots or watersheds can be much greater. 

Chemical, cultural, and mechanical weed control practices have been relatively successful ia reducing yield losses from weeds (448). However, herbicide-resistant weed populations, soil erosion, pesticide persistence ia the environment, and other problems associated with technologies used (ca 1993) to control weeds have raised concerns for the long-term efficacy and sustainability of herbicide-dependent crop production practices (449). These concerns, coupled with ever-increasing demands for food and fiber, contribute to the need for innovative weed management strategies (450). 

---