Pesticide movement within soil

Pesticide Movement within the Soil Column During Phytoremediation... 

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. 

Helling et al. (1971), also working with a Chlorella (C. sorokiniana), detected the movement of 11 triazine herbicides, nine phenylureas, and 13 other miscellaneous herbicides, together with two methylcarbamate insecticides, in a thin layer of soil supported on a chromatography plate. The Chlorella suspension was aspirated onto the soil-covered plate, which was then incubated at 100% relative humidity zones of pesticide movement were visible within 24 to 48 hr by algal inhibition. The authors foimd this procedure to be adaptable to detection of movement of many other herbicides and some insecticides and fungicides. 

Persistence of pesticides in the environment is controlled by retention, degradation, and transport processes and their interaction. Retention refers to the abihty of the soil to bind a pesticide, preventing its movement either within or outside of the soil matrix. Retention primarily refers to the sorption process, but also includes absorption into the soil matrix and soil organisms, both plants and microorganisms. In contrast to degradation that decreases the absolute amount of the pesticide in the environment, sorption processes do not affect the total amount of pesticide present in the soil but can decrease the amount available for transformation or transport. 

As pointed out by Westlake and San Antonio (46) such factors as temperature, light, humidity, air movement, volatility of the compounds, and microorganism activity are influencing the breakdown and mechanical dispersion of residues on plant surfaces and in soils. However, within plants and animals, probably the most important single factor in pesticide breakdown and transformation is through enzymatic action. As shown in Figure 1, when a pesticide is applied to plants, animals, soils, water or air, there are many factors that may effect chemical changes, and the rates of such alterations will depend on the nature of the compound and the particular environmental conditions to which... 

The accurate and precise analysis of pesticides is a critical requirement for the registration and use of pesticides throughout the world. Parent molecules, key metabolites and chemical breakdown products must be identitied and studied in well designed laboratory and Held research trials. Environmentally sound management practices rely on significant amounts of information about the levels and movements of pests, pathogens and specific chemical treatments within the environment The methods available for such analysis have become extremely sophisticated and sensitive in response to the need to detect lower and lower levels of contaminants in crops, water, soil, and farm animals. 

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