Pesticide dissipation in soil

Gustafson, D.I. and Holden, L.R. (1990) Nonlinear pesticide dissipation in soil a new model based on spatial variability, Environ. Sci. Technol., 24(7) 1032-1038. 

Abiotic mechanisms for degradation of herbicides in soil, discussion, 15 Abiotic transformation processes of pesticide dissipation in soil hydrolytic reactions, 5 role in pesticide degradation, 4-5... 

Nash, R.G. (1989) Models for estimating pesticide dissipation from soil and vapor decline in air. Chemosphere 18(11/12), 2375-2381. 

We have found substantial evidence that the presence of vegetation can increase dissipation rates of pesticide residues in soil. Further evidence inches that vegetation may stabilize the pesticide residues, decreasing the potential for leaching and uptaJce into biota thus, phytoremediation may prove to be a valuable tool in clean up of moderately contaminated sites. However, several considerations should be made regarding this technology ... 

The first step in a wildlife exposure assessment is to document the occurrence and persistence of a pesticide in the study area throughout the study duration. Several articles in this book describe the experimental designs and best practices to conduct field crop and environmental dissipation (air, soil and water) studies. This article presents methods to quantify spatial and temporal distributions of pesticide presence in ecosystems following normal application and resultant exposure of nontarget wildlife. 

The key results of the environmental transport and persistence analyses for these 12 pesticides are summarized In Table IV. The processes Included In soil dissipation half-lives could be biodegradation, hydrolysis, oxidation, reduction, soil photolysis, and volatilization. Note that biodegradation may be other than first-order, whereas the other processes are usually first order or pseudo-first-order. 

According to the vendor, the hydrolytic terrestrial dissipation (HTD) process is an ex situ process for the treatment of soils contaminated with toxaphene (a chlorinated pesticide) and other pesticides in soils. The process utilizes metal-catalyzed alkaline hydrolysis reactions, ultraviolet (UV) light, and reducing or oxidizing agents to remove chlorine from the contaminant. 

Despite the low average consumption of pesticides, in the sporadic reports available, it could be seen that even the roadside dusts, rural and urban soils and the underwater sediments are contaminated. Many pesticides are degrading the Indian environment, even though faster dissipation and possible degradation of POPs chemicals like HCHs and DDTs were observed in Indian soils by the tropical climate of India (Pillai, 1986). Such a phenomenon of dissipation in the dry season was substantiated by Ramesh et al. (1991) in the river sediments (Fig. 9.2). Further, the relative flux of residues into the aquatic environment is smaller than the amount volatilized to the atmosphere in tropical countries like India (Tanabe et al., 1991). 

Thin-layer radiochromatography (radio-TLC) is widely applied for a variety of environmental studies involving radiolabeled pesticides, such as plant uptake from soil, bioaccumulation in fish, dissipation from soil, metabolism in soil, plants, and fish, and environmental fate. The determination of the lipophiUdty of pesticides is important because their bioaccumulation and tendency for degradation and biotransformation are related to lipid solubility. TLC has advantages for lipohilicity studies compared to traditional partition coefficient measurement in an octanol-water system. 

As more sensitive analytical methods for pesticides are developed, greater care must be taken to avoid sample contamination and misidentification of residues. For example, in pesticide leaching or field dissipation studies, small amounts of surface soil coming in contact with soil core or soil pore water samples taken from further below the ground surface can sometimes lead to wildly inaccurate analytical results. This is probably the cause of isolated, high-level detections of pesticides in the lower part of the vadose zone or in groundwater in samples taken soon after application when other data (weather, soil permeability determinations and other pesticide or tracer analytical results) imply that such results are highly improbable. 

The time of year in which a pesticide is applied significantly affects its dissipation rate due to temperature, moisture, and solar-irradiance effects on abiotic and biotic dissipation processes. For example, dissipation rates for agrochemical applications made in the springtime are normally greater than those observed for fall (autumn) applications. Thus, the timing of agrochemical applications made in field soil dissipation studies should closely match those occurring under acmal-use conditions. 

The pesticide component of SWRRB takes into account the fate of the chemical applied under field conditions For example, the amount of pesticide actually reaching the ground after application over a plant canopy is calculated. Further, field dissipation of the chemical by photolysis on leaf surfaces as well as degradation in the soil is accounted for with the pesticide component of SWRRB. Leaching of the pesticide below the top 1cm of soil is also computed and runoff corrected for such losses. Further, adsorption of the pesticide to soil surfaces and sediment is taken into account by SWRRB. 

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. 

Knowledge of the relative role of vaporization in pesticide field dissipation has been markedly advanced by development of methods for measuring the vertical flux of vapors from a soil or soil-crop field plot. The basis and techniques for this method have been presented by Caro et al.(74) and Parmele et al. (7), and summarized recently by Taylor (6). The equation... 

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