Leaching soil diffusion

Pesticides in the soil environmnet exhibit essentially two types of the behaviour either they can persist in the soil or they can disappear from the soil, depending on adsorption and desorption, leaching and diffusion, evaporation and decomposition. The movement of pesticide in the soil starts by the leaching and diffusion, and via evaporation they enter the atmosphere... 

Land disposal sites result in soil contamination through leachate migration. The composition of the substances produced depends principally on the type of wastes present and the decomposition in the landfill (aerobic or anaerobic). The adjacent soil can be contaminated by direct horizontal leaching of surface runoff vertical leaching and transfer of gases from decomposition by diffusion and convection. The disposal of... 

DBCP. The predictions suggest that DBCP is volatile and diffuses rapidly into the atmosphere and that it is also readily leached into the soil profile. In the model soil, its volatilization half-life was only 1.2 days when it was assumed to be evenly distributed into the top 10 cm of soil. However, DBCP could be leached as much as 50 cm deep by only 25 cm of water, and at this depth diffusion to the surface would be slow. From the literature study of transformation processes, we found no clear evidence for rapid oxidation or hydrolysis. Photolysis would not occur below the soil surface. No useable data for estimating biodegradation rates were found although Castro and Belser (28) showed that biodegradation did occur. The rate was assumed to be slow because all halogenated hydrocarbons degrade slowly. DBCP was therefore assumed to be persistent. 

The objectives of this study were to (a) determine the mobilities of the herbicides, alachlor (2-chloro-2, 6 -diethyl-N-(me-thoxymethyl)acetanilide), butylate (S-ethyl diisobutylthiocarba-mate), and metolachlor (2-chloro-N-(2-ethyl-6-methyl phenyl)-N-(2-methoxy-l-methyl ethyl) acetamide in the laboratory using soil leaching columns and soil thin-layer vapor diffusion techniques,... 

The good correlation of the results of vapor diffusion and leaching experiments for butylate, alachlor, and metolachlor with their physical properties has given support to the value of physical property measurements to predict pesticide movement in the soil. 

Transport of the herbicides by vapor diffusion on moist soil was shown to be directly related to vapor pressure and inversely related to water solubility. Transport of the herbicides by leaching was shown to be inversely related to the Freundlich adsorption coefficient which in turn was directly related to the octanol/water partition coefficient and inversely related to water solubility (16). 

The schematic diagram illustrating the decreasing source method for diffusion transport determination of any organic pollutant in solution or leached from COMs is shown in Fig. 3. The soil-solid sample is contained between two re-... 

In leaching field-aged residues of Atrazine and Metolachlor from a soil column, a model with a single diffusion parameter underestimated desorption at early times and overestimated desorption at late times. 

A branched pore leaching model as applied to release of water-soluble carbon from soil incorporates reaction to soluble compounds coupled with pore diffusion within the solids and leaching into the bulk solution. Application of such a model appears to describe hemicellulose hydrolysis reasonably well but not significantly better than chemical reaction only or simple leaching models. 

Chemicals may enter groundwater as landfill leachates or from deep-well injection of hazardous wastes, leaching from soil and water, or septic tanks. Diffusion and advection are the typical mechanisms of chemical transport in groundwater. Groundwater may be taken up via human use or empty onto the surface waters via a natural spring. 

The Rooting-Zone Soil Root-zone soil includes the A horizon below the surface layer. The roots of most plants are confined within the first meter of soil depth. In agricultural lands, the depth of plowing is 15-25 cm. In addition, the diffusion depth, which is the depth below which a contaminant is unlikely to escape by diffusion, is on the order of a meter or less for all but the most volatile contaminants. Soil-water content in the root zone is somewhat higher than that in surface soils. The presence of clay in this layer serves to retain water. Contaminants in root-zone soil are transported upward by diffusion, volatilization, root uptake, and capillary motion of water transported downward by diffusion and leaching and transformed chemically primarily by biodegradation or hydrolysis. 

Khadkikar and Basavaiah (2004) have recognised five types of Terrae rossae, ranging from karstified limestone to soil development, that have developed as a result of leaching and residual accumulation of limestone as well as the input of aerosols. The palaeosols commonly have a sharp upper contact but a diffuse lower boundary and residual lumps of weathered aeolianite are present. 

Soil scientists are very interested in the relation between the concentration of an ion in the soil solution and the amount retained by the soil. It is this relation which determines the rate of diffusion of a nutrient to a plant root and which also determines the possibilities of leaching losses from the soil. 

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