Soil, evaporation rate

Air/soil Evaporation rate Soil adsorption constant Vapor pressure Vapor density Octanol/water partition coefficient Solubility Particle size... 

The released ammonia forms a pool of refrigerated liquid which evaporates by heat transfer from the soil. A constant mass value was assumed for the evaporation rate and a heavier-than-air gas dispersion model was used. 

Most gasoline constituents are volatile organics. Volatilization depends on the potential volatility of the compounds and on the soil and environmental conditions, which modify the vapor pressure of the chemicals. Factors affecting volatility are water content, clay content, surface area, temperature, surface wind speed, evaporation rate, and precipitation. 

For military purposes, unmodified vesicants are classified as persistent. Under proper conditions, agents can remain hazardous in soil and even in water for several years. Limited solubility slows the hydrolysis of liquid agents. Some hydrolysis products are highly toxic and extremely persistent. Evaporation rates range from near that of light machine oil to that of heavy motor oil. 

The simulations of volatilization were conducted using the complete model described by Jury et al. (33) where each chemical is present in the soil at a uniform concentration of 1 kg/ha to a depth, L, in the soil and is allowed to volatilize through a stagnant air boundary layer for a specified time period in the presence or absence of water evaporation. The standard conditions or common properties assumed in the simulations are the same as those indicated in Jury et al. (35, 36), i.e., air diffusion coefficient, 0.43 m /d water diffusion coefficient, 4.3 X 10-3 m /d atmospheric relative humidity, 50% temperature, 25°C soil porosity, 50% bulk density, 1.35 g/cm3 soil water content, 0.30 organic carbon fraction, 0.0125 amount of pesticide in soil, 1 kg/ha depth in soil, 1 or 10 cm water evaporation rate, 0, 0.25, or 0.50 cm/d. 

Table II shows the calculated cumulative volatilization after 10 days as affected by depth of pesticides in soil (L) and water evaporation rate (E) expressed as a percent of the 1 kg/ha initially present in the soil at t = 0. Soil water content (0) was assumed to be equal to 0.30, and the organic carbon content of the soil (foe) equal to 0.0125. The volatilization rates shown in Table II are for the ideal conditions and high water evaporation rates assumed in the simulations. They are undoubtedly the upper limits of volatilization to be expected from forest soils. Volatilization was increased greatly by evaporating water, particularly for the compounds with low Kjj values and increasing soil depth decreased volatilization.

Figure 4. Calculated volatilization flux versus time for selected forest pesticides as affected by depth (L) within the soil at water evaporation rate E = 0.25 cm/day.

The most frequently mentioned type of anthropogenic salinity is caused by saline irrigation water. If the amount of applied water is insufficient to leach salts out of the soil profile, high evaporation rates may cause an upward water movement. The result is the accumulation of soluble salts in the topsoil and at the soil surface. This process is often accompanied by sodicity, an excess of monovalent ions (mostly sodium), which destroys soil structure. 

Pesticides are subject to considerable loss by evaporation when they are thinly spread over large areas of crop exposed to moving air. In this situation they are subject also to biochemical, photochemical, and solution losses which make it difficult to assess directly evaporation under field conditions. The rate of evaporation of water is easily determined and has been the subject of much experiment. The relationship of loss of pesticide to loss of water from the same surface can be verified by laboratory experiments. Crystallization and solution in leaf substances exert some effect also. When the pesticide is distributed in the soil, evaporation of water can accelerate that of the water-soluble pesticide the mechanism lies in capillary flow of solution and not in the evaporation process itself. 

Under field conditions water evaporation normally occurs from the same surfaces as evaporation of pesticide. Any temperature effect caused by the former will therefore affect the latter, and in this repect the state of affairs is simpler. On the other hand, leaves can restrict water evaporation, and the soil surface is very complex hence, other complications appear under field conditions. All these factors, however, will make the pesticide evaporation rate (if the pesticide is fully exposed on an outer surface) faster than that calculated from the water rate. 

A value of 6 tons/acre-day as the potential evaporation rate from wet soil is conservative (low). If the pesticide is fully exposed—not dissolved, or adsorbed—its rate of loss will be less than expected by multiplying the 6 tons by the vapor pressure of the pesticide and the square root of its molecular weight and dividing by half this product for water (the half allowing for average 50% humidity of the air initially). 

Transport and Transformation of Chemicals A Perspective. - Transport Processes in Air. - Solubility, Partition Coefficients, Volatility, and Evaporation Rates. - Adsorption Processes in Soil. - Sedimentation Processes in the Sea. - Chemical and Photo Oxidatioa - Atmospheric Photochemistry. -Photochemistry at Surfaces and Interphases. -Microbial Metabolism. - Plant Uptake, Transport and Metabolism. - Metabolism and Distribution by Aquatic Animals. - Laboratory Microecosystems. - Reaction Types in the Environment. -Subject Index. 

As mentioned earlier, high salt contents in water (e.g., Na, Ca, and Mg sulfates) in contact with soils produce salinization (i.e., excess salinity). Other causes of salinization include high water tables, high evaporation rates, and low annual rainfall. Excessive salinity makes it more difficult for plant roots to take in water, which considerably reduces crop yields and also degrades the quality of shallow groundwater and of surface water. 

Evaporation usually exceeds rainfall on an annual basis in the Florida phosphate region (8). However, the infiltration rate of water into dry soil or phosphogypsum is about 1.3 cm/h 19). This greatly exceeds the evaporation rate. 

The positive correlation (r = + 0.85) between 5 C and 5 0 values (Fig. 22) of the eogenetic Lunde carbonates may be related to relatively rapid near-surface precipitation caused by evaporation and CO2 degassing (e.g. Salomons et al., 1978 Schlesinger, 1985 Salomons Mook, 1986 Spotl Wright, 1992), which increases the enrichment of C and 0 isotopes. However, it is believed that at depths of a decimetre the evaporation rate is substantially reduced, and there is thus very little opportunity for significant 0 enrichment in soil water before the next rainfall causes sufficient infiltration to obliterate this effect (Hellwig, 1973). 

The cations in productive agricultural soils are present in the order Ca2+ > Mg2+ > K+ > Na+. Deviations from this order can create ion-imbalance problems for plants. High Mg, for example, can occur in soils formed from basaltic serpentine rocks. The Mg inhibits Ca uptake by plants. High Na occurs in soils where water drainage is poor and evaporation rates exceed rainfall. High Na creates problems of low water flow and availability in sods. Low Ca occurs in acid soils. 

In the subsoils of arid and semiarid soils, Ca commonly precipitates as cakite (CaCC>3) rather than being leached away. It is found as indurated layers (caliche and other local names) in many arid soils and as more diffuse CaC03 in Aridisols and Mollisols. Precipitation of CaCCTj in soils is affected by the rates of soil water movement, CO2 production by roots and microbes, CO2 diffusion to the atmosphere, and water loss by soil evaporation and plant transpiration. CaCC>3 layers are also derived from upward movement and evaporation of Ca-rich waters. Calcium carbonate accumulations can amount to as much as 90% of the mass of affected soil horizons. Gypsum precipitates in some arid soils, despite being about 10 x as water soluble as Ca carbonate. 

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