Apparent solute soil-water

The last two assumptions are the most critical and are probably violated under field conditions. Smith et al. (3) found that at least a half-hour was required to achieve adsorption equilibrium between a chemical in the soil water and on the soil solids. Solution of the diffusion equation has shown that many volatile compounds have theoretical diffusion half-lives in the soil of several hours. Under actual field conditions, the time required to achieve adsorption equilibrium will retard diffusion, and diffusion half-lives in the soil will be longer than predicted. Numerous studies have reported material bound irreversibly to soils, which would cause apparent diffusion half-lives in the field to be longer than predicted. 

Adams, F., Burmester, C., Hue, N. V., and Long, F. L. (1980). Comparison of column-displacement and centrifuge methods for obtaining soil solution. Soil Sci. Soc. Am. J. 44, 733—735. Amoozegar-Fard, A. D., Nielsen, D. R., and Warrick, A. W. (1982). Soil solute concentration distribution for spatially varying pore water velocities and apparent diffusion coefficients. Soil Sci. Soc. Am.J. 46, 3—9. 

To evaluate the hypothesis that the undersaturation of the soil water may be due to the kinetics of the calcite solution processes, calculations of the solution rate were performed using both the data of Morse (1978) and Plummer et al. (1979) and are shown in Table III. It is apparent that there is little agreement, either in the overall average rates or in the calculated change... 

Electrical conductivity (or its mathematical inverse, resistivity) of a soil solution is strongly correlated with total salt content. Therefore, laboratory methods involving solution or saturated paste conductivity are often used to assess soil salinity. Electrical conductivity measurements of bulk soil (designated as ECa for apparent electrical conductivity) were also first used to assess salinity. Resistivity and conductivity measurements are also useful for estimating other soil properties, as reviewed by and. Factors that influence ECa include soil salinity, clay content and cation exchange capacity (CEC), clay mineralogy, soil pore size and distribution, soil moisture content, and temperature. ° For saline soils, most of the variation in ECa can be related to salt concentration. In non-saline soils, conductivity variations are primarily a function of soil texture, moisture content, bulk density, and CEC. The theoretical basis for the relationship between ECa and soil physical properties has been described by a model where ECa was a function of soil water content (both the mobile and immobile fractions), the electrical conductivity of the soil water, soil bulk density, and the electrical conductivity of the soil solid phase.Later, this model was used to predict the expected correlation structure between ECa data and multiple soil properties. ... 

Retention of organic contaminants on subsurface solid phase constituents in general is not completely reversible, so that release isotherms differ from retention isotherms. As a consequence, the extent of sorption depends on the nature of the sorbent. Subsurface constituents as well as the types of bonding mechanisms between contaminants and the sohd phase are factors that control the release of adsorbed organic contaminants. Saltzman et al. (1972) demonstrated the influence of soil organic matter on the extent of hysteresis. Adsorption isotherms of parathion showed hysteresis (or apparent hysteresis) in its adsorption and desorption in a water solution. In contrast, smaller differences between the two processes were observed when the soils were pretreated with hydrogen peroxide (oxidized subsamples) to reduce initial organic matter content. The parathion content of the natural... 

For both soils studied, comparison of calcium-effluent histories predicted by the solution to Equation 6 with those obtained from experimental columns gave good agreement only for the lowest flow rates. For the three higher water fluxes, more apparent dispersion was observed than could be explained by predictions that assume local equilibrium. Examples of these comparisons are shown in Figures 1 and 2. 

The problem of the suspension effect can be avoided in the glass electrode-reference electrode cell by inserting the reference electrode with its salt bridge junction into the supernatant, above the settled soil colloidal particles. In any event, the effect can be anticipated to produce an important error only in soil suspensions with a large content of colloidal material and low salt concentration. If the pH is measured in a salt such as 0.01 A/CaCb, the suspension effect is suppressed, apparently by the ability of the excess cations in soil solution to lower the tendency for from the salt bridge to diffuse to exchange sites. This is one reason that soil pH is sometimes measured in 0.01 MCaCb rather than in distilled water. 

The apparent diffusion coefficient, D, was determined for the particular leaching conditions of each of the thirteen experiments. This was accomplished using the measured chloride breakthrough (effluent concentration) curve and the analytical solution to Equation 7 with Kd==0. Examples of the observed and calculated chloride concentrations (determined by adjustment of D until a best fit was obtained) are presented for three different experiments (Experiments 7, 8, 11) in Figures 2-4. Values of D and the pore water velocity (v) determined for each experiment are presented in Table III. The value of D increased for cases with large v, and was different between soils for any particular v. This is consistent with the basic relationship be-... 

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