Dan Zaslavsky

Solute diffusion coefficient in the internal medium of a new gel based controlled release fertilizer

Abstract The diffusion of solutes in a new controlled release device was investigated and in situ measurements of constant and variable diffusion coefficients were obtained. The new controlled release device consists of a dry mixture of fertilizer and gel forming thickener contained in a nonpermeable coating having at least one opening. Water penetrates into the device through the opening, forms a gel and dissolves the fertilizer, which is then released by Fickian or non-Fickian diffusion mechanisms. Based on measurements of the penetration of water and the dissolution of fertilizer, the pseudo-steady state transport equation was solved and the solute diffusion coefficient was calculated. The computation of the diffusion coefficient was possible solely because a dual boundary condition exists at the dissolution front. An analytical solution was developed assuming a constant diffusion coefficient. A numerical solution was obtained for the case where the diffusion coefficient is concentration dependent. Two thickeners were tested: sodium carboxymethylcellulose (Na-CMC) and sodium polyacrylamide (Na-PAM). It was found that the solute diffusion mechanism in Na-CMC gels is Fickian-like and can be approximated by a constant diffusion coefficient. The solute diffusion in Na-PAM gels showed non-Fickian behavior and was estimated numerically using the variable diffusion coefficient. For comparison with the theoretical solutions, a dialysis cell was used to simulate the conditions in the gel formed inside the device and to evaluate the effects of thickener concentration on the diffusion coefficient. Reasonable agreement was found between the pseudo-steady state solutions and the dialysis cell experimental results.

Lateral Flow in a Layered Profile of an Infinite Uniform Slope

A numerical solution was used to calculate vertical infiltration of rainfall into a periodic layered profile of an infinite uniform slope. The calculated pressure-head distribution within the profile was then used to evaluate transient un saturated lateral flow in the slope which is not limited to the existence of an impervious barrier. Because of the pressure-line distribution within the layers, the horizontal component of the lateral flow changes direction from one layer to another. As a result, the net discharge across the profile can be directed upstream during the initial stages of infiltration into a relatively dry profile. Over longer times it is always directed downstream, and its value is proportional to the slope. This indicates that the layered soil profile behaves as an anisotropy media on the point scales and on the average. For a two-periodic layered profile the total volume that passes in the direction parallel to the slope is very nearly proportional to the square of the total rainfall, and it is represented versus the accumulated rainwater by one curve for all rainfall intensities. Thus the total rainfall is controlling the rate of lateral flow rather than the rainfall intensity. The thickness of the soil layers has only a secondary effect on lateral flow. The net volume that moves horizontally is a potential source for water accumulation at concave parts of a regular slope. This water accumulation can form saturated or near-saturated wedges that may contribute to surface-runoff generation at these partial areas for cases of low rainfall intensities and short ponding times. These wet zones of the hillslope may also contribute massive recharge of water and chemicals (if exist) to groundwater in narrow columns.

SMALL-PERTURBATION SOLUTION FOR STEADY NONUNIFORM INFILTRATION INTO SOIL SURFACE OF A GENERAL SHAPE

A steady state solution is developed for three-dimensional infiltration where both the water flux at the soil surface and the soil surface topography are nonuniform. By identifying a small parameter, which is defined as a ratio between the vertical and the horizontal characteristic lengths, the solution of the linearized equation can then be found by using perturbation methods. The spatial variation of both the soil surface topography and the water flux entering the profile appears only in parametric form in the equations and boundary conditions to all orders of approximation, thus allowing easy evaluation of the solution. The water accumulation due to soil surface topography is related to the inner produce between the gradients of the surface elevation and the rate of infiltration at the soil surface. Therefore the nonuniformity of the soil surface should be accompanied by a spatial nonuniformity of the applied flux in order to produce lateral variations in the solution. This method can be used to evaluate the water accumulation in the soil due to general rainfall or irrigation distributions. It can also be used for design purposes in cases where one or both sources of nonuniformities in the horizontal directions can be controlled, as for example, in an irrigation system in a field with a given topography.

Field evaluation of methods of incorporating soil conditioners

Abstract A comparison was made of three methods of applying soluble anionic polymers to improve the structure of field soils: spraying and rototilling, spraying and mixing, and application through a drip irrigation system. Structural state improvement was determined in terms of changes in aggregate stability to water obtained from wet sieving. It was found that the most efficient method of adding soluble conditioners in the field was by irrigation. Conditioning by drip irrigation increased the mean diameter of water stable aggregates in the wetting zone around the drippers where higheset rood densities are found. Mixing soil and conditioner in a concrete mixer was effective in improving soil structure but was tedious. Application of this method was found suitable for conditioning of limited amounts of soil, and could be considered only for small operations. Repeated spraying followed by rototilling was found to be the least effective in terms of soil structure improvement. This method was also much less convenient and more complicated than application by irrigation.

FLOW IN A SOIL LAYER UNDERLINED BY AN IMPERMEABLE MEMBRANE

The present article elaborates the idea of using natural or artificial slopes as contributors of water to lands under cultivation. The process of lateral flow in a sloped soil layer bordered by a soil surface and an impermeable membrane is considered. Solutions are presented in nondimensional form. An interesting phenomenon, which broadens our ideas about unsaturated filtration, was obtained.

SATURATION-UNSATURATION TRANSITION IN INFILTRATION TO A NON- UNIFORM SOIL PROFILE

A BRIEF REVIEW IS PRESENTED OF THE PROBLEM OF STEADY INFILTRATION INTO SOIL. EXPERIMENTS FOR DETERMINING THE TRANSITION FROM UNSATURATED TO SATURATED SOIL WERE CONDUCTED IN A COLUMN OF SAND, INSTALLED WITH A SERIES OF TENSIOMETERS. DIRECT MEASUREMENTS SHOWED THAT THE HYDRAULIC CONDUCTIVITY IN THE COLUMN WAS NOT UNIFORM. THE COLUMN WAS TREATED AS A GRADUALLY VARYING PROFILE ACCORDING TO THE GIVEN EQUATIONS. THE EXPERIMENTS CLEARLY DEMONSTRATE THE VALIDITY OF THE THEORY OF TRANSITION. IT WAS SHOWN THAT THE PRESENCE OF GROUND WATER DOES NOT INFLUENCE THE INFILTRATION RATE IN ANY WAY, UNLESS IT COMES VERY NEAR THE INTERFACE BETWEEN LAYERS. IT IS CONCLUDED THAT IN THE PRESENCE OF CONTROLLED LAYERS AT THE SURFACE, MEASUREMENTS WITH INFILTRATION RINGS NEED NO BUFFERING AT THE CIRCUMFERENCE TO PRESERVE VERTICAL FLOW. INFILTRATION RATE IS UNAFFECTED BY THE FLOW DIVERGENCE BELOW THE CONTROL LAYER. THE CONCEPT OF A CONTROL LAYER AT THE POINT OF WATER ENTRY MAY BE QUALITATIVELY EXTENDED TO TWO DIMENSIONAL AND THREE DIMENSIONAL FLOW. A FIGURE IS PRESENTED WHICH ENABLES ONE TO DEFINE SOME EQUIVALENT LAYER OF FLOW RESISTANCE AND AN EQUIVALENT HYDRAULIC CONDUCTIVITY. THIS APPROACH IS USEFUL FOR EVALUATING LEAKAGE FROM RESERVOIRS WITH COMPLEX TOPOGRAPHY AND FOR COMPUTING INFILTRATION RATES IN AREAS OF ARTIFICIAL GROUND WATER RECHARGE.

Flocculation of clay suspensions by an anionic soil conditioner

Abstract Ca- and Na-montmorillonite and Na-kaolinite were flocculated from suspension by two anionic soluble polymers. Light absorbance at 280 nm served as degree of flocculation measure. A prerequisite for flocculation was that salt concentration in clay suspension should exceed 1.0 and 0.2 meq. l−1 for Na- and Ca-montmorillonite, respectively. Furthermore it was shown that the clay had to adsorb a minimum amount of polymer: 0.8 mg/g for a 0.735 g l−1 Na-montmorillonite suspension and 1.0 mg/g for a 0.436 g l−1 Ca-montmorillonite suspension. Saturation adsorptions of polymers on Na- and Ca-clays were estimated to be 4.6 and 7.0 mg/g. These levels of adsorption stabilized the suspension and prevented flocculation. Na-polymetaphosphate (Calgon) added to a Na-montmorillonite suspension after the polymer addition retarded flocculation whereas its addition at a level >2.7 mg/g prior to the polymer addition prevented flocculation. Increasing the pH of the Na-kaolinite suspension from 6.5 to 8.2 considerably reduced its flocculation by the polymer. The effects of both pH and Na-polymetaphosphate indicate the importance of the positively charged sites on the clay in adsorption of anionic polymers and contribute to understanding of flocculation mechanisms.

INFILTRATION THROUGH SOIL WITH A SLIGHTLY PERMEABLE BURIED MEMBRANE

This article elaborates upon the idea of using membranes to slow down the loss of water from the root zone. A number of reasons lead us to the conclusion that the use of slightly permeable membranes is the optimal solution to achieve this goal. We consider the simultaneous process of filtration and evapotranspiration in a soil profile bordered on one side by a soil surface and on the other side by a permeable membrane placed parallel to it. Solutions are presented in nondimensional form. Concrete examples are given of the use of these solutions in practical applications.