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To predict the effects of water or leachate infiltration into unsaturated field soils requires measurements of both the field-saturated hydraulic conductivity (K fs ) and the matric flux potential (Φ m ). Measurement of these soil parameters commonly requires steady-state infiltration rates, which, in the slowly permeable clay liners that are found underlying municipal landfills, can require several weeks or months to be attained. The transient, capillary-dominated, early-time method of determining K fs and Φ m has been found to provide a much faster, yet still reliable estimate of these flow parameters. Here, an analytical solution was utilized for early-time, one-dimensional infiltration under the conditions of an initial constant head followed by a falling-head phase. Six laboratory trials were conducted using precisely compacted clay cores to determine K fs and Φ m during both early-time and steady-state flow domains. The results of these trials showed a favorable comparison between these two techniques, with the early-time constant-head and falling-head methods producing K fs estimates that were 60 and 75% of their respective steady-state values. This suggests that the early-time determination of K fs and Φ m on slowly permeable media has a definite practical advantage over lengthy steady-state analyses. A new, expeditious procedure for determining steady-state K fs values of highly compacted soil cores, involving use of a Mariotte-equipped pressure plate chamber, was also developed and validated.

The nuclear industry uses computer models to calculate and assess the impact of its present and future releases to the environment, both from operating reactors and from existing licensed and planned waste management facilities. We review four soil models varying in complexity that could be useful for environmental impact assessment. The goal of this comparison is to direct the combined use of these models in order to preserve simplicity, yet increase the rigor of Canadian environmental assessment calculations involving soil transport pathways. The four models chosen are: the Soil Chemical Exchange and Migration of Radionuclides (SCEMR1) model; the Baes and Sharp/Preclosure PREAC soil model, both used in Canada's nuclear fuel waste management program; the Convection-Dispersion Equation (CDE) model, commonly used in contaminant transport applications; and the Canadian Standards Association (CSA) derived release limit model used for normal operations at nuclear facilities. We discuss how each model operates, its timestep and depth increment options and the limitations of each of the models. Major model assumptions are discussed and the performance of these models is compared quantitatively for a scenario involving surface deposition or irrigation. A sensitivity analysis of the CDE model illustrates the influence of the important model parameters: the amount of infiltrating water, V; the hydrodynamic dispersion coefficient, D; and the soil retention or partition coefficient, Kd. The important parameters in the other models are also identified. This work shows we need tested, robust, mechanistic unsaturated soil models with easily understood and measurable inputs, including data for the sensitive or important model parameters for Canada's priority contaminants. Soil scientists need to assist industry and its regulators by recommending a selection of models and supporting them with the provision of validation data to ensure high-quality environmental risk assessments are carried out in Canada. Key words: Soil transport models, environmental impact assessments, model structure, complexity and performance, radionuclides 137Cs, 90Sr, 129I

Keywords: Physique du sol Reference HYDRAM-ARTICLE-1997-013View record in Web of Science Record created on 2005-10-11, modified on 2016-08-08

The determination of the field-saturated hydraulic conductivity, K fs , Using a Guelph Pressure Infiltrometer, typically relies on the establishment of steady-state flow. For media with very low values of K fs , such as the compacted day liners of landfills, the establishment of steady-state flow may take months. It then becomes opportune to rely on early-time observations of infiltration to infer K fs . The time domain in which early-time data analysis is valid can, for the first time, be delineated by equating the flux from both the early-time and steady-state flow expressions. This new delineating boundary value will be indicated by t cr . Thin will be the critical time at which the early-time behavior gradually transits into the steady-state time domain. There is, however, a time zone around t cr in which neither the early-time analysis nor the steady-state analysis is appropriate, as flow is neither purely dominated by capillary forces nor gravitational forces. Results of t cr delineations are presented as a function of two field and clay liner parameters: K fs and Δθ, the change in volumetric water content between initial and field-saturated conditions. Graphical interpretation reveals the existence of the inversely proportional relationships: critical time vs. D fs and critical time vs. applied pressure head. Also evident is the directly proportionate response of the critical time to change in Δθ. We find that the present form of the critical time expression can be reduced to previous formulations concerning characteristic times. We offer these derivations as proof of the validity of this new critical time parameter.

An effective field measurement of the air-entry and water-entry soil water pressure heads gives an improved description of the hydraulic properties of soils. One study obtained measurements of air-entry soil water pressure heads using a modified falling head infiltrometer. This study then estimated the value of the water-entry soils water pressure head from this value. Now direct field measurements of both the air-entry and water-entry soils water pressure heads can be easily obtained using an external tension bottle attached to the Guelph Pressure Infiltrometer at the conclusion of quasi-steady state infiltration readings.

Mathematical theories describing the movement of water and solute away from sources of limited spatial extent mostly assume some form of three-dimensional flow domain, but few have been rigorously tested, mainly because of the lack of experimental data. The objectives of this study were to obtain water flow and solute transport measurements during three-dimensional, axially symmetric water flow, using time domain reflectometry (TDR), and to test some of the existing theories of three-dimensional flow and transport using the measured data. A sandy soil was packed into a Plexiglas box fitted with curved TDR probes at different radial distances from one corner. A constant flux of water was applied at the corner of the box, and a pulse of KCI added as a tracer. Measurements of volumetric water content, θ, and resident solute concentration, C R , as a function of time and radial distance were made along four flowlines. The utility of a quasilinear solution was evaluated by comparing measured and predicted profiles of θ at steady state, and the distribution of solute travel times along various transects. Along the various streamlines, nonlinear least-squares analysis of measured solute travel times (t * ) produced estimates of the slope (α) of the exponential hydraulic conductivity function that were comparable to independently determined values. Estimation of α from t * along a single flowline, using a quasilinear solution, offers potential for field application. This technique could be used as a first step in the investigation of contaminant migration from point sources, or used in trickle-irrigation design.

Transport properties vary considerably over small distances in most soils. The stochastic streamtube model offers one approach to incorporating heterogeneity into transport predictions. This study tested the ability of the streamtube concept to predict transport in heterogeneous fields using measurements from undisturbed columns. Fifty undisturbed columns (0.15-m diam. by 1.5 m deep) were taken every 0.4 m from a 20-m-long transect in a loamy sand soil with variable horizon thickness. Each core was instrumented at 0.1-m intervals with time domain reflectometry probes to measure resident fluid concentrations of a conservative (Cl - ) tracer under steady flow conditions. Large-scale concentration curves of Cl - from solution samplers and coring were obtained from field experiments conducted on the same soil under similar boundary conditions. Differences were observed in the solute spread and mass recovery, but not in the centers of mass. Horizontal scale dependence of transport was observed in the field but not in the columns. This suggests that a higher dimensionality of transport, probably along the horizon interfaces, may be responsible for the observed scale dependence in the field. Although the stochastic streamtube model gave good predictions of the center of mass, it does not appear to be a realistic physical analogue for describing solute dispersion in soils with spatially variable layer thickness

The field-saturated hydraulic conductivity Kfs can be obtained from measurements of the long-time steady flow rates from ring infiltrometers by assuming either that the soil is a “Green and Ampt” soil that has a relationship between hydraulic conductivity K and soil water pressure ψ, such that K = Kfs, 0 > ψ > ψf; K → 0, ψƒ < ψf, or that the soil is a “Gardner” soil with an exponential relationship K = Kfs exp(αψ), where α is a constant. It is shown that infiltration into a Green and Ampt soil from a surface source with zero head is the same as that for a Gardner soil with α → ∞ (gravity dominant) and with a surface source at a head |ψfs|. The shape factor G calculated for Gardner soils using a numerical solution of Richards's equation is used to calculate Kfs and α from ring infiltrometer tests based on Q = [aH/G + a/(αG) + πa2]Kfs, where Q is the steady state flow rate, H is the constant ponded head, and a is the ring radius. The equivalent G factor for Green and Ampt soils calculated using electric analogue solutions of Laplace's equation is shown to agree very well with the numerical solution. The slope of the linear relationship between the shape factor and the depth of insertion of the ring divided by the radius of the ring is shown to have a slope approximately equal to 1/π.

A new procedure is presented for in situ determination of saturated and near-saturated hydraulic conductivity [Kfs and respectively] from a sequence of steady infiltration measurements made at several tensions on a single infiltration surface. The method applies to tension infiltration from either a surface disk or from within a ring inserted a small distance into the soil. The analysis employs a modification of Wooding's solution for infiltration from a shallow pond, combined with numerically determined shape factors that account for the interaction effects between flow geometry and soil properties. The method is found, via numerical simulations, to have an overall accuracy within about ±7%, regardless of whether the predicted function is flat with an indistinct air-entry value, steep with a distinct air-entry value, or very steep with no air-entry value. Some important practical features of the method are that it does not require measurement of the often-difficult square-root-of-time infiltration behavior, no special considerations are required regarding the thickness of the contact layer, it minimizes the effect of local spatial heterogeneity by using only one infiltration surface per set of measurements, it avoids the use of potentially unstable simultaneous-equations solution procedures, and it can be applied to tension infiltration from both disk and ring infiltrometers. View complete article To view this complete article, insert Disc 5 then click button8

A new analysis of steady, ponded infiltration from within a single ring takes soil hydraulic properties, ring radius, depth of ring insertion, and depth of ponding into account. It also provides a means for determining the field-saturated hydraulic conductivity (Kfₛ) and the matric flux potential (φₘ). The analysis employs numerically determined shape factors (G) that are found to depend significantly on ring radius (a) and depth of ring insertion (d), but only slightly on depth of ponding (H) and soil hydraulic properties. As a consequence, averaged G values (Gₑ) can be developed for specified d and a that apply to a wide range of ponded heads and soil types. Procedures for calculating Kfₛ and φₘ are based on G or Gₑ, and on the ponding of one, two, or multiple H levels in the ring. Test calculations based on Gₑ suggest that Kfₛ can be obtained with an accuracy of about ±20% for H = 0.05 to 0.25 m and α = 1 to 36 m⁻¹, where α is the soil parameter of the exponential hydraulic conductivity-pressure head relationship. A similar level of accuracy (using Gₑ) is obtained for φₘ when α is small (α ≤ 4 m⁻¹), and when both α and H are large (α > 4 m⁻¹, H ≥ 0.20 m). Significant errors in φₘ can occur, however, when α is large but H is small. Potentially important features of this single ring method include low sensitivity of the Kfₛ calculation to errors in Gₑ, reduced measurement errors resulting from small-scale soil variability, and the ability to pond large heads in order to increase flow rates in low-permeability materials. LRRC Contribution no. 89-57.

SUMMARY A procedure is proposed for serial sectioning of impregnated porous media, for photographing the sections under the microscope, and for digitizing the image using an IBM PC (XT) with a digitizing board. The result is a three-dimensional matrix containing the enhanced digitized image in a binary form of O's and I's which represent solid and pore spaces respectively. The procedure was tested on three different porous materials: (1) a pack of glass beads, 170–350 μm; (2) Berca sandstone; and (3) Elora clay loam soil. Each of these was impregnated, forty-six to eighty cross-sections were prepared by close parallel polishing of 10–50 μm per section, and then the sections were photographed and digitized. Examples of the real and digitized cross-sectioned samples are given as well as an example of reconstructed images in a plane perpendicular to the plane of sampling. It is proposed that the method of serial sectioning and automated digitization of porous media provides a powerful tool for further three-dimensional geometrical and topological investigation of pore space to be used in models of fluid transport phenomena.

The Guelph permeameter method is used in 0.02-m- and 0.03-m-radius wells to measure in situ the field-saturated hydraulic conductivity (K10) and matric flux potential (φm) of a heterogeneous, anisotropic, structured loam soil. The K10, estimates, obtained using both Richards (GP-R) and Laplace (GP-L

Three-dimensional information on the pore space in porous media, generated either in a continuous or in a discrete manner, was transformed into a geometrical-topological network system of intersecting ellipsoids, using random processes for the investigation of pore size and the reconstruction of the pore structure. It was assumed that: (i) pore space and pore size can be described by an orthogonal three-dimensional system; (ii) pore unit shape can be described by an ellipsoid; (iii) the reconstruction of the medium by randomly packing ellipsoids in space and allowing them to intersect will preserve the topological-geometrical properties of the original medium. The rationale for this approach lies in our inability to describe analytically the pore space geometry and topology of a natural porous medium. Even if this feat could be accomplished, the use of such information for fluid flow studies in porous media would be impractical or impossible because of the complicated boundary conditions imposed by the irregular geometry of the pore space. Instead, the natural pore geometry has been replaced with a geometry that can be handled mathematically, even if only approximately, and the network structure of the pore space is also replaced with a network that can be handled. These concepts find applications in Part II. The results presented here confirm that the proposed concepts have a sound basis. The advantage offered by this study with respect to research of pore space geometry and topology is that it offers an automated and computerized method for obtaining a relatively simple statistical representation of a porous medium.

Six different approximations of the soil sorptivity were applied to three soils at different initial water contents. Parlange's approximation obtained by optimization was found to give excellent results. A new parameter was found to be quite constant over a range of initial water contents and is useful in estimating the sorptivity for changing initial water contents. An exponential diffusivity function was also used to approximate the sorptivity.

The Guelph permeameter method was assessed using laboratory and numerical procedures. Sand tank experiments using air-dry Caledon sand confirmed the theoretical basis of the method, that flow out of a small, uncased well into unsaturated soil rapidly decreases to a steady rate (Qs) and involves a finite wetting region. Saturated flow and field-saturated flow experiments found that the truly saturated hydraulic conductivity Ks, (no entrapped air present) for repacked Caledon sand was approximately a factor of 3.5 greater than the corresponding field-saturated hydraulic conductivity, Kfs (entrapped air present). Horizontal infiltration experiments, which were conducted primarily to measure the metric flux potential (φm) of air-dry Caledon sand, suggested that the discrepancy between Ks and Kfs was the result of an approximate 17% undersaturation due to the entrapped air. Application of the Laplace (K1fs) and Gardner (φ1m) analyses to the sand tank results yields approximately a factor of 12 overestimate of Kfs by K1fs, but only about a 10% overestimate of φm by φlm. This indicates that capillarity (parameterized by φm) dominated field-saturated flow (parameterized by Kfs,) in the sand tank experiments. A numerical model based on Richards' equation was able to accurately simulate the sand tank results using Kfs from the field-saturated flow experiments and φm from the horizontal infiltration experiments. Qs and C values were predicted within ≤2% of the experimental values. The numerical results suggest that Kfs, (rather than Ks) and φm are appropriate parameters for describing flow out of a well into unsaturated, structureless soil; and that the integrally correct representation of the K(ψ) relationship provided by the exponential form of Gardner (1958) and by the method of calculating α, can be adequate for describing both transient and steady flow. A three-member family of numerically derived, unsaturated flow C value curves representing “sand,” “loam and structured clay,” and “unstructured clay” appears to compensate adequately for the neglect of the gravity-capillarity interaction in the Guelph permeameter equations. Sensitivity analyses indicate that for the Richards analysis the correct choice of C value curve from the three-member family of curves is least critical for the hydraulic parameter of primary importance in describing the flow. When field-saturated flow is the dominant component of flow out of the well, the error induced in Kfs, by incorrect choice of C is on the order of ≤30% for the flow conditions analyzed. When capillarity is the dominant component of flow out of the well, the error induced in φm by incorrect choice of C is on the order of ≤10% for the flow conditions analyzed. The sensitivity analyses also confirm that the Laplace analysis is reasonably accurate when the field-saturated component of flow dominates, and that the Gardner analysis is reasonably accurate when the capillarity component of flow dominates.

A procedure for microcomputer-based digitization of images is proposed using an IBM-PC microcomputer and a TECMAR digitizer board. Using cross sections of impregnated porous materials such as glass beads, sandstones and soils, procedures are described to determine the porosity, pore (chord) size distribution, and permeability of porous materials. The estimation of root length and width is also carried out using image digitization. A final example is the measurement of eroded soil areas from aerial photographs.

The saturated hydraulic conductivity, Ks, was measured on a loamy sand, a fine sandy loam, a silt loam and a clay at four 100-m2-area sites in southern Ontario. Twenty measurements of Ks were obtained by each of three different measurement techniques at each of the four sites. The techniques included: (1) the air-entry permeameter method; (2) the constant head well permeameter method using the Guelph Permeameter; and (3) the falling-head permeameter method applied to small soil cores. The Ks data were found to be better described by the log-normal frequency distribution than by the normal frequency distribution. Statistical comparison of the mean Ks values [Formula: see text] indicated significant differences between some or all of the methods within each site. This site-method interaction was interpreted in terms of the influence of macropores and air entrapment on each of the measurement techniques. The measured Ks values ranged over an order of magnitude on the sand, one to two orders of magnitude on the loams, and three orders of magnitude on the clay. The [Formula: see text] estimates averaged over the three methods were: 3 × 10−5 m∙s−1 for the sand; 2 × 10−6 m∙s−1 for the loams and 1 × 10−7 m∙s−1 for the clay. Although all techniques were able to discriminate between the three soil types, the best choice of method for any particular situation appears dependent on the required type and accuracy of the Ks measurement, soil type, and the various practical constraints on the investigation. Key words: Air-entry permeameter, Guelph Permeameter, falling-head permeameter, spatial variability, macropores, entrapped air

A series of functionally terminated poly(perfluoroalkylene oxides), a new class of curable, liquid, fluorinated polymers, has been prepared by the photopolymerization of perfluorooxydipropionyl fluoride, O(CF2CF2COF)2, and related perfluoroacyl fluoride monomers. Polymerization proceeds by ultraviolet-induced scission of CF2COF groups and coupling of the CF2—radicals thus formed. Polymer molecular weight and functionality increase with photolysis time. The acid fluoride-terminated polymers (Mn 1000–5000, functionality 2–5) are liquids possessing low Tg (about −60°C) and excellent, thermal stability. Other functional end groups (e.g., COOH, COOR, CH2OH, etc.) have been prepared using standard reactions.

A series of miscible displacement experiments was conducted in an unsaturated glass bead medium maintained at a constant average water content during each displacement. The variation in the form of the breakthrough curve with decreasing water content was not large and was not related in a simple way to the water content; however, there was consistent shift of the breakthrough curve to the left of the relative concentration value of 0.5 and 1 pore volume and a lone tail or slow approach to the final relative concentration of 1.0. Some of these effects may be attributed to the presence of stagnant liquid in the pores. (11 refs.)

A new apparatus for studying miscible displacement processes in soils is described. Both the soil water content and the pore water velocity can be controlled within certain ranges. The usefulness of this technique is illustrated by a brief description of: (1) the mixing of Cl− during miscible displacement in a glass bead medium; (2) the movement and adsorption of Atrazine, a herbicide, in soil; and (3) the movement and microbiological nitrification of NH4+−N to NO3−−N during unstaturated flow conditions in soil.

W. R. Gardner's recent method for determining capillary conductivity from the transient outflow of a slab following a small step-change of pressure has been extended to cases in which the impedance of the supporting membrane or porous plate must be taken into account. From this theoretical result a practical method is developed employing all of the experimental data (not just the exponential tail) for determining K values accurate within the inherent limits of the data.

Synopsis A summary is presented on the mechanisms of dispersion of miscible contituents in the liquid phase of porous materials. The transport is considered from both a microscopic (within the pore spaces) and macroscopic (an average of a representative number of pores) point-of-view. An attempt has been made to point out some of the problems associated with the use of the physically measurable macroscopic variables when investigating the basically microscopic phenomenon of dispersion. Some simple reactions between soil materials and chemical substances are discussed and some applications to tracer studies are presented.

A series of experiments have been conducted at a constant temperature on the intrusion of salt from a reservoir containing a fixed volume of solution into an underlying long vertical column of porous material, initially saturated with pure water and sealed at the bottom. The variation with time of the salt concentration in the reservoir was measured for different types of porous materials, different initial concentrations, and different mechanical treatments of the solution. The experimental results, substantiated by the theoretical analyses, indicate that the main contributor to the influx of salt into the porous material is the convective dispersion arising from the onset of horizontal variations of the salt concentration and of the vertical velocity component. The rate of this process is governed by a characteristic time where ρU and ρL are the initial densities in the reservoir and in the porous material, respectively; K is the hydraulic conductivity of the porous material with respect to the liquid initially saturating it; and H is a characteristic head of the solution. Approximate expressions have been derived for the rate of decrease of the salt concentration in the reservoir, for the influx of salt into the porous material, and for the advances of the salt front within it.

The macro- and micromorphological characteristics of an Oneida soil profile are reported, together with the results of chemical and hydrological determinations. The data indicate that porosity decreased with depth m this profile, that clay migration was a dominant process, and that dithionite- and oxalate-extractable iron distribution was similar to that reported for Gray-Brown Podzolic soils. An electron microprobe analysis of a dark, thin-section feature in the Bt1 horizon showed it to contain 5–10% Fe, varying amounts of aluminum, low levels of manganese, and traces of phosphorus.

A miscible displacement technique was used to study the movement of EDTA and Zn EDTA in soils. EDTA moved quite readily through the soil. The EDTA from a Zn EDTA complex also moved readily through the soil cores; however, the Zn was replaced to a large extent. If the Zn EDTA complex remained intact, then it moved through the soil with the liquid phase. These data have implications on the use of chelated metals for plant nutrition. If the desired element remains in the chelate complex, it could move with water and thus can be leached out of a soil. If the complex breaks down in the soil, the metal can react with the soil. In either case, the desirable element may not remain available to plants.