Tim W. J. Bauters

Physics of water repellent soils

Although it is generally well known that water repellent soils have distinct preferential flow patterns, the physics of this phenomenon is not well understood. In this paper, we show that water repellency affects the soil water contact angle and this, in turn, has a distinct effect on the constitutive relationships during imbibing. Using these constitutive relationships, unstable flow theory developed for coarse grained soils can be used to predict the shape and water content distribution for water repellent soils. A practical result of this paper is that with a basic experimental setup, we can characterize the imbibing front behavior by measuring the water entry pressure and the imbibing soil characteristic curve from the same heat treated soil. q 2000 Elsevier Science B.V. All rights reserved.

Lateral expansion of preferential flow paths in sands

The stability and persistence of preferential flow paths in sands can determine the flow paths of subsequent infiltration events. We have measured the evolution of preferential flow paths in a slab of sandy soil using an array of tensiometers and light transmission. The pressure and water content measurements show that the nonuniform moisture content exists even when the potentials are equalized horizontally and that then are the result of hysteresis in the soils pressure-saturation relationship. The equalization of potential takes place over several days, if at all, and is consistent, initially, with estimates of vapor transport out of the finger cores. Once the soil is wet enough, the remainder of water movement takes place in liquid films. Hysteresis produces another interesting situation when the pack is drained. We find that the wetter portions of the soil can be at a lower potential than the drier portions, resulting in a horizontal driving force for a flow of water from the drier to the wetter soil.

Measurement of fluid contents by light transmission in transient three-phase oil-water-air systems in sand

Most three-phase flow models lack rigorous validation because very few methods exist that can measure transient fluid contents of the order of seconds of whole flow fields. The objective of this study was to develop a method by which fluid content can be measured rapidly in three-phase systems. The method uses the hue and intensity of light transmitted through a slab chamber to measure fluid contents. The water is colored blue with CuSO4. The light transmitted by high-frequency light bulbs is recorded with a color video camera in red, green, and blue and then converted to hue, saturation, and intensity. Calibration of hue and intensity with water, oil, and air is made using cells filled with different combinations of the three fluids. The results show that hue and water content are uniquely related over a large range of fluid contents. Total liquid content is a function of both hue and light intensity. The air content is obtained by subtracting the liquid content from the porosity. The method was tested with static and transient experiments. Measurements made with the light transmission method (LTM) and synchrotron X rays of the static experiment agreed well. In the transient experiments, fingers were formed by dripping water on the surface in a two-dimensional slab chamber with partially oil-saturated sand. The LTM is able to capture the spatial resolution of the fluid contents and can provide new insights in rapidly changing, three-phase flow systems.

Numerical simulation of experimental gravity-driven unstable flow in water repellent sand

Abstract Laboratory experiments related to gravity-driven unstable flows in water repellent porous media contained in two-dimensional chambers have been reported [Bauters, T.W.J., DiCarlo, D.A., Steenhuis, T.S., Parlange, J.-Y., 1998. Preferential flow in water-repellent sands. Soil Sci. Soc. Am. J. 62, 1185–1190]. These experiments demonstrate that water repellency has a significant impact on the stability of flow. As a follow up to these experiments, numerical solutions of the Richards equation for a two-dimensional domain are derived to examine the effect of water repellency on flow characteristics. Of particular interest is the development of gravity-driven unstable flow conditions caused by water repellency. The degree of water repellency of the porous medium is manifested in the water saturation—capillary pressure and water saturation—hydraulic conductivity relationships for the porous medium. To derive the numerical solutions, parameters closely representing the flow domain boundary conditions and the porous medium properties in the experiments of Bauters et al., were employed. In this paper we present the results of simulations for two cases: a water wettable sand and an extremely water repellent sand. The numerical solution for the water wettable sand led to a stable flow condition, while for the water-repellent sand the flow was unstable as manifested by the development of a single finger of flow. A new feature of these modeling results, in comparison to previous modeling results for gravity-driven unstable flow, is that the water pressure inside the finger core is positive. In testing the numerical solutions we compared the solution results to the laboratory results in terms of flow patterns, water pressure at a single reference point, and wetting front velocity. The degree of agreement between the laboratory results and the numerical solutions in terms of these measures is quite good.

HIGH-SPEED MEASUREMENTS OF THREE-PHASE FLOW USING SYNCHROTRON X RAYS

Accurate monitoring of flow instabilities, which can occur when nonaqueous phase liquids (NAPLs) flow through porous media, is an important component of predicting the transport and fate of these compounds in the subsurface. In particular, flow situations in which three mobile phases (such as water, NAPL, and air) exist in the porous media are inherently complex. Unfortunately, the relatively low source intensities and the nontunable source energies make traditional dual gamma techniques unsuitable to study flow instabilities which can change within seconds. We present an alternate technique, which uses synchrotron X rays from the Cornell High Energy Synchrotron Source (CHESS) to measure three-phase fluid saturations on the time scale of seconds. Using the harmonic content resulting from X ray diffraction, we obtained a high-intensity X ray beam consisting of distinct tunable energies. Three-phase saturations were measured on 5-s timescales during fingering of light NAPL into regions of dry and water wet sandy soil. In the water wet soil the oil finger was less saturated, slower, and wider than the same finger in the dry soil. The results yield insights into the nature of three-phase preferential flow.

Dual-energy synchrotron X ray measurements of rapid soil density and water content changes in swelling soils during infiltration

Understanding soil swelling is hampered by the difficulty of simultaneously measuring water content and bulk density. A number of studies have used dual-energy gamma rays to investigate soil swelling. The long counting time of this technique makes it impracticable for studying the rapid changes in moisture content and soil swelling shortly after infiltration is initiated. In this paper, we use the dual-energy synchrotron X ray to measure, for the first time, the water content and bulk density changes during the fast, initial phase of the swelling process. Ponded infiltration experiments were performed with two soils: a bentonite-sand mixture and a vertisol. Swelling curves and hydraulic diffusivity were determined. Deformation was very rapid immediately after water application and then became progressively slower. The hydraulic diffusivity decreased with time, which can partially explain the very rapid decrease in infiltration rates observed in the field.

Visualization and Measurement of Multiphase Flow in Porous Media Using Light Transmission and Synchrotron X-Rays

Abstract: Non‐aqueous phase liquids enter the vadose zone as a result of spills or leaking underground storage facilities, thus contaminating groundwater resources. Measuring the contaminant concentrations is important in assessing the risk to human health and the environment and to develop effective remediation. This research presents the development and application of the light transmission method (LTM) for three‐phase flow systems, aimed at investigating unstable fingered flow in a soil‐air‐oil‐water system. The LTM uses the hue and intensity of light transmitted through a slab chamber to measure fluid content, since total liquid content is a function of both hue and light intensity. Evaluation of the LTM is obtained by comparing experiments with LTM and synchrotron X‐rays. The LTM captures the spatial resolution of the fluid contents and can provide new insights into rapidly changing, two‐phase and three‐phase flow systems. Application of the LTM as a visualization technique for environmental and physical phenomena is noted. Visualization by LTM of groundwater remediation by surfactants as well as visualization of model cluster growth and fractal dimensions was also explored.

Surfactant-induced changes in gravity fingering of water through a light oil

Gravity-driven preferential flow (fingering) can greatly affect how one fluid displaces another in the subsurface. We have studied the internal properties of these preferential flow paths for water, with and without surfactants, infiltrating into oil saturated porous media using synchrotron X-rays, and miniature tensiometers to characterize fluid content and pressure relationships. We also used a light transmission technique to visualize overall flow pattern. Capillary pressure and water content decrease behind the front, similar to fingers in air-dry sand, with quantitative differences for five different surfactants with surface tensions ranging from 4–21 g/s2. Using unstable flow theory, the finger widths, capillary pressure drops within the fingers, finger tip lengths, and finger splitting dynamics were scaled successfully with interfacial tension, fluid density, and the contact angle using the fingers in air–water systems as the reference.