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IN SITU, ON-SITE AND LABORATORY MEASUREMENTS OF SOIL AIR PERMEABILITY: BOUNDARY CONDITIONS AND MEASUREMENT SCALE

Iversen, B. V.1; Schjønning, P.1; Poulsen, T. G.2; Moldrup, P.2

Articles

The measurement of soil air permeability is a fast and easy method that can be used in different ways to characterize the soil. An air permeameter was constructed in order to measure air permeability (ka) in situ, on-site (exhumed soil samples), and in the laboratory on a wide range of Danish agricultural soils. Two different sizes of sample rings were used (100 cm3 and 3140 cm3). The device was initially tested in the laboratory on repacked soil samples to evaluate dependency of ka on sample size. The results showed consistent values of ka for both sample sizes, indicating only little scale effect. In the field, air permeability was measured in situ and on-site using large sample rings. Air permeability in situ was determined by using a "shape factor" taking into account boundary conditions at the lower end of the ring while assuming isotropic soil conditions. An expression for the shape factor developed by Liang et al. (1995) was used. The results from the two measurement methods compared well, indicating reliable air permeability values using the expression of Liang et al. for the soils studied. Air permeability in structured soil measured using exhumed samples of different size showed that small samples generally yielded lower values and higher variability in ka than large samples, in accordance with the concept of a representative elementary volume.

The quality and reliability of prediction by simulation models describing subsurface processes rely heavily on the quality of the input data relating to the soil characteristics. The increased use of distributed models in connection with geographical information systems has increased the amount of data needed and focused on knowledge of the spatial variability of soil physical parameters. When modeling dynamic water transport through the unsaturated zone, the saturated hydraulic conductivity (Kw) is an essential parameter. Measurements of Kw are often time demanding, and the quality of the measurements may not be proportional to the amount of time used. Determination of Kw from more easily obtainable and/or readily available soil properties has been proposed in different studies (Gimenez et al., 1997; Mckenzie and Jacquier, 1997; Poulsen et al., 1999; Timlin et al., 1999). Relating Kw to (intrinsic) air permeability (ka) has been proposed by Schjønning (1986), Riley and Ekeberg (1989), Blackwell et al. (1990), Loll et al. (1999).

Air permeability can be used to determine the pore characteristic of soils. Blackwell et al. (1990) used ka and air-filled macroporosity to characterize the soil and used this relationship to identify changes in soil structure caused by soil management practices and biological activity. Ball (1981) used gas diffusivity, ka and air-filled porosity to describe the continuity and tortuousity of macropores in the soil. Roseberg and McCoy (1990) measured ka at different water contents at and near saturation to analyze macropore behavior of the soil. Knowledge of ka is also useful in relation to modeling soil vapor extraction systems for remediation of soils contaminated with volatile organic compounds (Poulsen et al., 1996; Moldrup et al., 1998).

Several studies have proposed methods for measuring ka in situ (Kirkham, 1947; Grover, 1955; Steinbrenner, 1959; Green and Fordham, 1975; Bowen, 1966 and 1985; Fish and Koppi, 1994). Grover (1955) developed a simple air permeameter that consisted of a hollow float connected to the soil sample by a tube and an annular water-filled reservoir. Steinbrenner (1959) developed a portable air permeameter with the purpose of measuring macroscopic pore space in situ. Green and Fordham (1975) developed an air permeameter with a diameter of 5.1 cm to be used for soil samples. The permeameter consisted of a small cylinder containing compressed air controlled by precision pressure regulators. By means of this device, ka could be measured with the sample placed in situ in the soil or with the core exhumed. Bowen (1966, 1985) used a modified version of Grover's (1955) air permeameter, where the process of reading the instrument was improved by incorporating a sensitive flowmeter and manometer. Fish and Koppi (1994) constructed an air permeameter with a sample diameter of 18 cm and measured the pressure difference through the soil sample with a digital manometer.

When measuring ka in situ (i.e., with the sample still in place in the soil), the air pressure at the lower end of the sample is not known because the air still has to flow through an (unknown) volume of soil before it reaches the soil surface. Measurements of ka carried out by Green and Fordham (1975) showed that the flow rate for exhumed cores was 1.4 to 2.5 times larger than for in situ cores using the same pressure difference; they concluded that ka measured in situ required careful interpretation of the results. The consequence of the lack of boundary conditions means that a "shape factor" has been introduced in the calculation of ka, talking into account the geometry of the flow lines when the air leaves the lower part of the measuring cylinder in the soil. From experimental data using an electrolytic model (Frevert, 1948), Grover (1955) produced a nomogram for estimating the shape factor for different sample diameters and sample depths. Kirkham et al. (1958) later discovered an error in Grover's nomograms, such that the ratio between the shape factor and the sample diameter was four to five times too large, in agreement with results from Boedicker (1972). Liang et al. (1995) carried out a thorough investigation using finite element modeling to describe airflow through a homogeneous isotropic soil medium and developed an expression for determining the shape factor. Liang et al. tested their new expression in the laboratory on repacked fine loamy sand in a metal soil container that was 30.5 cm in diameter and 34.3 cm high. They found that calculated ka values were close to the actual measured values. The validity of the shape factor expression developed by Liang et al. has, however, not been tested for a range of soil types and soil horizons.

The objective of this study was to develop a portable air permeameter capable of measuring air permeability in situ, on-site (exhumed soil samples), and in the laboratory using two different sizes of core samples (100 cm3 and 3140 cm3). A second objective was to test the shape factor expression by Liang et al. (1995) under field conditions for different agricultural soils and to test scale dependency between the two measurement scales.

1Danish Institute of Agricultural Sciences, Department of Crop Physiology and Soil Science, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark. B.V. Iversen is corresponding author. E-mail. bo.v.iversen@agrsci.dk

2Department of Civil Engineering, Aalborg University, Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark

Received June 16, 2000; accepted September 5, 2000

© 2001 Lippincott Williams & Wilkins, Inc.