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Published online 20 November 2006
Published in Vadose Zone J 5:1257-1263 (2006)
DOI: 10.2136/vzj2006.0063
© 2006 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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ORIGINAL RESEARCH

Field Testing of a Soil Corer Air Permeameter (SCAP) in Desert Soils

Karletta Chief*, Ty P. A. Ferré and Bart Nijssen

Department of Hydrology and Water Resources, Univ. of Arizona, Tucson, AZ 85721. B. Nijssen, currently at 3TIER Environmental Forecast Group, Inc., 2001 Sixth Ave., Suite 2100, Seattle, WA 98121
* Corresponding author (kchief{at}hwr.arizona.edu)

Received 24 April 2006.



   ABSTRACT
TOP
 EXECUTIVE SUMMARY
 ABSTRACT
INTRODUCTION
 THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Measurements of air permeability are useful both directly, to assess gas phase movement through soils, and indirectly as a proxy for soil hydraulic conductivity. Several designs have been presented for field-based air permeameters; however, none are suitable for use in gravelly desert soils. We designed an air permeameter compatible with a standard soil corer to facilitate insertion into desert soils. The soil corer air permeameter (SCAP) uses digital components to measure flow rates under low-pressure gradients to improve accuracy, ease of use, and portability. The SCAP allows the extraction of undisturbed soil samples for laboratory analysis, providing direct comparisons of air permeability (ka) with other soil physical and hydraulic properties. The soil sample can be extracted before measuring ka, thus removing the need for a shape factor to account for divergent flow. In an analysis of the SCAP's performance, field testing showed that digital components operate well under field conditions; however, spanner wrench insertion holes must be sealed to provide correct ka measurements. The Jalbert and Dane shape factor for in situ ka is applicable for tilled, unstructured soils. However, significant soil-specific variability exists in the shape factor, so we recommend measuring ex situ ka in the field, where possible. Ex situ field ka and laboratory hydraulic conductivity (Ksat) measurements were compared and air to water permeability ratios were calculated to determine structural changes due to water saturation. For soils that could be extracted with minimal structural change, we found good correlation between ka and Ksat and reasonable agreement with previously published results.

Abbreviations: JDSF, Jalbert and Dane shape factor • SCAP, soil corer air permeameter


   INTRODUCTION
TOP
 EXECUTIVE SUMMARY
 ABSTRACT
 INTRODUCTION
THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
AIR PERMEABILITY DESCRIBES the ability of air to move through the subsurface and gives indirect information about the hydraulic properties and structure of the soil (Klinkenberg, 1941; Loll et al., 1999; Fish and Koppi, 1994; Reeve, 1953). Quantification of air permeability can be important for the design of soil vapor extraction efforts and for providing optimal aeration of agricultural soils (Moldrup et al., 1998; Glinski and Stepniewski, 1985). In addition to these direct uses, it may be possible to estimate the saturated hydraulic conductivity based on air permeability measurements (Iversen et al., 2001a). This offers practical advantages over more cumbersome, time-consuming direct measurements of this important hydraulic property. Several designs have been described for air permeameters (Grover, 1955; Steinbrenner, 1959; Green and Fordham, 1975; Fish and Koppi, 1994; Iversen et al., 2001b; Jalbert and Dane, 2003). Iversen et al. (2001b) designed an insertion air permeameter that makes use of a relatively large diameter (20-cm diameter by 20-cm height) hand-driven ring for characterizing agricultural soils. Jalbert and Dane (2003) designed a surface-contact air permeameter as well as an air permeameter that is inserted into the soil. We have developed an alternative design for an air permeameter for insertion into desert soils, which are often too hard to allow manual insertion and too gravelly to allow surface measurements. The SCAP was developed to work with commercially available soil corers, making use of their driving mechanisms and sample retainer cylinders. In addition to being suitable for use in desert soils, this design provides a standard soil sample that can be subjected to further laboratory measurements.

We analyzed the performance of the SCAP. First, we examined the accuracy of using digital flow meters under field conditions and determined measurement errors due to instrumentation, specifically the effects of spanner wrench insertion holes common in standard soil core barrels. We then examined the applicability of published shape factors to the SCAP design for determining in situ ka. Finally, we compared air permeability measurements made with the SCAP to hydraulic conductivity measurements made on soil samples recovered with the SCAP to determine whether there is a correlation between these parameters.


   THEORY
TOP
 EXECUTIVE SUMMARY
 ABSTRACT
 INTRODUCTION
 THEORY
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Air permeability is defined as the ability of soil to transmit air through interconnected air-filled pores under an imposed air pressure gradient. Like hydraulic conductivity, air permeability is a function of volumetric water content, porosity, pore size distribution, and pore geometry (Roseberg and McCoy, 1990). Unlike water flow, the flow of gases through porous media has been shown to exhibit non-Darcian "slip flow" in that velocities near the pore wall are nonzero (Klinkenberg, 1941). Weeks (1978) demonstrated that "slip flow" is only significant for soils with air permeability <0.01 µm2, such as soils with high silt and clay fractions (Springer et al., 1995). For most soils, air permeability can be measured by imposing controlled, isothermal, steady-state, laminar air flow conditions and applying Darcy's law.

This study focused on the use of SCAP for the direct measurement of air permeability using the Darcian air flux equation for soil moisture at field capacity or drier. Assuming that the air permeability is equal to the intrinsic permeability under relatively dry conditions, the Darcian air flux density, q [L/T], can be related to the measured air permeability, ka [L2], the fluid dynamic viscosity, {eta} [M/LT], and the sum of the change in pressure P [M/LT2] across a distance z [L], and the product of the fluid density {rho} [M/L3] and the acceleration due to gravity g [L/T2]:

Formula 1[1]
Because gases have a very low density, the gravitational term {rho}g is <1% of the pressure term and is negligible (Springer et al., 1995), so Eq. [1] can be simplified to

Formula 2[2]
The flux can be expressed as the flow, Q [L3/T], per unit area perpendicular to flow, As [L2] through a path length, L [L]. The pressure gradient can be defined using the discrete pressure difference between the inlet and outlet of a permeameter, Pi and Po. Solving for ka gives

Formula 3[3]
With the SCAP, air permeability measurements can be made in situ, with the instrument inserted in the soil, or ex situ, after the soil corer has been removed from the soil (Fig. 1 ). For ex situ measurements, the outlet pressure is atmospheric pressure. For in situ conditions, the outflow pressure is higher than atmospheric, and air only reaches atmospheric pressure after flowing back up to the soil surface. As a result, the path length, L, over which the pressure decreases from the inlet pressure to atmospheric pressure, is not equal to the inserted height of the column (H). Given the design of the SCAP, which facilitates the extraction of soil samples for other measurements, ex situ measurements can be made rapidly in the field, requiring little more time than in situ measurements.


Figure 1
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  		Fig. 1. Air flow lines associated with ex situ and in situ air permeability measurements. Ex situ measurement is made on a stand, as shown.

 
The effects of divergent flow beyond the outflow end of an inserted air permeameter are addressed by assuming that the flow paths are the same for all measurement conditions, allowing for the introduction of a constant shape factor, A [L]. The shape factor is defined as As/H, reducing Eq. [3] to

Formula 4[4]
For in situ measurements, Liang et al. (1995) developed a finite element air flow model (ANSYS F) to estimate the shape factor based on the air permeameter diameter and height of insertion into a homogeneous and isotropic medium (valid for diameter/height ratio <10). More recently, Jalbert and Dane (2003) used a two-dimensional unsaturated flow finite element model (HYDRUS2D, Simunek et al., 1996) to estimate the shape factor in a larger simulation volume, leading to

Formula 5[5]
where D is the diameter of the ring [L], and H is the height of ring inserted into the soil [L].

In addition to the effects of slip flow, air permeability under air-saturated conditions, ksa, can be higher than water permeability under water-saturated conditions, ksw, due to air entrapment, shrinkage, and crack formation (Gullich and Werner, 1984; Al Jibury and Evans, 1965). Reeve (1953) demonstrated through field and laboratory experiments that the ratio of ka to water permeability, kw, is related to the stability of the soil structure. Specifically, a ratio >1 indicates that the soil has experienced slaking, swelling, dispersion, or other processes during wetting. These considerations suggest that while air permeability may be used to estimate the intrinsic permeability, and therefore the saturated hydraulic conductivity, site-specific conditions can lead to a complex relationship between these properties.


   MATERIALS AND METHODS
TOP
 EXECUTIVE SUMMARY
 ABSTRACT
 INTRODUCTION
 THEORY
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Air Permeameter
The SCAP design is based on the air permeameter presented by Iversen et al. (2001b) with modifications to address the difficulty that we experienced in inserting their relatively large-diameter ring (20 cm) into the rugged terrain that is typical of the semiarid southwestern USA. Further modifications were made to improve the portability of the system for use on remote sites. The SCAP prototype was designed for use with any standard 5.175-cm (2 1/4-inch) o.d. soil core sampler (Fig. 2 ) (Product no. 200, Soilmoisture Equipment Corp., Santa Barbara, CA). Specifically, the SCAP is a custom-made stainless steel 8.255-cm (3 1/4-inch) o.d. cap that threads onto the soil core barrel, using a rubber O-ring to achieve an air-tight seal. The cap has a 0.635-cm (1/4-inch) o.d. air inlet and pressure measurement ports with Tygon tubing. A diffuser is placed at the bottom of the air inlet port to minimize disruption of the soil surface during measurement (Fig. 3 ).


Figure 2
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  		Fig. 2. Standard soil core sampler with 3-cm-length soil retainer cylinders.

 

Figure 3
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  		Fig. 3. The soil corer air permeameter (SCAP) attached to a soil core barrel. Top insert: inner SCAP showing air diffuser and rubber O-ring. Bottom insert: underside view of SCAP showing air diffuser and pressure port.

 
The first steps when using the SCAP are identical to those followed for standard soil core sampling. First, three soil retainer cylinders of 3-cm (1.181-inch) length are inserted into the soil core barrel and the handle is screwed on. After surface litter is removed, the barrel is carefully pounded to be perpendicular to the surface so that horizontal movement is minimized (Fig. 4 and 5A ). The handle is then removed and the soil is packed around the outside and inside edges of the inserted barrel to minimize preferential flow along the barrel (Fig. 5B). Then, the SCAP is attached to the soil barrel and the digital manometer and flow meter are attached to their corresponding ports (Fig. 4 and 5C) (Catalog No. A-68603–00, Cole Palmer, Vernon Hills, IL; Models 50SD and 50-12-1, McMillan Co., Georgetown, TX). Air is released at a low rate by adjusting the outflow of the tank until the pressure stabilizes at a pressure <10 cm of water. This step is quick and usually takes <30 s. Then, the corresponding flow rate is noted. Laminar flow conditions can be confirmed by taking measurements at different pressures to ensure that there is a linear relationship between flow rate (Q) and pressure (P).


Figure 4
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  		Fig. 4. Illustration of field setup for measuring in situ air permeability using the soil corer air permeameter.

 

Figure 5
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  		Fig. 5. Photographs of (A) gently pounding soil core barrel into soil, (B) connecting the soil corer air permeameter (SCAP) to the soil core barrel, and (C) connection of the digital instruments to the SCAP.

 
The SCAP has an inserted depth (H) of 10.5 cm, inside diameter (D) of 5.3 cm, and a volume of 231.7 cm3. A typical flow rate for a low-permeability soil (10 µm2) is 0.3 L/min at a pressure of 5 cm of water. A typical flow rate for a high-permeability soil (201 µm2) at the same pressure is 6 L/min. The SCAP measures air permeability values across a large range from approximately 1 to 1000 µm2. Once the air permeability measurements are completed, the undisturbed soil sample is removed following the procedures described for the soil core sampler. The retrieved sample can be analyzed further in the laboratory (e.g., for gravimetric moisture content or particle size distribution).

Field Measurements
Air permeability measurements were taken at the soil surface (A horizon) on four agricultural soils and one alluvial deposit in Tucson, AZ (northernmost 34, southernmost 30, easternmost –113, westernmost –109). These measurements were made on soils with moisture contents at field capacity or drier. The soils sampled were: Anthony very fine sandy loam (coarse-loamy, mixed, superactive, calcareous, thermic Typic Torrifluvent); Vinton fine sandy loam (sandy, mixed, thermic Typic Torrifluvent); Gila very fine sandy loam (coarse-loamy, mixed, superactive, calcareous, thermic Typic Torrifluvent); Pima loam (fine-silty, mixed, superactive, calcareous, thermic Typic Torrifluvent); and stream-channel material from Rillito Creek (gravelly coarse sand). Table 1 provides detailed information regarding the five sites and taxonomic identification (Soil Survey Staff, 1993). At each agricultural site, in situ air permeability measurements were taken along a 14- to 18-m transect at 2-m intervals; the measurement interval at the alluvial site was 3 m along a 24-m transect. At each measurement point, air permeability was measured at five pressures (3–7 cm of water). There were four field trials during which air permeability was measured at the sites (Data Sets 1–4).


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  		Table 1. Site information and taxonomic identification of four agricultural soils and one alluvial deposit in Tucson, AZ.

 
For Data Sets 1 and 2 at the agricultural sites, in situ measurements were made with a mechanical flow meter and a digital flow meter at each sampling location. Many standard soil corers have holes in the sides of the core barrels to allow insertion of a spanner wrench to remove a tightly screwed handle. (The model tested had two 3.5-mm-diameter holes, one on each side of the corer barrel). To determine the effects of these small insertion holes on an unaltered soil corer, in situ measurements for Data Sets 1 and 2 were made with the holes in the core barrels left uncovered. Ex situ measurements were made with the holes unsealed and temporarily sealed by placing a fingertip over each hole. Each of the first two field trials produced four transects with 7 to 10 air permeability measurement points for each.

Two soil corer barrels were used for Data Set 3 to measure in situ and ex situ air permeability for the agricultural soils. The first barrel was the standard unsealed type used with soil corers and the second barrel was sealed semipermanently with J-B Weld (Product no. 8265-S, J-B Weld Co., Sulfur Springs, TX). The third field trial resulted in eight transects (two parallel transects per site) with 10 measurement points per transect. Data Set 4 consists of only in situ air permeability measurements for the Rillito Creek stream-channel deposits along a nine-point transect (Table 2).


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  		Table 2. Measurement descriptions for Data Sets 1 through 4.

 
Laboratory Measurements
The middle soil retainer cylinder was collected for each in situ air permeability measurement for all field trials. The volume of soil recovered was 66 cm3. Bulk density ({rho}b), gravimetric moisture content ({theta}m), volumetric moisture content ({theta}v), and porosity ({phi}) were determined for each soil sample based on oven drying. The soil samples collected for Data Sets 3 and 4 were analyzed for particle size distribution according to the USDA classification (Table 3). Organic matter percentage was measured for all samples except the Rillito Creek stream-channel deposits. The Ksat was measured using the Reynolds soil core tank method (Reynolds et al., 2002), which allows batch soil sample saturation for either falling head or constant head permeameter measurements. Helium sparging was used to de-aerate gypsum water (0.005 M CaSO4·H2O) and soil samples were saturated slowly for 12 h from below. No biological inhibitors were added.


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  		Table 3. Soil properties and particle size distribution of four agricultural soils and one alluvial deposit in Tucson, AZ, for Data Sets 3 and 4.

 

   RESULTS AND DISCUSSION
TOP
 EXECUTIVE SUMMARY
 ABSTRACT
 INTRODUCTION
 THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Confirmation of Laminar Flow Conditions
At each measurement point, flow rate (Q) and pressure (P) were measured at five pressures (3–7 cm of water) to confirm laminar flow conditions. All measurements revealed linear relationships between Q and P. Figure 6 is an example confirming a laminar flow regime for in situ ka measurements along the Anthony transect.


Figure 6
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  		Fig. 6. Linear relationships of pressure and Darcian flow for in situ air permeability measurements at 10 points along the Anthony transect (Data Set 3). All R2 values exceed 0.999.

 
Validating the Accuracy of the Digital Flow Meter Used in the Field
The digital flow meter has an accuracy of 1.5% of the full scale, whereas a typical mechanical flow meter has an accuracy of 5% of the full scale. The use of a digital flow meter allows improved accuracy as well as easier field operation; however, the electrical components of digital flow meters can be sensitive to moisture and particulate matter that may exist in the applied air. To reduce the effects of these environmental factors, a moisture trap and a particulate air filter were installed in the tubing connected to the digital flow meter (Fig. 4). Although there were some slight differences for the lowest permeability values, there is a strong 1:1 relationship (r = 0.998) between measurements made using digital and mechanical flow meters (Fig. 7 ), demonstrating that the digital meters could be used under field conditions.


Figure 7
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  		Fig. 7. Comparison of air permeability (ka) values obtained using mechanical and digital flow meters for Data Sets 1 and 2 (n = 71).

 
Measurement Errors due to Instrumentation (Soil Corer)
Although the retainer cylinders inside the barrel actually cover the spanner wrench insertion holes along the barrel, the presence of the holes significantly impacted the air permeability measurements (Fig. 8 ). Measurements made in the Anthony soil during the first measurement trial and in the Pima soil for both trials show good agreement among the sealed and unsealed measurements. We suspect that all other measurement conditions were impacted by increased short-circuit flow through the holes in the core barrels, leading to higher ka values for the unsealed corers. As a result, we recommend that the holes in the soil corer be sealed when using the SCAP.


Figure 8
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  		Fig. 8. Comparison of air permeability (ka) values obtained from an unsealed soil corer and a sealed soil corer for Data Sets 1 and 2 (n = 71).

 
Applicability of the Jalbert and Dane Shape Factor
The Iversen et al. (2001b) permeameter, which has D/H ratio of 2, is well described by the Liang et al. (1995) shape factor. However, the D/H ratio of the SCAP is 0.5, which tests the lower bounds of the applicability of the Liang shape factor (Liang et al., 1995). In contrast, the Jalbert and Dane (2003) shape factor has been shown to approach the ex situ shape factor as the D:H ratio approaches 0. Therefore, the Jalbert and Dane shape factor (JDSF) should be more applicable to the SCAP design. The Liang shape factor calculated for the SCAP is 0.0068 m and the JDSF is 0.0186 m. The best fit (minimum root mean squared error) of all paired in situ and ex situ sealed-barrel measurements gave a shape factor of 0.0102 m. Soil-specific calibration for the Anthony, Vinton, Gila, and Pima soils gives shape factors of 0.0091, 0.0183, 0.0157, and 0.0086 m, respectively. All of these fitted values are less than the JDSF, which would lead to underestimation of ex situ ka using the JDSF (Fig. 9 ). Closer examination however, shows that the JDSF agrees most closely with the soil-specific values determined for the Vinton and Gila soils. These sites were being used during the experiments, so the soils had been tilled, leading to more homogeneous conditions than for the fallow Anthony soil and heavy Pima loam. That is, the JDSF, which was developed for homogeneous soils, gives reasonable values for homogeneous conditions; however, the JDSF may underestimate the in situ ka for the more structured soils. Based on these results, we recommend site-specific calibration or ex situ measurement, where possible. However, if it is likely that extraction will cause unacceptable structural damage to the sample, or if maximizing the data collection rate is critical, we recommend using the JDSF, especially in relatively homogeneous soils.


Figure 9
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  		Fig. 9. In situ vs. ex situ air permeability (ka) measurements for four agricultural soils in Tucson, AZ, for Data Set 3 (n = 40) using the Jalbert and Dane (2003) shape factor.

 
Predicting Saturated Hydraulic Conductivity from Air Permeability
Previous research has shown a correlation between laboratory-measured ka and Ksat values made on the same soil sample volume in the range of –50 to –100 cm of water (Loll et al., 1999; Iversen et al., 2001a). Comparison of the SCAP-measured ex situ field ka (sample volume of 232 cm3) and the laboratory-measured Ksat (sample volume of 66 cm3) showed mixed results (Table 4). In general, it can be assumed that the sample volumes for ka and Ksat determination are comparable. Caution should be used, however, if measurements are made in highly structured soils or across distinct soil horizons. For our measurements, the air to water permeability ratios (ka/kw) for the Gila and Pima soils are very high, indicating that structural changes occurred during water saturation (Reeve, 1953; Whelan et al., 1995). The remaining soils (Anthony and Vinton soils and the Rillito Creek material) show reasonable agreement (Fig. 10 ) with the findings of Iversen et al. (2001a).


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  		Table 4. Mean and standard deviation for air (ka) and water (kw) permeabilities and soil properties for Data Sets 3 and 4. Values in parentheses are the range of ±1 standard deviation for the geometric mean.

 

Figure 10
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  		Fig. 10. Log air permeability (ka) vs. log saturated hydraulic conductivity (Ksat) graphs for ex situ measurements for four agricultural soils and in situ measurements for one alluvial deposit (n = 39).

 

   CONCLUSIONS
TOP
 EXECUTIVE SUMMARY
 ABSTRACT
 INTRODUCTION
 THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
We analyzed the performance of the SCAP, designed for simultaneous measurement of air permeability and extraction of soil samples in desert soils. Field testing showed that the digital components of the SCAP operate well under field conditions, but the spanner insertion holes on the sides of standard soil corer barrels must be sealed to provide meaningful in situ air permeability measurements. The shape factor described by Jalbert and Dane (2003) gives a good approximate correction for in situ measurements for tilled, unstructured soils. There is significant soil-specific variability in the shape factor, however, so we suggest that measurements be made ex situ, where possible. Finally, we compared ex situ field air permeability and laboratory hydraulic conductivity measurements. The results can be used to identify soils that experience structural changes during water saturation. For those soils that do not show these effects, we found good correlation between ka and Ksat and reasonable agreement with previously published results.


   ACKNOWLEDGMENTS
 
This research and publication was made possible through funding from the National Science Foundation Small Grant Exploratory Research Program, the State of Arizona Proposition 301 Water Economic Development and Sustainability Program, and the National Science Foundation Science and Technology Center (NSF STC) of Sustainability of Semi-Arid Hydrology and Riparian Areas (SAHRA) Research Center. We would also like to acknowledge Dr. Bo V. Iversen (Dep. of Agroecology, Danish Inst. of Agricultural Sciences, Research Centre Foulum) and Dr. Per Moldrup (Environmental Engineering Section, Dep. of Life Sciences, Aalborg Univ.) for the use of the Iversen et al. (2001b) air permeameter and published air permeability and saturated hydraulic conductivity measurements.


   REFERENCES
TOP
 EXECUTIVE SUMMARY
 ABSTRACT
 INTRODUCTION
 THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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