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Published online 20 November 2007
Published in Vadose Zone J 6:849-854 (2007)
DOI: 10.2136/vzj2007.0049
© 2007 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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ORIGINAL RESEARCH

Field Measurements of Air and Water Pressures in a Heterogeneous Forest-Reclaimed Lignitic Mine Soil

Edzard Hangena,* and Horst H. Gerkeb

a Dep. of Preventive Soil Protection and Soil Monitoring, Bavarian Environment Agency, Hans-Högn Strasse 12, 95030 Hof, Germany
b Institute of Soil Landscape Research, Leibniz-Centre for Agricultural Landscape Research (ZALF), Eberswalder Strasse 84, 15374 Müncheberg, Germany
* Corresponding author (edzard.hangen{at}lfu.bayern.de).

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.


Received 14 March 2007.



   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
Soil matric and air pressures can significantly affect each other especially if the soil-gas phase becomes disconnected from the atmosphere. Such effects were presumed to be one possible explanation for observed preferential flow patterns in a mine soil. The objective of this study was to evaluate interactions between soil-air and water pressures in situ under field conditions. In a 2.5-m-wide mine soil pit, nine soil air pressure probes were horizontally installed 27 cm apart in a row at 80 cm depth and augmented by two rows of tensiometers, one row 10 cm below and one row 10 cm above the air probes. Field data were recorded for 3 wk in autumn 2001. At two of the nine measurement positions, deviations of soil air from atmospheric pressure were observed. Solely at these two positions, water-saturated conditions were found and drainage could be collected using a cell lysimeter, indicating that these air probes were disconnected from aboveground air. After accounting for soil temperature effects, the fluctuations in soil air pressure were found to correspond with those of the ambient soil water pressure during infiltration events. The gradual decrease of soil air pressure after the event might be induced by suction effects of draining soil water in the flow finger with the soil water pressures remaining above the air entry pressure of the soil. Results suggest that effects of flow on soil air pressure can occur locally. In this heterogeneous mine soil, the air–water pressure interactions were restricted to soil regions that coincided with preferential flow paths.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
Interactions between air and water pressure in soil are often neglected when studying water and solute movement in soils (Touma and Vauclin, 1986). This simplifying assumption is justified in homogeneous porous media, when air can freely escape during infiltration events (Wang et al., 1998). If air cannot escape, air pressure builds up relative to atmospheric pressure and affects the water pressure and movement (Hammecker et al., 2003). The differential soil air pressure depends, among other factors, on variations of the soil temperature (Adams et al., 1996; Renault et al., 1998). In heterogeneous soils, where, for instance, an infiltration front may divert along a hydrophobic soil region, the increasing air pressure may lead to lateral movement of soil water toward zones of lower air pressure and may favor the initiation of preferential flow patterns (Ritsema et al., 1998). The movement of soil water can influence that of soil air, thereby affecting the transport of gaseous components, such as oxygen toward plant roots (Flühler and Läser, 1975) and microorganisms (Renault et al., 1998). Gas transport processes in soil may further affect air-stripping strategies for the remediation of volatile contaminants in the unsaturated zone (Rojstaczer and Tunks, 1995) or the release of climatic relevant nitrous oxide in the course of irrigation cycles (Clough et al., 2003). Similar reports on in situ observations of fluctuations in soil air pressure together with soil temperature and water movement are rather limited. To the best of our knowledge, spatial interactions between soil air and soil water and possible effects on gas transport have not been addressed for forest-reclaimed lignitic and acidic mine soils. Dye tracer observations (Hangen et al., 2004) and field cell-lysimeter analyses (Hangen et al., 2005) suggested that preferential water movement for forest-reclaimed lignitic mine soils was caused by a combination of the effects of seasonally and spatially variable water repellency and small-scale spatial heterogeneity and anisotropy of hydraulic properties (Gerke, 2006). Under such conditions, interactions between soil air and water pressure are likely to occur since the coarse-textured mine soil containing lignitic components in the form of dust and fragments develops spatially distributed water repellency on drying (Gerke et al., 2001). The objective of this study was to identify and explain possible interactions between the gas phase and the liquid phase in a lignitic mine soil using simultaneous in situ measurements of soil air pressure, atmospheric air pressure, soil water pressure, and soil temperature. In addition, the spatial variability of these interactions was assessed to improve understanding of air and water movement in mine soils.


   Materials and Methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Measurements were performed at a mine dump (Bärenbrück) near Cottbus, Germany. This afforested mine soil is classified as Anthroptic Regosol (FAO/ISSS/ISRIC, 1998) consisting mainly of fine sand (0.063- to 0.2-mm particle diameter) mineral fraction and residual lignitic components ranging from dust- to larger-sized fragments of up to 10 cm in diameter (Buczko and Gerke, 2006). The pH values are about 2 below the 40 to 50 cm depth due to oxidative sulfide weathering, and pyrite contents of the fresh overburden sediments are about 1.5% by mass (Neumann, 1999). In 1978 the upper 40 cm of the soil were ameliorated with CaO-containing flue ash from a nearby lignite power plant. The mine soil was reforested with Pinus nigra (Arnold) in 1982. Most of the lyC horizon underneath a 2-cm-thick Ai horizon consists mainly of dumped tertiary overburden spoil sediments that contained residual lignite of about 10% by volume. An undisturbed mine soil profile (1.65 m wide x 1.1 m deep), which was underpinned by a cell lysimeter (interception area: 1.65 m x 2.5 m) (Hangen et al., 2005), was investigated and instrumented. Two rows of nine pressure transducer tensiometers with thermoreceptors (UPT6; UP Inc., Cottbus, Germany) each were horizontally installed at 70- and 90-cm soil depth, respectively (Fig. 1 ). The manufacturer had individually calibrated these: manufacturer-reported measurement error for soil water pressure was 0.3% over the –800 to +800 hPa measurement range; error for soil temperature was 0.5% over the –30 to +70°C measurement range. The lateral spacing between the tensiometers corresponded with the midpoint of the underlying lysimeter cells (Fig. 1; Hangen et al., 2005) and the observed mean distance between preferential flow fingers determined during a dye tracer infiltration experiment (Hangen et al., 2004). The tips (i.e., ceramic cups) of the tensiometers were positioned about 1 m within the undisturbed soil block to minimize boundary effects. Nine soil air pressure probes (KB-811091-A; UIT Inc., Dresden, Germany) were similarly installed in a row at the 80-cm depth (Fig. 1). The air volume inside the 1-m-long air-pressure probe was 21 cm3. These instruments recorded the differential soil air pressure (i.e., soil air pressure minus ambient atmospheric pressure) within a measurement range of –20 hPa to +20 hPa. The manufacturer's reported overall measurement error of the air probes (i.e., including nonlinearity, hysteresis, repeatability, and temperature effects) over the operating temperature range (+5 to +50°C) was 1% of the full-scale measurement interval, and drift effects due to nonlinearity accounted for 0.1% of the full-scale measurement range. Drift tests we performed after the field-experiment under atmospheric pressure conditions (0 hPa) showed a total recorded pressure increase of about 0.005 hPa in 17 d. The nine probes were individually calibrated by exposure to the pressure steps of 0, –10, and –20 hPa, respectively, for 10-min intervals with a logging resolution of 10 s. The individual calibration functions (Table 1) were slightly different from a general one provided by the manufacturer. Oxygen concentrations in the soil air were monitored (among other positions) at the 70- and 90-cm soil depths using 1-m-long, perforated brass tubes that were horizontally installed 33 cm from outermost soil air probes (Fig. 1). The oxygen values were determined weekly using an infrared-based through-flow device (MultiwarnII, Dräger Inc., Lübeck, Germany) with a manufacturer-reported measurement accuracy of ± 0.07% by volume. In the unsaturated two-phase air-water soil pore system, the soil water pressure, p, on a volume basis (hPa) is here referred to as (e.g., Touma and Vauclin, 1986)

Formula 1[1]
where pa is differential soil air pressure and pm is soil matric pressure. In this study, p was measured with tensiometers and pa was measured with soil air probes. The symbols p70 and p90 as used below, denote soil water pressures at 70 and 90 cm, respectively. To account for soil temperature, T, effects on the differential soil air pressure of entrapped air, the pressure–temperature relation suggested by Renault et al. (1998) was applied. The temperature-corrected differential soil air pressure, pat, for the 80-cm soil depth was obtained using soil temperatures measured at two depths:

Formula 2[2]
where T70 and T90 are soil temperatures (K) at 70 and 90 cm, and t0 and t are starting and ending times, respectively; factor 3.5 describes soil air pressure change of ~0.7 hPa for change in soil temperature of about 0.2 K with no volumetric change of air volume assumed. Values of p70, p90, T70, T90, and pa were simultaneously logged in 10-min intervals (DL2, Delta-Instruments, Cambridge, UK). Atmospheric air pressure, patm, was recorded every 15 min at a meteorological station (WS 2000, ELV Inc., Leer, Germany) located about 5 km northeast of the experimental plot. Precipitation was quantified using a recording tipping-bucket device (RG 50, UP Inc., Cottbus, Germany) that was installed in an open land area about 30 m northeast of the plot.


Figure 1
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  		FIG. 1. Scheme of investigated mine soil cross-section indicating the positions of horizontally installed measurement devices and pressure probes. For a detailed description of the cell lysimeter, refer to Hangen et al. (2005).

 

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  		TABLE 1. Coefficients of the general and the individual calibration functions pa = aU + b, where pa is soil air pressure (hPa), U is voltage (mV), and a and b are linear regression coefficients. Coefficient of determination: r2.

 

   Results and Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
A complete assessment of all nine locations of soil air pressures was limited to a period between 30 August and 24 September 2001. During this investigation period, decreases in atmospheric air pressure typically corresponded with the onset of major rainfall events, which occurred on 1, 4, 8, 10, 13, and 18 September, with a maximal intensity of 4.2 mm h–1 on 1 September (Fig. 2 ). Depth-averaged soil temperatures decreased from 18.7 to 13.6°C at 70 cm and from 18.2 to 13.5°C at 90 cm (Fig. 3 ). For seven of nine measurement positions (positions 1, 3, 4, 6, 7, 8, and 9), the differential soil air pressure steadily approximated 0 hPa, which indicated that the connectivity between atmospheric air and soil air was unimpeded (Fig. 4b ). Although a minimal air volume in the air probe shaft is desirable to optimize the accuracy of soil air pressure measurements (e.g., Flühler and Läser, 1975; Adams et al., 1996), the 21-cm3–probe volume might have reduced the sensitivity of the signal under field conditions. In seven out of nine cases (measurement positions 1, 3, 4, 6, 7, 8, and 9), soil water pressure at 70 and 90 cm indicated unsaturated conditions. Positions 2 and 5 showed periods of water saturation at both measurement depths (Fig. 4a and 4c). Water saturation possibly triggered the fluctuations in the differential soil air pressures. Previous studies showed that distinct fluctuations in differential soil air pressure can be caused by infiltration-induced water saturation in near-surface soil, which, in turn, leads to a disconnection between ambient atmospheric air and soil air (Wang et al., 1998; Renault et al., 1998). To focus on interaction between soil air and soil water pressure, only the temperature-corrected (Eq. [2]) pressure data from measurement positions 2 and 5 (Fig. 4b) for the period 10–21 September will be further analyzed. The correction for soil temperature was limited to the time interval starting on 10 September, for which a disconnected soil air phase and thus minimal volumetric variations (Renault et al., 1998) could be presumed. During this period, soil temperatures gradually decreased by about 3°C at 70 cm and 2.5°C at 90 cm depth (Fig. 3). Despite their horizontal distance of 81 cm, the soil air pressure probes at positions 2 and 5 displayed similar signals between 10 and 18 September (Fig. 5 ). Besides positive values of differential soil air pressure on 10 September, maximum values occurred on 18 September (on the second half of that day). These maxima were more pronounced at measurement position 5 compared with position 2 (Fig. 5). From late on 19 September until the termination of the measurements, differential soil air pressures at both measurement positions were about –18 hPa (Fig. 5).


Figure 2
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  		FIG. 2. (A) Atmospheric pressure and (B) precipitation during the period of investigation.

 

Figure 3
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  		FIG. 3. Soil temperatures at (A) 70 cm and (B) 90 cm soil depth for all nine measurement positions (see Fig. 1). Soil temperature at position 7 in 90 cm soil depth was not recorded because of transmission problems.

 

Figure 4
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  		FIG. 4. Soil water pressures at (A) 70 cm and (C) 90 cm soil depth and (B) differential soil air pressure at 80 cm soil depth for all nine measurement positions (see Fig. 1). Soil water pressure at position 8 in 90 cm soil depth was below the minimum measurement threshold of –800 hPa and therefore is not depicted here.

 

Figure 5
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  		FIG. 5. Differential soil air pressure after temperature compensation at measurement positions 2 and 5.

 
In a similar study that compared time series of atmospheric and soil air pressures for an 80-cm-deep instrumented observation hole on a bentonite grout (Rojstaczer and Tunks, 1995), negative values of differential soil air pressures of up to –20 hPa were observed for a period of more than 1.5 mo after pronounced rainfall events. Field soils are often assumed to possess connectivity between soil air and the atmosphere due to continuous macropores such as root channels or worm tubes (e.g., Flühler and Läser, 1975). The root zone of this mine soil, however, was basically limited to the upper 40 cm of the amelioration horizon (Hangen et al., 2001). Thus, preferential flow through root-zone macropores appears to be an unlikely cause for the heterogeneous wetting and drying of the mine soil profile (Fig. 4a and 4c). Rather than macropores, the inclined subsurface-dumping structures of the mine spoil material may have induced the observed tensiometer signals (Fig. 4a and 4c). Along anisotropies of the dumping structures, soil water that originally percolates vertically could be redirected diagonally. Results of soil water drainage patterns as well as subsequent three-dimensional soil sampling of the mine soil section support the assumption that water could accumulate in the region near the interface between two inclined layers corresponding to a funneling effect (Hangen et al., 2005). Apparently, percolation at these preferential flow regions was sufficient to entrap soil air at measurement positions 2 and 5, while no such effect was evident at the other measurement positions (Fig. 4).

When comparing the temporal courses of the temperature-compensated pat values at 80-cm soil depth with those of the ambient soil water pressures, p, at 70 and 90 cm soil depth (i.e., directly above and below the air pressure probes), an overall correspondence becomes obvious for position 2 (Fig. 6a ) and position 5 (Fig. 6b). Following a general decline in soil water pressures from 10 to 18 September, soil water pressures showed a rapid increase that corresponded with a rapid increase in differential soil air pressure. While pat values at position 2 reached a maximum of –6 hPa on 18 September (Fig. 6a), the pat values at position 5 increased to +1 hPa (Fig. 6b). Effects such as the decline of the pore space due to swelling (Clough et al., 2003) can be excluded here for this predominantly coarse-textured soil. Although hydrostatic values of p of up to +10 hPa were observed at position 2, 70 cm depth (Fig. 6a), the "bursting pressure" limit (Wang et al., 1998) was apparently not reached because the data do not indicate an abrupt decrease in soil air pressure. Instead, the soil air pressures at the 70 and 90 cm depths decreased almost congruently to the decline in soil water pressures at 90 cm (Fig. 6a and 6b). This congruent decline over several days may be induced by the suction effect of a moving wetting front, as discussed by Renault et al. (1998) and Flühler and Läser (1975). In the present study, the longevity of negative soil air pressure values may reflect the soil air disconnection from atmospheric pressure, although the soil is progressively draining as indicated by tensiometer values at 70 and 90 cm soil depth (Fig. 6). Note that such suction effect at positions 2 and 5 can only be explained to occur within a preferential flow finger of sufficient size (i.e., lateral extension). Air entry values of soil water pressure for the soil matrix, p*, were estimated to be about –30 hPa for the sandy lignitic-dust component (Buczko and Gerke, 2005a,b) and were experimentally determined to range from –600 to –60 hPa for the lignitic fragments (Einecke, 2005). Using water retention data for the studied mine spoil (Scherzer, 2001; 97.5 cm depth), we calculate a p* value of about –120 hPa from the following relation (Dingman, 1994):

Formula 3[3]
where {theta}PWP is the water content at permanent wilting point (m3 m–3), {phi} is porosity (m3 m–3), and b is an empirical factor, which equals 4.05 for fine sand (Dingman, 1994); site-specific values were {theta}PWP = 0.1452 (m3 m–3) and {phi} = 0.4766 (Scherzer, 2001). The p*-value estimations of –120 hPa (Eq. [3]) and –60 to –600 hPa (Einecke, 2005) are both smaller than the measured p values (Fig. 6a and 6b). Therefore, although the measured p values are negative, they fall in the range (0 to about –60 hPa), where soil pores will remain water filled and air entry will be negligible. Similar observations were made in laboratory experiments, which showed that coarse sand lenses were vented only after the soil water pressure of the surrounding fine sand dropped to about 15 hPa less than atmospheric pressure (Dunn and Silliman, 2003). Other factors that could explain the decrease in differential soil air pressure values include the consumption of certain soil air constituents, particularly oxygen, in the course of root respiration (Flühler and Läser, 1975) or pyrite oxidation (Elberling et al., 1998). As stated above, root oxygen uptake can be neglected at the investigated soil depths. Concerning oxygen depletion due to ongoing pyrite oxidation, measurements at the two different positions and depths (Fig. 1) did not indicate a decrease in soil-air oxygen content with time (Fig. 7 ). Between 10 and 19 September, an increase in oxygen content (1.4% by volume) was measured at one of the 90-cm-depth locations (Fig. 7). Elevated O2 contents in 70 cm soil depth on the right-hand side likely reflect soil heterogeneity. Thus, it did not appear possible to relate the oxygen concentration data directly to either of the two air-entrapment zones (i.e., positions 2 and 5) because the oxygen and pressure measurement locations were separated by distances of more than 60 cm.


Figure 6
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  		FIG. 6. Soil water pressures (p) at 70 and 90 cm and temperature-compensated differ- ential soil air pressure (pat) at 80 cm for (A) measurement position 2 and (B) position 5 (see Fig. 1).

 

Figure 7
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  		FIG. 7. Oxygen content in soil air at 70 and 90 cm soil depth at left- and right-hand side of investigated mine soil block (see Fig. 1).

 

   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
For a heterogeneous forest-reclaimed lignitic mine soil, interactions between soil water and air pressures were observed at two out of nine measurement positions during a rainfall period in early autumn 2001. These two positions were most likely located within preferential flow channels. The differential soil air pressure changes in these isolated zones coincided with saturated or near-saturated moisture conditions measured directly above and beneath the associated soil air probe. The observed gradual decrease of relative soil air pressures within the entrapped air volumes may result from the suction effect during drainage periods as long as water pressures remain above the air entry value of the mine soil. We did not observe decreases in soil air pressure that could be attributed to oxygen consumption from pyrite oxidation, but the lack of such observations may have been due to the sensitivity of field techniques that were used. The study provides field data for two-phase flow modeling in pyritic mine soils. Smaller-scale measurement devices and improved sensor configurations that minimize soil disturbances could further improve detection of transient soil air pressure fluctuations and possible air entrapment during infiltration in mine soils.


   ACKNOWLEDGMENTS
 
This project was financially supported by the Deutsche Forschungsgemeinschaft (DFG), Bonn, through grants INK4/B1-1 and GE990-2/1. The authors thank Wolfgang Schaaf and Reinhard F. Hüttl, Chair of Soil Protection and Recultivation, Brandenburg University of Technology, Cottbus (BTUC), for supporting the project. We appreciate the help in the field and during installation provided by Tao Pan and Bernhard Böttcher from the Chair of Soil Protection and Recultivation, BTUC. We thank Gerhard Kast, UP Inc., Cottbus, for putting his laboratory at our disposal for recalibrating the soil air pressure probes. We also thank B.J. Andraski and three anonymous reviewers for helpful comments.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 





This Article
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