Vadose Zone Journal 1:310-315 (2002)
© 2002 Soil Science Society of America
Notes
Advances in Tensiometry for Long-term Monitoring of Soil Water Pressures
J. B. Sissona,
G. W. Gee*,b,
J. M. Hubbella,
W. L. Brattonc,
J. C. Ritterb,
A. L. Wardb and
T. G. Caldwelld
a Geosciences Research Department, Idaho National Engineering and Environmental Laboratory, Bechtel Idaho, Incorporated, Idaho Falls, ID 83415
b Hydrology Group, Environmental Technology Division, Pacific Northwest National Laboratory, Richland, WA 99352
c Applied Research Associates, Inc., Richland, WA 99352
d Desert Research Institute, Reno, NV 89512
* Corresponding author (glendon.gee{at}pnl.gov)
Received 17 July 2001.
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ABSTRACT
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Soil water pressures, measured over space and time, are needed to predict the direction of water flow and chemical transport in the vadose zone. Advanced tensiometers (ATs), which utilize a water-filled porous cup connected directly to a pressure transducer, can be installed at almost any location and depth using standard drilling techniques such as auger drilling, but these methods can significantly disturb the site. For sites where minimal disturbance is desired, alternate approaches for tensiometer placement have been sought. To test installation techniques and performance longevity, advanced tensiometers were placed into the ground at a test site near Richland, WA using two different installation methods, auger drilling and a drive-cone push technique. The tensiometers were subsequently monitored for nearly 2 yr without refilling or recalibration. The data indicated that tensiometers placed by the auger technique took several months to equilibrate, while the cone push units came to equilibrium within 24 h following their installation. Soil water pressures always remained above -90 cm pressure head (-90 mbar) at depths >90 cm. At the greatest depth (730 cm), positive then negative pressures were observed as the water table was lowered and the soil drained. The results suggest that for our test conditions (coarse sandy soil, no vegetation), soil water pressures stay well within the tensiometer range and unit gradient conditions persist, indicating a draining profile. Advanced tensiometers, placed either by auger or cone penetrometer, provide a robust and reliable method for long-term monitoring of soil water pressure profiles.
Abbreviations: AT, advanced tensiometer • BGS, below ground surface • CPT, cone-penetrometer technology • DCT, drive-cone tensiometer • PVC, polyvinyl chloride
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INTRODUCTION
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TENSIOMETERS ARE USED to measure soil water pressure in the operational range from 0 to -700 cm pressure head (0 to -700 mbar) (Gardner et al., 1922; Richards, 1931; Richards et al., 1937; Cassel and Klute, 1986). Tensiometers can also be used to measure positive pressures in soils that are saturated and thus can act as piezometers for monitoring perched water bodies or water table elevations. Advanced tensiometers (Sisson and Hubbell 1999) incorporate the pressure-transducer directly into the porous cup to minimize the length of the water column that hydraulically connects the pressure transducer to the cup. By minimizing the water column length, the tensiometer performance is enhanced by virtually eliminating several problems encountered in conventional tensiometers, namely, excessive thermal noise, sluggish response, and limited depth placement (Hubbell and Sisson, 1996, 1998; Sisson and Hubbell, 1999). Placement of the pressure transducer, porous cup, and the water column at depths where diurnal temperature fluctuations are dampened minimizes the problem of thermally affected fluid movement into and out of the water column. In addition to reducing the noise level from temperature fluctuations, the length of time between refillings of the tensiometer was extended from once per week to once per year or longer, depending on the depth of placement and the in situ soil water pressures. Finally, the short length of the sealed portion of the AT makes it possible to place the unit at almost any depth in the subsurface.
Only limited data are available detailing soil water pressure profiles at the U.S. Department of Energy's Hanford Site near Richland, WA (Gee, 1987). Such information is needed to determine flow direction of water borne contaminants and to evaluate numerical models used to predict subsurface contamination. At contaminated sites like Hanford, there is a need to minimize any disturbance of the buried waste because radioactive contaminants can be brought to the surface, causing worker exposure and increasing both risk and costs. We developed a technique to emplace tensiometers at depth with minimal soil disturbance by hydraulically pushing the tensiometer into the soil with a drive-cone technique. This study reports the application of ATs for waste site monitoring using both auger-and drive-cone techniques. The study also investigated the robustness and reliability of the emplaced sensors after more than 1 yr of operation and illustrates how the data obtained can be used to describe the downward flux of meteoric water in coarse sands at the Hanford Site.
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MATERIALS AND METHODS
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The test site is located about 7 km north of Richland, WA at the Department of Energy's Hanford Site. The test location has been used in the past to study water balance at a simulated waste site and employs a set of large drainage-type lysimeters. Figure 1 is a schematic of the lysimeters used in the study. The lysimeters were constructed in 1978 and were instrumented for long-term water balance monitoring and modeling, and the soils have been assessed for hydraulic properties by a variety of techniques, including transient flux (instantaneous profiling) and permeameter tests in the field and steady-state column tests in the laboratory (Gee, 1987; Rockhold et al., 1988; Fayer and Gee, 1992; Gee et al., 1992; Tyler et al., 1999). The lysimeters have a diameter of 2.7 m and depth of 7.6 m and are constructed from vertically oriented corrugated galvanized steel culverts with an impermeable (concrete) base. The two lysimeters (north and south) are filled with screened (<12 mm) sandy sediment (95% sand, 3% silt, 2% clay) stockpiled during site excavation. The third caisson, located between the two lysimeters, is used to provide physical access to the backfilled lysimeters and as an instrument shelter. Precipitation that percolates though the south lysimeter accumulates at the impermeable base of the lysimeter and is routed into the instrument caisson where it is measured with a small tipping spoon (Rain-O-Matic, Pronamic Co. Ltd., Sikeborg, Denmark). A tipping bucket rain gauge (Model TE525WS, Campbell Scientific, Inc., Logan, UT) was used to monitor precipitation at the site. A Campbell Scientific 10X (Logan, UT), multiplexer, and phone line are located in the instrument caisson to collect, store, and transmit data from the pressure transducers, tipping rain gauge.
Figure 2 shows the general construction of the AT, as described by Hubbell and Sisson (1998). The rigid inner guide pipe that connects from land surface to the transducer was replaced with flexible HDPE tubing (12.7 mm i.d.). The components of the AT include the ceramic cup, water-filled chamber, and a pressure transducer apparatus. The pressure sensors used in the tensiometers are Honeywell, Micro Switch 26PCCFA6D with a ±1060 cm head (15 psig) range (referenced to atmospheric pressure). The AT differs from standard tensiometers in that the pressure transducer is positioned at the ceramic cup, which allows the entire tensiometer to be placed at depths where the effects of temperature changes are lessened.
The ATs were installed in the south lysimeter at 90, 170, 230, 430, 550, and 730 cm below ground surface (BGS) in a 21-cm-diam. borehole augured into the lysimeter from land surface in August 1999. The ATs were filled with water and transducers on the same date as installation. The ATs were periodically refilled with water during the first 6 mo of operation by pulling the flexible inner guide pipe upward a few centimeters to release the transducer gasket, allowing water to flow into the lower water reservoir and then pressing the gasket downward to form the tensiometer seal. After the first 6 mo of operation the units have not been serviced. Each AT was placed at its final depth inside the 15.4-cm-i.d. hollow stem auger and backfilled using the sand obtained from drilling. The sand was compacted while backfilling. While no direct measurement of compaction was made on the backfill, repeated measurements of compaction of this soil in the laboratory and in other field tests indicated that the density of the backfill with standard compaction effort is approximately 1.6 g cm-3 and ranges from 1.5 to 17 g cm-3.
The drive-cone tensiometer (DCT) is a new innovation (Fig. 3), built and designed at the INEEL and installed by Applied Research Associates (Richland, WA). The DCT functions in an identical manner as the AT and uses the same pressure sensor. The innovation allows the Advanced tensiometers to be installed using cone-penetrometer technology (CPT). Cone-penetrometer technology is a technology that is becoming more common in the geotechnical and environmental communities as an alternative to standard rotary drilling techniques (Bratton & Timian 1995). As opposed to drilling where a borehole is excavated, the CPT creates a borehole by pushing the soil aside, making room for the instrumented probe without bringing drill cuttings to land surface. Because of this insertion technique, CPT techniques are limited to unconsolidated soils. Penetrations have been conducted as deep as 100 m in mine tailings and typically achieve 30 to 60 m in many soils.
The installation of the DCT using CPT consists of screwing a tensiometer well tip onto threaded sections of 5-cm (2.0-inch) schedule 80 polyvinyl chloride (PVC) pipe. The PVC is pushed with a 4.44-cm (1.75-inch) CPT push rod inside the PVC until the tensiometer tip reaches the desired depth (Fig. 2). Once the final depth is reached, the CPT push rods are removed, leaving open access to the porous tip section of the tensiometer unit. Water is poured into the well to a height above the machined surface in the DCT (
12 cm) and a pressure transducer inserted into the tensiometer unit. A rubber stopper mounted around the pressure transducer seats on the machined surface inside the tensiometer unit. This creates a seal so the pressure transducer can measure the soil water pressure. The DCT is constructed of stainless steel with a 0.2-µm porous stainless-steel membrane (Mott Metallurgical Co., Farmington, CT). The DCTs were installed at a depth of 700 cm BGS in the north lysimeter and in the backfill adjacent to this lysimeter (Fig. 1) in March 2000. In a manner similar to the ATs, the DCT units were refilled with water once after about 6 mo and were not serviced after that time. Data were collected from the tensiometers and tipping rain gauges on an hourly basis and recorded.
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RESULTS AND DISCUSSION
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Figure 4 shows the soil water pressure data obtained over approximately 2 yr of monitoring. Observed pressure pulses suggest that wetting occurred to at least the 210-cm depth over the late fall through winter time periods in both 2000 and 2001. The response of the AT unit at the 90-cm depth shows an oscillatory pattern over the August to December 2000 time period; then, as the wetting front apparently reached the depth of the tensiometer cup, the oscillations disappeared until late fall of 2001 when it reoccurred. The oscillations may be caused by changing weather conditions (near surface thermal and pressure changes), but at present their pattern is not clearly understood.
Figure 4 also shows precipitation record for the period of October 1999 through December 2001. Cumulative precipitation was nearly 400 mm during the 2 yr of testing, with the bulk of the precipitation coming during winter months. There is a pronounced increase in precipitation in December through January 2000 compared with the rest of the monitoring period.
Wetting events occurred during each of the three winters for 2000, 2001, and 2002. Penetration of the wetting front as noted by distinct pressure changes was detected to the depth of 210 cm. In February 2000, the 90-cm tensiometer indicated a significant increase in soil water head from -130 to -40 cm pressure head (-130 to -40 mbar) over a 6-wk period. In contrast, the 150-cm tensiometer shows a lag in response of about 7 wk and a damping with depth in the amplitude of the pressure front. The soil water pressure response is damped with depth so that soil water pressure at 430 cm does not change despite gravity effects. The soil water pressures tend to approach -40 cm. Soil water pressures (or heads) are more stable with increasing depths and approach a gravity drainage value for the material. The wetting front response in March 2000 corresponds with precipitation in December and January 2000 (Fig. 4). The wetting front in early 2001 appears to be in response to a more diffuse precipitation event. There is no distinct increase in precipitation but there is a fairly distinct increase in cumulative water storage and a very distinct increase in soil water pressure with depth to at least 210 cm. The soil water pressure data suggest that for the sandy material, the water builds up until sufficient hydraulic conductivity is reached to release the water to drain into the underlying sediments. This suggests that distinct wetting front events can be detected and tracked with depth by tensiometers even if the precipitation record shows a diffuse rainfall pattern and doesn't indicate significant rainfall events. The peak of the pressure pulse from the advanced tensiometers moved from 90 to 150 cm in 20 d, then from 150 to 210 cm in 39 d, and from 210 to 430 cm in about 120 d.
The coarse sand layer at 6 m did not appear to have a significant effect on the distribution on soil water pressure with depth; however, the 730-cm tensiometer showed a deviation in the slope due to the formation of perched water above the gravel and concrete base of the lysimeter. Interestingly, the tensiometer at 730 cm had higher soil water pressure readings than anticipated, suggesting that the gravel may be acting like a soil water barrier. The 730 cm tensiometer readings responded to drainage of the lysimeter when the outflow line was lowered twice, once in March and again in November 2000 (Fig. 4).
The data show that unit gradient conditions prevail at this site (e.g., total head values are parallel to the 1:1 line). This is consistent with the observation that the lysimeter has been draining for the past 20 yr. Recent measurements indicate that for the past several years the average drainage rate from the lysimeter has been 55 ± 10 mm yr-1, so a simple estimate of the unsaturated conductivity using the Darcy equation equates directly to the measured flux because the head gradient is unity (Fig. 5).
Figure 6 shows the time series of the soil water pressure from March 2000 to April 2001 from the DCTs. The DCT unit inside the lysimeter was at the same soil water pressure as the sediments outside the lysimeter, suggesting that hydraulically the pressure conditions were similar inside and outside the lysimeter. These measurements compare favorably with the ATs installed by the hollow stem auger in the north lysimeter.
After placement in the ground, the response of the DCT units was almost instantaneous (Fig. 6). The difference between the cone-placed and auger-placed tensiometers is that in the auger-drilled tensiometer nest that soil was backfilled around the units. The soil was not preconditioned; it was simply taken from the auger cuttings and probably had air-dried some before being placed back down the hole. The backfill apparently required several weeks to equilibrate with the surrounding sediments. The two DCTs only vary about 10 cm over the year period (Fig. 5).
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CONCLUSIONS
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Advanced tensiometers can be installed using either hollow-stem auger or drive-cone technologies. One advantage of the drive-cone placement is that no cuttings are brought to the surface, reducing the risk of contamination exposure during sensor placement at waste sites. Another advantage is the rapid attainment of pressure equilibrium. Soil water pressures obtained from the DCT were found to reach steady values within a few days following their installation. In contrast, ATs installed with the hollow stem auger method required several weeks before steady soil water pressures were observed. Advanced tensiometers proved to be robust and reliable, operating for nearly 2 yr without servicing. Monitoring of soil water pressure over a 7.6-m-deep soil profile for an extended time at our test site has shown that the pressure pulse from winter rains annually reached 210 cm but was damped below that depth. Unit gradient conditions persisted throughout the course of the study, consistent with observed drainage. The unsaturated hydraulic conductivity of the sediment also could be estimated directly from the drainage flux because the pressure gradient was known.
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ACKNOWLEDGMENTS
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This work was conducted under the direction of the Pacific Northwest National Laboratory with funding from the U.S. Department of Energy's Hanford Science and Technology Initiative and the Ground Water/Vadose Zone Integration Project. Battelle under Contract DE-AC06-76RL01830 operates Pacific Northwest National Laboratory for the U.S. Department of Energy.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
