Published online August 16, 2005

Improved Tension Infiltrometer for Measuring Low Fluid Flow Rates in
Unsaturated Fractured Rock
Paolo Castiglione, Peter J. Shouse,* Binayak Mohanty, David Hudson,
and Martinus Th. van Genuchten

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

ABSTRACT

similar fractured rock formations elsewhere. One possible means for obtaining such estimates is the use oftension infiltrometers.
Tension infiltrometers have long been used in soil hydrology to measure field soil infiltration rates and/or toestimate various infiltration parameters such as the sorptivity and the saturated hydraulic conductivity (Perroux and White, 1988). Others have used tension infiltrometers to characterize flow through soil macroporesat or near saturation (e.g., Wilson and Luxmoore, 1988;
Lin and McInnes, 1995; Jarvis and Messing, 1995; Mohanty et al., 1996). Unfortunately, tension infiltration
experiments are much more problematic to conduct infractured rock than in most soils. Extremely low infiltration rates are expected in fractured rocks, particularly
at high tensions, thus making accurate measurements
difficult. Because of low rock permeabilities and longequilibration times, various environmental factors normally neglected during tests with soils may not be negligible for rock formations.
Recent attempts to use tension infiltrometers to measure water flow properties of welded tuff layers exposed
along the Exploratory Study Facility (ESF) at Yucca
Mountain (a tunnel constructed to provide access along
the eastern edge of the potential repository area) showedthat traditional soil infiltrometer designs are inadequate
to accurately describe flow in fractured rock media (Hudson et al., 2000, unpublished data). Many fractures, measured in micrometers, are still fully functional at infiltrometer supply tensions above 100 cm of water. These
fractures are most likely responsible for the recharge
rates of 5 to 10 mm yr⫺1 (1.6 ⫻ 10⫺10 to 3.2 ⫻ 10⫺10 ms⫺1) estimated to occur at Yucca Mountain (e.g., Flintet al., 2002). Tensions needed to fully characterize theflow regime of these fractures are unattainable using current soil infiltrometer technology. Furthermore, becauseof extremely low infiltration rates and long equilibration times at especially higher tensions, evaporative
fluxes that inevitably occur between the disk and thering (normally neglected during tests in soils) may become important error sources when measurements are
made on fractured rock. As well, temperature fluctuations causing pressure variations at the rock surface
may reduce the accuracy of the measurements. Finally,
the need to periodically refill the reservoir tower, which
requires an operator and induces severe disturbances
to the infiltration process, can become an importantpractical problem. Hence, the primary objective of thisstudy was to develop a tension infiltrometer that overcomes some of these limitations and can be used tomeasure infiltration rates at levels thought to occur at
the Yucca Mountain site.

The search for a permanent storage facility for the geological disposal of high-level nuclear waste has motivated extensive researchduring the past several decades to characterize and predict fluid flow
into and through unsaturated fractured rock. Tension infiltrometer
experiments are extremely useful to investigate infiltration into fracture networks, but are difficult to perform using commercially available equipment developed mostly for soils. Our objective was to develop
a tension infiltrometer suited for accurate measurements of infiltration
into fractured rock at very low flow rates and for long equilibration
times. We constructed several prototype instruments from porous
stainless-steel membrane, stainless-steel casing, acrylic tubing, several
temperature-compensated pressure transducers, solenoid valves, anda data logger for automated control and data acquisition. An automated refill system was also developed to facilitate long unattended
equilibration periods typical in infiltration experiments on unsaturated
fractured rock. Results show that the improved design reduces temperature effects on the infiltration rate, allows for much longer periods ofunattended operation (auto-refill), and reduces evaporation from theinfiltrometer. The estimated upper flow-rate limit of our new infiltrometer is about 1 mm d⫺1, based on the conductance of the porous steel
membrane (11 mm d⫺1). We were able to make measurements of thefluid flux as low as 10 mm yr⫺1 at a pressure head of about ⫺110 cm.

Plans to use the unsaturated zone of Yucca Mountain, Nevada as a permanent storage facility for thegeological disposal of high-level nuclear waste has motivated extensive research aimed at understanding fluid
flow processes into and through unsaturated fractured
rock formations (e.g., NRC, 2001; Bodvarsson et al., 2003;
Wu et al., 2004). Yucca Mountain, located in a semiarid region of southern Nevada, is characterized by alternating
welded and nonwelded volcanic tuff layers. Generally,
nonwelded units have relatively high matrix porosities
and permeabilities and lower fracture densities, whereas
the welded layers, such as those that may host the envisioned repository, have lower porosities and higher fracture densities. One major challenge is to estimate the unsaturated hydraulic properties of the major hydrologic
units of Yucca Mountain (e.g., Flint et al., 2001) or
P. Castiglione, Land Resources and Environmental Science Dep.,
Montana State Univ., 334 Leon Johnson Hall, Bozeman, MT 59717;
P.J. Shouse and M.Th. van Genuchten, USDA-ARS, George E. Brown,
Jr. Salinity Lab., 450 W. Big Springs Road, Riverside, CA 92507; B.
Mohanty, Dep. of Biological and Agricultural Engineering, 301
Scoates Hall, Texas A&M Univ., College Station, TX 77843-2117; D.
Hudson, U.S. Geological Survey, MS 423, 1180 Town Center Drive,
Las Vegas, NV 89134. Received 17 Sept. 2004. *Corresponding author(pshouse@ussl.ars.usda.gov).
Published in Vadose Zone Journal 4:885–890 (2005).
Notes
doi:10.2136/vzj2004.0135© Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

Abbreviations: ESF, Exploratory Study Facility.

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Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

OPTIMAL DESIGN CRITERIA FOR ROCKTENSION INFILTROMETER
The design specifications for a rock tension infiltrometerare somewhat different from those for a soil tension infiltrometer. The infiltration process in fractured rock is expected toproceed much slower than that in most soils, especially at tensions of about 20 cm or higher. In our own studies (e.g., Wanget al., 1998; Simunek et al., 1998) we have measured equilibrium infiltration rates of about 10⫺8 m s⫺1 (≈1 mm d⫺1) forclay soils to about 10⫺4 m s⫺1 (10 m d⫺1) for sandy soils, which
required equilibration times from several minutes to several
days depending on soil type and the applied tension. By comparison, infiltration rates for rocks may be as low as 10⫺10 m s⫺1(≈3 mm yr⫺1) or less, with required equilibration times of 1 to
2 mo. Also, tension infiltrometer experiments on fractured
rock will be most useful if they can be performed at relatively
high tensions (e.g., tensions of 100 cm or more to characterize
flow in fractures that are only a few micrometers wide).
An infiltrometer for characterizing water flow in fractured
rock formations needs to be made from robust materials toallow operation of the infiltrometer under relatively unusualenvironmental conditions (Kilbury et al., 1986; Kilbury, 1984;
ASTM, 2002). For example, exposed rock surfaces are often
jagged, with relatively sharp edges that may easily damage
the porous nylon membrane materials frequently used in commercially available infiltrometers. The membrane itself may also
limit the effective tension applied to the rock surface. Whilethe nylon material on the disk may have a relatively low bubbling (air-entry) pressure, stretching and clamping material tothe plastic base may cause distortions in the pore size and reduce the contact angles, thus significantly increasing the bubbling pressure of the instrument.
Kilbury et al. (1986) gave specifications for a robust double
ring infiltrometer for measuring saturated water flow in fractured rock. Several of their design concepts have been adapted
to our tension infiltrometer. One of these addresses evaporation from the infiltrometer. Rock tension infiltrometers must
be vapor tight to limit or eliminate evaporation from the device
during very long equilibration times. Evaporation, if present,
would be measured as an equivalent infiltration flux and hence
would compromise the accuracy of the infiltration data. The
second adaptation addresses the usually uneven rock surfaces
which require a cap material that makes good contact between
the rock and the infiltrometer membrane. Also, the cap itself
must facilitate a relatively high flux rate, or at least a ratethat equals or exceeds that of the rock. The anticipated longequilibration times coupled with limited access to the test site,
warrant an instrument that is capable of refilling itself. The
device must also have data acquisition and control systems so
it can take readings over time and initiate refilling. The needfor high-accuracy measurements requires that the reservoir
tube diameter must be made as small as possible to maximize
the ratio of the disk to the reservoir diameter, thus enabling
accurate readings. Accuracy further requires that the disk be
as large as possible so a reasonable number of fractures atthe surface is included in the infiltration measurements at agiven pressure head. The integration of fractures and matrix
pores of differing size should lead to more representative
infiltration data.

MATERIALS AND METHODS
Tension infiltration experiments were performed in the laboratory on a large sample of volcanic tuff (Fig. 1) extracted
from the Yucca Mountain ESF site. The lithologic characteristics of the block, in particular the matrix porosity and thetypology of fractures, were very similar to those observed insitu near Alcove 8 (e.g., Wang and Bodvarsson, 2003). The
final design of the tension infiltrometer was tested on the large
rock sample (Fig. 1), as well as on smaller blocks in a controlledtemperature room.

Infiltrometer Disk
The infiltrometer disk was designed to transmit water to therock surface at high tensions, to prevent evaporation during
infiltration experiments, and to be made from robust materials. A schematic of our final design is shown in Fig. 2. The disk
was made of a thick stainless-steel lamina at the top (30-cm
diam.), a metal ring (4 cm high, 27.5-cm diam.), and a 0.32cm-thick (1/8 in) stainless-steel porous membrane (Mott Corporation, Farmington, CT) at the bottom (Kilbury et al., 1986;
Kilbury, 1984; ASTM, 2002). The parts were welded together
to form a chamber. The porous membrane is characterized bya relatively narrow pore size distribution, having a mean poresize of 0.2 ␮m, and a relatively high porosity of about 80%.
We measured a bubbling pressure of ⫺60 cm. The saturated
conductivity of the membrane was measured to be 1.3 10⫺7m s⫺1 (11 mm d⫺1). Previous infiltration studies (Hudson, 2000,
unpublished data) indicated the membrane conductivity isapproximately three orders of magnitude higher than the expected tension infiltration rate at a pressure head of ⫺50 cm.
However, the membrane conductivity was comparable to infiltration rates observed in saturated conditions (Hudson, 2000,
unpublished data), which suggests that the disk may not besuitable for measurements at or near saturation (Perroux andWhite, 1988). The top lamina contained two ports. One port wasconnected to the reservoir tower and the other to a fully
temperature-compensated Setra 230 pressure transducer (Setra Systems, Inc., Boxborough, MA) mounted on top of thedisk to monitor the pressure head during infiltration. The
second port also facilitated the removal of air from the disk.
A stainless-steel ring (4 cm high, 27.5-cm diam.) hosting
the contact material was placed over the rock surface. Having
a proper seal between the metal ring and the rock is criticalto prevent evaporation. For our laboratory experiments we
cut a circular groove into the rock with a diamond bit, andplaced the ring in the groove as shown in Fig. 2, but in the field
the ring simply rested on the bedrock. The field experimentrequired several applications of silicone to properly seal thering to the rock.
Because of the particular shape of the disk, the thick top
lamina on top of the metal ring forms a closed chamber thatprevents evaporative flux from the contact material. Sealing
was assured by using a soft rubber gasket glued beneath thelamina (Fig. 2) and by taking advantage of the weight of thedisk (≈14 kg). We note also that the contact material needs
to be slightly deformable to achieve a good seal at the disk–ringinterface, while at the same time ensuring good contact withthe rock surface. We found that diatomaceous earth material
with a saturated conductivity of 4 ⫻ 10⫺5 m s⫺1 (≈3.5 m d⫺1)
was suitable contact material. The pressure head of the contact
material was measured using a temperature-compensated pressure transducer tensiometer inserted through the ring. The tensiometer permitted monitoring of the advancing wetting front
within the contact material, thus providing possible additional
information for optimal analysis of the tension infiltrometerdata using inverse procedures (e.g., Simunek et al., 1998).
The effectiveness of the design in limiting or eliminating
evaporative losses was tested by gluing the contact material
host ring to an impermeable surface (acrylic sheet), then adding the contact material, the disk, and the Mariotte and reservoir towers, and subsequently adding water as in an ordinary887

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Fig. 1. View of improved tension infiltrometer disk setup on large fractured rock sample used in this study.

infiltration experiment. Once the diatomaceous earth was wet
and the infiltration process terminated, the entire assembly
was weighed and placed in an environmental chamber for
one-week periods at 15 to 20% relative humidity, with thetemperature set at 20, 30, or 40⬚C. These conditions permitted
estimates of the evaporation rate from the weight loss measurements. We observed an evaporation of 1.2 g of water after
1 wk at 40⬚C, while no evaporation could be measured at 20
and 30⬚C. Performance of the disk hence seemed to be morethan satisfactory, considering the ambient temperature (≈22⬚C)
and relatively high relatively air humidity (≈80%) conditionsin the ESF tunnel at Yucca Mountain.

Tension Infiltrometer
Our design of the Mariotte and reservoir towers (Fig. 3)
was based in part on an analysis of the effects of temperature
on infiltrometer performance (Castiglione et al., 2005) andtemperature-induced water flow dynamics within air pocket
tensiometers (Warrick et al., 1998; Butters and Cardon, 1998).

We refer to Castiglione et al. (2005) for a detailed analysis ofthese effects. The infiltrometer consisted of a single Mariotte
tower (150 cm tall) and a reservoir tower (180 cm tall), bothmade of small-diameter acrylic tubes. The diameter of thetubes was selected to be as small as possible (0.953-cm i.d.,
3/8 in), while still allowing free movement of air bubbles (Perroux and White, 1988). This was done to achieve maximum
accuracy in the measurements of the infiltration rate and tominimize the initial volume of the air pockets.
As shown by Castiglione et al. (2005), it is important tokeep the height of the air pocket in the Mariotte tower as
small as possible (≈1 cm) to limit temperature effects. Tube C(Fig. 3), connecting the reservoir and Mariotte towers, consisted of two segments separated by a normally open solenoid
Valve S3 (the function of this valve is explained below). The
horizontal segment of Tube C consisted of a 10-cm-long,
0.635-cm i.d. (1/4 in) tygon tube, while the vertical segment
was made of 0.318-cm i.d. (1/8 in) nylon tubing. As discussed
in more detail by Castiglione et al. (2005), an advantage ofthe above design is that it dampens pressure changes causedby temperature fluctuations in the reservoir tower air space.
The vertical air tube in the Mariotte tower was replacedby 12 external horizontal segments of tygon tubes (0.635-cm
i.d., 1/4 in), placed 10 cm apart at different depths, each having
a ball valve (Fig. 3). The tension applied to the rock surface
was regulated by choosing the proper segment for air entry,
and by keeping the remaining valves closed. This designmodification further reduces pressure variations associatedwith temperature fluctuations in the Mariotte tower air space(Castiglione et al., 2005).

Automated Refilling System

Fig. 2. Schematic of improved infiltrometer disk showing major components.

Before describing the automated refilling system, we first
briefly discuss disturbances induced by normal refilling of an
infiltrometer. The reservoir tower of soil tension infiltrometers
is usually refilled manually by removing a stopper or similar
device at the top and then filling the reservoir tower withwater. To preserve the tension applied at the soil or rock sur-

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888

VADOSE ZONE J., VOL. 4, AUGUST 2005

Fig. 4. Plots vs. time of (a) observed cumulative infiltration, (b) theapplied pressure head at the rock surface, and (c) ambient temperature using an early high-tension infiltrometer prototype. Notice
how the observed surface pressure heads are affected by the ambient temperature fluctuations.

Fig. 3. Schematic of Mariotte and reservoir towers used for the improved low-flux, high-tension infiltrometer.

face, the disk is usually disconnected from the tower (e.g., using
a valve or clamp) before removing the stopper. This procedure
establishes a positive pressure at the bottom of the reservoir.
When infiltration restarts, this positive pressure is inevitablytransmitted to the soil surface and hence will affect the infiltration rate. The pressure head within the infiltrometer disk isfurther affected by the thermodynamic status of the small
volume of air inevitably left at the top of the reservoir (whenusing rubber stoppers). The pressure, in fact, drops upon the
expansion of this volume of air as the water level in the reservoir descends. As infiltration proceeds, the pressure eventually
returns to the value before refilling. Only at this time does
bubbling start again in the reservoir tower, with the desired
tension at the soil surface finally being reestablished. The timeneeded to restore the original tension thus depends on theheight of the air pocket left upon refilling and on the rate of
expansion of the pocket (i.e., on the infiltration rate). While
this does not usually represent a problem for tension infiltrometer experiments on soils, the disturbances induced during therefill maneuver may last from many hours to several days withhigh-tension infiltration rates involving fractured rock.
To eliminate any disturbance, we designed the tension infiltrometer in such a way that no volume of air is left when thereservoir is filled. In this way the negative pressure at the surface is instantaneously reestablished as the infiltration is restarted. This is accomplished by connecting the reservoir tower
to a large tank (Fig. 3) and using the tank to refill the tower

from the bottom. The tank itself is placed at a sufficient height
above the rock or soil surface to completely fill the tower bygravity. A system of four solenoid valves (S1 through S4 in
Fig. 3) controlled by a data logger via a relay driver permits
the automatic refill. Valves S1 and S3 are normally open, while
the other two are normally closed (i.e., they are powered only
when the reservoir is being filled). When the water level inthe reservoir (monitored by the pressure transducer mounted
on top) is sufficiently low, the data logger controls the valvesin the following sequence. Valve S1 is closed (the system is
disconnected from the disk), Valve S3 is closed (reservoir andMariotte towers disconnected), and Valves S4 and S2 are open
(water can freely flow from the tank into the reservoir tower).
This process allows the water to rise rapidly in the tower until
it reaches the small Tube D (Fig. 3). The top of the reservoir
was designed in such a way that this refilling displaces all air
in the reservoir tower. Completion of the filling process isindicated by a positive head recorded by the pressure transducer as water reaches Tube D. To terminate refilling andreestablish the infiltration process, the valves are operated in
the following reverse sequence: S2 closes, S4 closes, S1 opens,and S3 opens. The entire refilling process using the above
design was found to require ⬍10 s.

APPLICATION TO FRACTUREDROCK SAMPLE
Figure 4 shows results of an infiltration experiment performed with an early prototype of the rock tension infiltrometer. Plotted are cumulative infiltration, the pressure headapplied at the rock surface, and the temperature data, all as
a function of time. The cumulative infiltration curve (Fig. 4a)shows a pronounced initial sorption phase characterized byhigh infiltration rates, followed by an approximately constantinfiltration rate starting at about 1 d. As indicated by thetensiometric data recorded within the contact material, thewetting front at this time reached the bedrock. Although asteady-state infiltration process seemingly had developed after
1 d, thus allowing approximate estimates of the infiltration
rate, the data were subject to persistent noise that clearlycorrelated to the daily temperature fluctuations (Fig. 4c). The
temperature effects were most evident from the measured
surface pressure heads (Fig. 4b), which followed a quasiperiodic pattern. Notice that temperature variations of about 4⬚C889

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Fig. 6. Steady-state infiltration rates measured with a traditional soil
tension infiltrometer (solid circles) and the improved low-flux,
high-tension infiltrometer (solid triangles).

Fig. 5. Plots vs. time of (a) observed cumulative infiltration, (b) applied pressure head at rock surface, and (c) ambient temperatureof the improved high-tension infiltrometer prototype. Notice thattemperature effects on the pressure head at the rock surface are
now minimized relative to the results in Fig. 4.

caused overpressures of more than 10 cm, which made theearly prototype unacceptable for long-term infiltration experiments on unsaturated fractured rock.
Upon drying the rock, a new series of experiments were
performed with the final prototype tension infiltrometer. Figure 5 shows in more detail data for infiltration with an appliedpressure head at the rock surface of about ⫺82 cm. Although
the temperature fluctuations during this experiment were
somewhat less severe as compared with the data in Fig. 4,
system response was now much better. Surface pressure headvariations were now ⬍1 cm, and without any noise in the
cumulative infiltration data (Fig. 5a).
The steady-state infiltration rates recorded for three surfacepressure heads (triangles) are plotted in Fig. 6. The figure
also includes data obtained for the same rock sample using
a traditional tension infiltrometer setup (Soil Measurement
Systems, Tucson, AZ) at higher pressure heads (circles). The
traditional setup was found to produce inaccurate results forpressure heads below about ⫺25 to ⫺30 cm and to fail completely below ⫺40 cm. We note that the two sets of data in
Fig. 6 should be comparable since disks with the same diameter
were used for the two sets of measurements.
The objective of this study was to develop a reliable technique to measure extremely small fluxes at relatively hightensions at the fractured rock surface. Although the analysis
and interpretation of the data in Fig. 6 are beyond the scopeof this paper, it is interesting to note the vastly different infiltration rates observed for the imposed range of tensions. Thereis little doubt that the rock matrix remained saturated throughout this range of tensions. Hence, differences in Fig. 6 must
be ascribed to different classes of fractures being involvedin the infiltration process. With the appropriate adaptations
described here, the tension infiltration technique therefore
represents a very useful tool for investigating flow in fractured
media. It also interesting to note that the lowest flux we measured with the improved tension infiltrometer was about 10
mm yr⫺1, corresponding to a pressure head of about ⫺110
cm (Fig. 6). This flux is approximately equal to the average
percolation rate estimated for Yucca Mountain (Flint et al.,2001, 2002). Our experience with the newly designed rocktension infiltrometer suggests that the applied pressure headcan be easily extended to about ⫺150 or ⫺160 cm, thus potentially providing additional data about the infiltration process
at rates far less than 10 mm yr⫺1. Such experiments, also if
performed in situ (e.g., in the ESF tunnel), are best implemented using relatively large porous stainless-steel membranedisks (e.g., 50-cm diam.).
ACKNOWLEDGMENTS
This study was based on work supported in part by SAHRA(Sustainability of semi-Arid Hydrology and Riperian Areas)
under the STC Program of the National Science Foundation,
Agreement no. EAR-9876800, and National Science Foundation Hydrology Grant no. EAR-0296158.

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