United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas NV 89193-3478
Research and Development
EPA/600/S4-88/001 May 1989
f/EPA Project Summary
Development of a Capillary
Wick Unsaturated Zone Pore
Water Sampler
K. W. Brown, J. C. Thomas, and M. W. Holder
Existing unsaturated zone soil
water samplers have several
deficiencies which jeopardize their
utility for field sampling. Suction
cups only function when a vacuum is
applied, and sample from an
unknown volume of soil. Pan
samplers only sample saturated flow.
A capillary wick sampler was
developed to overcome these
problems. Materials for its con-
struction were selected and tested
for conductivity, capillary potential
and chemical inertness. Break-
through curves for selected in-
organic ions and organic chemicals
were established In the laboratory.
No adsorption/desorption of these
chemicals was found for the capillary
wick sampler, the suction cup
sampler, or the pan sampler. Banks
of 8 capillary wick samplers were
installed in test plots of undisturbed
soils having sand, slit loam and clay
textures. Bromide breakthrough
curves were determined at each
location. The data were used to
determine the number of samplers
required to characterize the flow of
contaminants resulting from a uni-
form application to the soil surface.
These results indicated that to
achieve 95% confidence, 31 sam-
plers would be required in the sandy
soil, 6 in the silt loam soil and 2 in
the clay soil.
The experimental plots were
drained and samples were collected
over a range of soil moisture
contents and soil moisture poten-
tials. It was demonstrated that the
wick sampler does adequately collect
soli solution samples from soils
having soil moisture potentials
ranging from 0 to -60 x 10-4 MPa.
The capillary wick sampler is an
improvement over existing samplers
since it does not require continuous
suction to provide continuous
samples and because It can collect
samples of flow which takes place
when the soil is unsaturated. While
the sampler collects volumes repre-
sentative of the flux at potentials of
-50 x 10-4 MPa, convergence at
greater potentials and divergence at
lower potentials prevent its use as a
tool for measuring flux of water or
contaminants.
This Project Summary was devel-
oped by EPA's Environmental Moni-
toring Systems Laboratory, Las Vegas,
NV, to announce key findings of the
research project that Is fully docu-
mented In a separate report of the
same title (see Project Report
ordering Information at back).
Introduction
Samples of water moving through the
unsaturated zone between the soil
surface and the groundwater table are
needed to detect and monitor any mobile
contaminants which may be moving
toward the groundwater. Soil water
samples collected from the unsaturated
zone could provide early warning of
potential groundwater pollution arising
from agricultural activities such as
fertilizers, pesticides, and salts; industrial
activities such as hazardous waste piles,
landfills, pits, ponds, and lagoons; and
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commercial activities such as buried fuel
storage tanks.
Present techniques for unsaturated
zone pore water sampling consist of
suction cup or pan (free drainage)
samplers (EPA 1977).
Each of these sampling techniques
have several flaws which make them
difficult to use and cast uncertainty on
the validity of the collected samples. This
project was, therefore, undertaken to
develop an improved unsaturated zone
soil water sampler which would over-
come some, if not all, the shortcomings
of the present samplers.
Methods and Materials
Laboratory Study Procedures
A laboratory study was conducted to
document the ability of three types of
samplers to sample known con-
centrations of organic and inorganic
chemicals. To accomplish this, three
samplers of each type; suction cup
samplers, glass block pan samplers, and
capillary wick samplers were tested by
exposing them sequentially to distilled
deionized water, water with a known
concentration of organic or inorganic
chemical, and finally distilled deionized
water again. The first solution contained
200 mg Cd L.-1 and 220 mg NO3 L-1,
added as CdNOs. The second test
solution contained 100 mg Br L'1,
added as KBr. The third solution
contained 13.4 mg L'1 trichloroethylene,
40.0 mg L-1 toluene, 9.9 mg L-1
ethylbenzene, and 13.6 mg L-1
naphthalene. Samples of the effluent
containing organics were placed in 22
mL headspace vials sealed with a Teflon
coated silicone rubber septum and
aluminum crimp cap and stored on ice
until analysis. Because the physical size
and shape of the various lysimeters
differed greatly, it was necessary to
normalize the data. This was done by
dividing the volume of collected sample
by the surface area of the sampler to
obtain what will be hereafter referred to
as an equivalent depth. Sampling
intervals were equivalent to 0.1, 0.2, 0.3,
and 0.5 cm equivalent depth for the
background distilled deionized water and
0.1, 0.2, 0.3, 0.5, and 1.0 cm equivalent
depth for the test solution and the
following distilled deionized water.
Capillary Wick Sampler Design
Three capillary wick samplers were
constructed, as shown in Figure 1.
Materials used to construct the capillary
wick sampler were chosen for (1); th
non-reactivity to inorganic and orgai
compounds and (2); the absence
substrate for microbial growth. Five <
diameter Pyrex tubing was epoxied to t
glass plate. Additional sections of pyi
tubing were attached to one anotr
using teflon lined gaskets. A 5 liter pyi
bottle was used as a sample collect!
chamber. A 1.27 cm diameter gla
wicking was chosen for use in this stu
due to its capillary rise of 54 cm and hi
saturated conductivity of 10-2 c
sec'1. The woven glass cloth used
the sampler was used on the surface
the sampler to cover the exposed wick.
Sample Analysis
Bromide and nitrate analyses we
done using specific ion electrodes (Ori
No 94-35A and 93-07, respective!'
Cadmium analyses were done using
Perkin Elmer model 603 Atom
Absorption Spectrophotometer and a
acetylene flame.
Organic analyses were done using
Hewlett-Packard (HP) 19395A Hea
space Sampler coupled to an HP g
chromatograph (GC) model 5890, whi
was in turn coupled to an HP ma
selective detector (MSD) model 5970.
HP high performance (SE-52) cros
Soil Surf ace
Glass Plate
Glass Wick Enclosed in Tube
Airvent and Overflow
Collection Chamber
Figure 1. Schematic diagram of the unsaturated zone capillary wick sampler.
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linked 5% phenyl methyl capillary
column with a film thickness of 0.11 iim,
an internal diameter of 0.20 mm, and a
length of 25 m was used for all organic
analyses. The tuning calibration com-
pound used was perfluorotributylamine
(PFTBA).
Field Study Procedures
Sampler Construction
Glass plates, 0.96 cm thick were
purchased and cut to 30 cm x 30 cm for
use in the Weswood silt loam and the
Lufkin clay. Plates 25 cm x 25 cm were
used in the Padina sand. For field
installation, 5 cm diameter plastic tubing
was substituted for the pyrex tubing.
Additionally, 3.8 L glass bell jars were
substituted for the pyrex sample col-
lection chambers.
Sampler Installation
Eight samplers were installed in each
of three soils; the Weswood silt loam, a
fine silty, mixed thermic fluventic usto-
chrept; the Padina sand, a loamy,
siliceous, thermic grossarenic paleustalf;
and the Lufkin clay, a vertic albaqualf,
fine, montmorillonitic thermic using the
following procedure.
A plot area 2.4 m by 3.7 m was
delineated and a trench 4.3 m long 1.2 m
wide and 1.5 m deep was excavated with
a backhoe along one long side of the plot
area. Four excavations conforming to the
dimensions of the sampler were dug
horizontally 46 cm into the trench
sidewall and samplers were placed into
the excavations. The area around and
beneath each sampler was then carefully
backfilled and compacted to prevent
cavities and/or damage to samplers due
to soil shifting. The nylon sample
removal tubing was encased in 2.54 cm
diameter PVC pipe which acted as a
conduit to the soil surface.
To facilitate drainage, a 60 cm
diameter sump was placed in the center
of the trench and surrounded by a 15 cm
gravel bed across the bottom of the
remainder of the trench. A continuous
sheet of plastic was placed against the
sampler sidewall and across the top of
the gravel bed and the trench was
backfilled and compacted. A parallel
trench 2.4 m from the sampler sidewall
of the initial trench was dug and the
procedure repeated. Two end trenches
were then dug, filled with a 15 cm layer
of gravel, lined with plastic and then
refilled and compacted. The resulting
plot was a 2.4 m x 3.7 m monolith of
undisturbed soil lined with plastic. A set
of gamma probe access tubes (5 cm
diameter, 0.9 m deep) were installed to
allow measurement of water content.
Two sets of tensiometers were installed
at depths of 30, 61, and 91 cm. A
piezometer was installed to record the
water table level. A wooden frame was
placed behind the plastic lining and
around the plot to a height of 20 cm
above the soil surface. A temporary rain
shelter was constructed over the plot and
water was ponded to a standing depth of
15 cm until the plot became saturated.
Upon saturation, a solution containing
100 mg Br L'1 as KBr was applied to
determine Br- breakthrough curves for
each sampler. Samples were collected
continuously until the Br concentration in
the samples equaled that of the applied
solution. Bromide measurements were
made as previously described in the
laboratory section. After breakthrough,
the plot was covered with plastic and
allowed to drain.
During drainage, soil moisture content
and soil moisture potential measure-
ments were made. Water table level and
sample volume measurements were
made and recorded. Measurements were
continued until no sample was collected
for a 24 hour time interval. During this
time, the soil surface was covered with
plastic to prevent upward flux of water.
This procedure was repeated 5 times on
the Padina sand and 3 times on the
Weswood silt loam.
Results and Discussion
Laboratory Study
When expressed on an equivalent
depth basis, all samplers achieved
breakthrough of inorganic ions by 0.3 cm
as shown in Figure 2. None of the
samplers had any initial inorganic
contaminants, and they did not
significantly alter the concentration of
any of the added ions. Measurement of
the dead volume of the different
samplers approximately accounted for
the majority of the 0.3 cm equivalent
depth required for the change in sample
concentration. Most of the maximum
concentrations measured were within 5%
of that applied and therefore were not
considered significant. Therefore, it
appears that in reference to the
samplers' ability to transmit the tested
inorganic ions, none of the samplers has
any advantage over the others.
All samplers were free of initial organic
contaminants and initial water samples
were all below the detection limits for
each of the four organics studied (Figure
3). Concentrations of trichloroethylene,
toluene, and ethylbenzene rose quickly
and were at or very near their maximum
by 0.5 cm equivalent depth. Once the
application of organics was terminated
and clean water was applied, the organic
concentrations dropped rapidly and by
0.5 cm equivalent depth were about one
tenth of the maximum concentration
measured. These data therefore indicate
that there is little to no adsorption and
desorption of organics on the sampler
materials. None of the organic concen-
trations measured reached the concen-
trations originally present in the stock
solution which was applied probably due
to losses to the atmosphere in and
around the samplers.
Field Study
Bromide Study
Normalized bromide breakthrough
curves for each of the three soils were
typical of what was expected. A relative
Br concentration of 0.5 was reached 26
to 27 hours after application began to the
Padina soil, in 12 to 15 hours for the
Weswood soil, and in 113 hours for the
Lufkin clay. Variation in bromide con-
centration collected for each sampler at
corresponding times can be attributed to
variable flow patterns within the plot and
was particularly pronounced in the
Padina sand.
The number of samplers required to
obtain an accurate estimate of the
chemical concentration in each soil
texture was determined using the
equation based on the 95% confidence
limits computed from the sample mean
as given by Snedecor and Cochran
(1978). Based on this equation, the
number of samplers needed for different
soils with different accuracy levels are
presented below.
Soil
Accuracy Level
5% 10% 15%
Number of Samplers
Sand
Loam
Clay
31 8
6 2
2 1
3
1
1
The number of samplers needed to
collect a representative sample in any
texture soil decreases as the accuracy
level decreases. Variation in number of
samplers needed between soil textures at
equivalent accuracy levels is due to
variation in population standard devi-
ations. The greatest number of samplers
needed occurs in the sand soil due to the
high standard deviation occurring in that
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Concentration
200
0 0.5,0 .5 7.0,0 .5
Volume/Surface Area fern)
Figure 2. Mean concentratiaon of Cd in
samples from pan samplers
sequentially exposed to solu-
tions containing 0, 200, and 0
mgCdL''
JO
15 20
Time (hours)
25
30
40
Figure 3.
Relative bromide concentration with time in samples from Padina soil. Data points
are mean of 7 values.
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soil texture. One possible reason for the
large population standard deviation in the
Padina sand is that there may be
numerous irregularities in texture within
the plot leading to differential flow
patterns in the soil. It may also be
possible that saturated flow through
macropores in the well structured loam
and clay soils was more uniform than
flow through the very poorly structured
sand. Some of the variability may also be
due to the 30% reduction in sampler size
needed to accommodate the high
saturated conductivity of the sand.
Soil Water Flux Measurements
For the Padina soil, sample collection
was near 3,000 cm3 hr1 at soil
moisture potentials near zero after which
the volumes decreased rapidly. At soil
moisture potentials between -25 to -85
x 10~4 MPa, the volume collected
ranged from 0 to 1360 cm3 hr'1. The
collection of samples at potentials drier
than -50 x 10'4 MPa and the large
standard deviations in the Bromide study
both indicate the presence of variable
flow patterns in this soil. From the data it
is apparent that when using these
samplers in a sandy soil the sample
collection frequency should be deter-
mined on a case by case basis, with
samples being collected at a frequency
sufficient to prevent divergence due to
overflow of the collection vessel.
The data from the Weswood were
similar in nature and again at wet
potentials (0 to +17 x 10"* MPa) the
sample collection rate ranged from 82 to
4,000 cm3 hr1. At potentials drier than
-10 x 10'4 MPa the collection rate
ranged from 0.1 to 22 cm3 hr1. Thus
sampling frequency for this soil ought to
range from daily at near saturation
conditions to weekly at drier soil
moisture conditions.
When expressed as fractional flow, it is
apparent that the samplers capture up to
11 times the actual soil water flux in the
Padina sand at soil moisture potentials
from 0 to -26 x 10'4 MPa. As the soil
moisture potential decreases, the fraction
of the flow captured crosses one at a
potential of -51 x 10'4 MPa and
decreases to 0 by a potential of -85 x
10'4 MPa. A similar trend was seen in
the Weswood soil except that the drop
was faster.
Advantages of the Capillary
Wick Sampler
1. The sampler collects an adequate
sample volume for laboratory analysis
over a range of soil textures, soil
moisture contents, and soil moisture
potentials.
2. It is possible to determine the number
of samplers needed to obtain a
representative sample of soil moisture
flux constituents in . various soil
textures.
3. Once sampler installation is complete,
it requires little energy input to a)
maintain the samplers and b) to
collect a sample.
4. The sampler can collect inorganic ions
and organic chemicals without altering
the concentration of the sample
constituents collected.
5. The sampler is composed of inert
materials which will not react with
inorganic ions or organic chemicals.
6. The sampler materials should not
present a substrate for microbial
growth.
7. The sampler provides continuous
suction without the use of a vacuum
system. Therefore, a sample can be
collected from any soil which has a
matric potential wetter than -70 cm
of water.
8. The sampler provides continuous
collection of leachate, therefore
periodic removal of samples should
provide an integration of the con-
centration of constituents passing a
given depth.
9. The capillary wick sampler is suitable
for use in all soil textures. Best results
are obtained in well structured soils,
i.e.,the loam and the clay, as opposed
to unstructured soils such as the sand.
Conclusions
A capillary wick sampler capable of
maintaining a potential of -50 x 10'4
MPa was designed, constructed from
commercially available materials, and
tested in both the laboratory and field.
Laboratory testing of the capillary wick
sampler showed no adsorption/
desorption of inorganic ions or organic
chemicals. Breakthrough curves of
inorganic ions and organic chemicals
were similar for the capillary wick
sampler, the suction cup sampler and the
pan sampler.
Field measurements over a range of
soil textures indicated that to collect a
sample representative of the chemicals
flowing in the soil solution within 95%,
90%, and 85% confidence intervals that
sandy soils would require 31, 8, and 3
samplers, loam soils 6, 2 and 1 samplers,
and clay soils 2, 1 and 1 samplers,
respectively. Use of the capillary wick
sampler to measure soil moisture flux is
not feasible. When soil moisture contents
are high, the sampler collects in excess
of 100% of the soil moisture which
should be intercepting the glass plate
due to convergence of soil moisture in
response to the potential generated by
the wick material. These data also
suggest that the capillary wick sampler
should not be used to determine soil
moisture flux in areas where a perched
water table exists within 50 cm below the
glass plate. Soil moisture samples can
still be collected but care should be taken
in data interpretation since back
siphoning from the water table into the
sampler may occur. As the soil dries to
-50 x 10'4 MPa, sample volumes
collected approach 100% of the soil
solution intercepted by the sampler. At
potentials drier than -50 x 10'4 MPa,
less than 100% of the soil solution which
should be intercepted by the sampler is
collected. Divergence of soil moisture
flow can also occur if the sampler
collection chamber is not emptied before
it fills completely.
References
EPA, 1977. Procedures Manual for
Groundwater Monitoring at Solid Waste
Disposal Facilities. U.S. EPA Office of
Solid Wastes. SW-616.
Snedecor, G. W. and W. G. Cochran.
1978. Statistical Methods. Iowa State
University Press. Ames, Iowa, pp.
516-517.
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K. W. Brown, J. C. Thomas, and M. W. Holder are with Texas A&"M
College Station, TX 77843.
Lawrence Eccles is the EPA Project Officer (see below).
The complete report, entitled "Development of a Capillary Wick Unsaturated Zone
Pore Water Sampler" (Order No. PB 89-129 100/AS; Cost: $21,95, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Las Vegas, NV 89193-3478
United States Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S4-88/001
0000 529 PS
U S eNVJR PROTECTION AGEHCT
REGION 5 LIBRARY
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