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

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