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Organic Geochemistry xxx (2009) xxx–xxx
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Organic Geochemistry
journal homepage: www.elsevier.com/locate/orggeochem
Stable isotopes applied as water tracers in column and field studies
Paul Koeniger a,1,*, Christian Leibundgut b, Timothy Link c, John D. Marshall c
a
LIAG Leibniz Institute for Applied Geophysics, Hannover, Germany
IHF Institute of Hydrology, Albert-Ludwigs-University, Freiburg, Germany
c
CNR College of Natural Resources, University of Idaho, Moscow, USA
b
a r t i c l e
i n f o
Article history:
Received 29 October 2008
Received in revised form 7 July 2009
Accepted 8 July 2009
Available online xxxx
a b s t r a c t
The stable isotopes deuterium (2H, D) and oxygen 18 (18O) were applied in water for use as tracers in column experiments and in two field studies. Their performance was compared against uranine and was
used to characterize saturated and unsaturated water movement and depths of plant water uptake. Deuterium and 18O are completely soluble and chemically and biologically stable. They are not subject to
radioactive decay like tritium, nor photodegradation and sorption processes, like uranine. The column
studies were conducted under saturated conditions; they explored variations of (i) flow rates (1.4–
3.5 ml min1), (ii) column lengths (0.5, 1.0, 1.5 m), and (iii) tracer concentrations (0.07, 0.14, 0.28 ml of
a 99.8% D2O solution). A one dimensional dispersion model was used to generate parameters that allowed
us to compare the tracers. The column experiments showed higher mean transport velocities and smaller
dispersion coefficients for deuterium in comparison to uranine. The first field study, on a rain dominated
floodplain, found unsaturated flow rates of 0.03–0.04 m day1. The second field study examined snowmelt infiltration on a loess soil and found unsaturated flow velocities of 0.002–0.004 m day1 over a
six month period. Plant samples taken from the soil plots during late spring and summer reflect decreasing soil water deuterium concentrations and indicate depths of plant water uptake. Stable isotopes of
water proved to be useful as a tracer in all studies and offer a suite of new possibilities in the field of biogeosciences because of the ability to directly label water molecules and to analyze small sample aliquots.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Tracer techniques have proven to be one of the most powerful
tools to characterize water residence times, flow and pollutant
transport in hydrological systems. Studies using stable isotopes
as naturally occurring tracers are numerous (e.g., Peters and Leibundgut, 1993; Adar et al., 1995; Leibundgut, 1995; Kendall and
McDonnell, 1998), whereas hydrological applications using isotope
labelled water with deuterium (2H2O, D2O), oxygen 18 (H218O), or
double-labelled water (2H218O) are relatively uncommon.
In groundwater systems, deuterium label was used as an applied tracer by Garnier et al. (1985), who injected 260 g 2H as labeled solution (2H2O) together with iodide (I), carbon 13
(H213CO3) and uranine during a radial flow tracer experiment over
a distance of 10 m to compare the behavior of the four tracer solutions in a field experiment. Garca Gutirrez et al. (1997) compared
uranine and deuterium (12 l D2O) injected during a saturated zone
* Corresponding author. Address: Stilleweg 2, 30655 Hannover, Germany. Tel.:
+49 511 643 3072; fax: +49 511 643 3665.
E-mail address: [email protected] (P. Koeniger).
1
Formerly in IHF Institute of Hydrology, Albert-Ludwigs-University, Freiburg,
Germany and CNR College of Natural Resources, University of Idaho, Moscow, USA.
tracer experiment over a distance of more than 20 m to gain insight into the field transport of tracers for a characterization of a
potential radioactive waste repository site. Maloszewski et al.
(1999) used deuterium (480 ml D2O 100%) as a tracer simultaneously with bromide (Br) and the fluorescent tracer eosine for
a groundwater test over a distance of 11 m to examine the role
of a major fault in the transport of water and pollutants through
fractured rock, and to demonstrate advantages of multi-tracer tests
for determining flow and rock parameters. This research indicated
the advantage of using conservative tracers in combination with
sorptive tracers to evaluate tracer loss by sorption versus loss by
diffusion into a matrix. Becker and Coplen (2001) report experiments in fractured crystalline bedrock (granite and schist) comparing deuterium, bromide and pentafluorobenzoic acid (PFBA). The
tracer transport in groundwater over a distance of 36 m was similar for all tracers; however PFBA showed a slight difference, probably due to its lower rate of molecular diffusion. Becker and Coplen
(2001) discuss injection and sampling, detection limit and density
problems as well as the applicability of deuterated water as a conservative groundwater tracer.
Unsaturated zone studies using deuterium as an applied tracer
are not as common as saturated zone studies. Lischeid et al. (2000)
used deuterium together with bromide and chloride (Cl) during
0146-6380/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
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an irrigation experiment under steady state flow conditions to assess uncertainty of tracer experiments under identical boundary
conditions. Schumann and Herrmann (2001) used D2O to identify
preferential flow pathways of water in the unsaturated zone on
microscale test plots during an irrigation experiment. Hangen
et al. (2005) used three tracers (bromide, terbuthylazine and
D2O) to assess preferential water movement and solute leaching
in a forest reclaimed lignitic mine soil. Other authors reported results of experiments carried out with deuterium under controlled
conditions in lysimeters (e.g., Russow et al., 1996; Schoen et al.,
1999; Mali et al., 2007). Russow et al. (1996) used D2O, bromide
and 15NO3 in three lysimeter experiments to estimate water movement and residence times, investigate nitrate displacement and
compare tracer behavior. Schoen et al. (1999) used D2O and reactive solutes simultaneously to analyze preferential flow in an
undisturbed lysimeter of 120 cm diameter. A tracer lysimeter
experiment using deuterium and uranine is reported by Mali
et al. (2007). Deuterium turned out to be more appropriate than
uranine to investigate water flow in the unsaturated zone of a
coarse gravel aquifer.
Several hydro-ecological studies in the soil–water–plant interface were published recently, where deuterium was used as tracer
for sap flow investigations (Kalma et al., 1998; Marc and Robinson,
2004), hydraulic redistribution (Brooks et al., 2002), transpiration
(Marc and Robinson, 2004) and radial water transport and storage
in trees (James et al., 2003; Meinzer et al., 2006).
Although tritium was the best labeling choice in the past, the
application of tritium is now restricted for environmental and
health concerns (see Moser and Rauert, 2005 for a recent review).
As a consequence dye tracers have been increasingly used in the
unsaturated zone. Dye tracers show a range of sorptive behaviors
but have been used to simulate nutrient and pollutant transport
in soils, aquifers and rivers. Uranine is one of the most widely used
fluorescent dye tracers in groundwater studies and is generally
considered to be a conservative (i.e. non-sorptive) tracer. It is easy
to handle, non-toxic, inexpensive and has excellent detectability,
however it cannot be used as a tracer for plant water uptake
studies.
An advantage of stable isotope tracers is that unlike tritium and
dye applications, they do not introduce radioactive or chemical
contaminants into the environment. By definition, they display
conservative behavior, as deuterium-labelled and (oxygen-18)
double-labelled water actually comprise the water molecule. They
can therefore be used to study interactions of hydrogeological and
ecological processes (i.e. unsaturated zone movement, plant water
uptake and hydraulic lift, xylem transport, leaf evaporation). Furthermore, stable isotope tracers are used to address contaminant
fate and transport, which is of interest for organic geochemists,
and a challenging research topic in the field of biogeosciences.
An understanding of snowpack processes is also important for
investigations of water resources, climate change and plant water
availability during the growing season. In numerous studies, naturally occurring stable isotopes were used for snow accumulation
and ablation studies, determination of snowmelt infiltration,
snowmelt contribution to runoff, and soil water movement (see
Koeniger et al., 2008 for a recent review). Labeling of snowmelt
infiltration with stable isotopes can be an advantage, since the tracer will not preferentially elute from the snowpack like ionic tracers, potentially alter the snow cover albedo, or undergo
photodegradation like dye tracers.
The general objectives of this work are to test the utility of applied deuterium for a variety of tracer applications. The specific
objectives are: (i) to systematically test deuterium against a commonly accepted dye tracer (Ur) in the laboratory, and (ii) to apply
and discuss deuterium in the field in (a) rain dominated, and (b)
snow dominated natural soil systems. To our knowledge, this is
the first study to report the use of applied isotopes to study snowmelt infiltration. Aspects of multi-tracer studies of plant water uptake are also discussed.
2. Experimental setup and methods
2.1. Column studies
Columns of 9 cm diameter and 0.5 m segment lengths were
used to compare deuterium and uranine under controlled, fully
saturated conditions. For an investigation of influence of column
length, multiple segments were connected to form 1.0 m and
1.5 m columns. The uranine molecule (C20H10O5Na2) has a diameter of about 1 nm and a molecular weight of 332.3 amu. Deuterated
water (D2O) is the size of the water molecule (0.2 nm) and when
dissociated to HDO, has a molecular weight of 19 amu. Nine column experiments were conducted in the laboratory of the Institute
of Hydrology in Freiburg, Germany (IHF). For uranine experiments,
1 ml was injected for each experiment and solutions with 0.04 g l1
were used except for the experiment with varying concentrations,
where 0.01, 0.02, 0.04 g l1 were used. For deuterium, 0.28 ml of a
99.8% deuterium solution were injected, and for the experiment
with varying concentrations, 0.07, 0.14, 0.28 ml were used. In total
we used 0.3 mg uranine and 2.2 ml concentrated deuterium for the
column experiments.
The protocol detailed by Klotz and Schimmack (1992) was
followed for preparation and filling of the columns. The substrate
was mixed to reflect the grain size distribution of a local aquifer that
consisted of 28% fine gravel (2–3.2 mm), 25% sand (0.63–2 mm), 31%
medium sand (0.2–0.63 mm), 10% fine sand (0.063–0.2 mm) and 6%
silt (0.002–0.63 mm), by weight. No clay, organic material or
contaminants were added to the substrate or introduced by mixing.
The material was homogenized and filled under wet conditions. The
columns were conditioned to saturated water flow conditions over a
period of 90 days until a constant flow rate was reached.
An evaluation of the column experiments was conducted using
a one dimensional dispersion model to compare the measured
with the calculated curves using only two fitting parameters, transit time (tt) and a dispersion parameter (PD) as described by Maloszewski et al. (1983) and Maloszewski and Zuber (1990). The
transport equation for one dimensional flow (Eq. (1)) and its analytical solution (Eq. (2)) are given as:
@C
@2C
@C
¼ DL 2 v @t
@x
@x
M
x
ðx v t2 Þ
Cðx; tÞ ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp Q
4 DL t
4 p DL t 3
ð1Þ
ð2Þ
where C denotes the tracer concentration (mg m3), DL is the longitudinal dispersion coefficient (DL = PD v x) in (m2 s1), v is the
flow velocity (m s1), M is the tracer mass (g), Q is the flow rate
(m3 s1), t is the tracer residence time (t = x v1) in (s) and x is
the distance in the direction of the flow (m).
2.2. Field study 1 – rain dominated system, southwestern Germany
A field scale irrigation experiment with deuterated water and
uranine was conducted in the flood plain of the upper Rhine valley
(47°560 0400 N, 7°360 0200 E, 201 m a.s.l.) to investigate unsaturated
zone water movement and infiltration depths of water under simulated precipitation conditions. The climate at this site can be described as rain dominated with a long term mean yearly
precipitation amount of 667 mm. Rainfall occurs throughout the
year with the highest amounts during the summer months of
May, June and July. Mean annual temperature is 9.8 °C and there
are typically few days with snowfall and almost no continuous per-
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iod of snow during the winter in this area, which is regarded as the
warmest climate in Germany.
The soil layer consisted of a 40 cm thick layer of fluvial deposits
of sand (40%), silt (50%) and clay (10%) with a field capacity of
approximately 120 mm (0.30–0.35 vol/vol) and a wilting point of
40 mm (0.11–0.13 vol/vol) followed by a deeper unsaturated zone
of sand and coarse gravel with a much lower field capacity
(0.04 vol/vol) (Sturm et al., 1996). The organic matter content of
the soil layer at the test site was described in detail by Trüby
(1983). Organic carbon and total nitrogen are typically distributed
in the soil layer. Maximum values of 33 mg Corg and 9.9 mg total
nitrate per g total substrate in the upper 0–5 cm decrease continuously with depth. An organic content of 7% was estimated for
the 0–5 cm soil layer. C/N ratios of 38, 17 and 14 were found in
depths of 2–0, 0–5 and 5–15 cm, respectively (Trüby, 1983). Root
systems and organic material are rare in the deeper soil layer.
The water table at this site is approximately 7 m below surface
due to flow regulation induced erosion and degradation of the river
bed and therefore had no influence on the experiment at this site.
The natural vegetation was altered due to the anthropogenically
induced groundwater decline and consists of a predominantly pine
forest (Pinus sylvestris L., Pinus nigra Arn.) with a mean height of
15 m and an understory of grasses (Melica mutans) (see Brandes
et al., 2007 for more details on the study site).
On two soil plots of 0.24 m2 each, a simulated precipitation
event of 26 mm occurred over 20 min. The event comprised
6.25 l of tracer solution containing 1.6 g l1 uranine and a deuterium concentration of +272‰ (0.4 ml of a 99.8% D2O solution per
plot). A higher concentration of uranine was used in comparison
to the column studies, because we expected sorptive loss on organic material. The plots were sheltered by a tent shaped suspended
plastic sheet after the tracer application to prevent additional rain
water infiltration, but to allow evaporation and transpiration to occur. Soil profiles were sampled on the day of injection for background isotope analyses and 12 and 35 days after injection by
soil excavation to a depth of 1.2 m. The samples were stored in
air tight plastic bags and transported to the laboratory for soil
water extraction. Soil moisture was measured gravimetrically after
drying soil aliquots for 24 h at 110 °C.
2.3. Field study 2 – snow dominated system, interior Pacific Northwest,
USA
A tracer experiment with deuterium and oxygen 18 was conducted on a loess soil near Moscow, Idaho (46°450 4200 N,
116°440 4800 W, 850 m a.s.l.), US to evaluate snowmelt infiltration
dynamics. In contrast to the site in Germany, this site receives a
large component of snowmelt input. Long term (30 yr) mean precipitation for the site is 706 mm and most of the precipitation occurs as snow during the winter months. Mean annual
temperature is approximately 8.3 °C. The summer months (May–
October) are hot and dry; during this time, where evaporation
and transpiration exceed precipitation and the resulting water deficit limits plant growth.
Four comparable 1 m2 soil plots were prepared on a flat meadow
(Lolium perenne L.) site. Tracer was applied at the base of a 0.33 m
thick snow layer. A syringe and stainless steel needle were used
to apply deuterated water (250 g D2O 70% per plot) on a predefined
pattern of 13 injection points on each plot to avoid disturbance of
the snow layer while establishing a homogenous distribution of
the labeled water. Snow water equivalent (SWE) was measured
on the 21st of January, 26th of January and 3rd of February 2007
with a Snowmetrics volumetric snow sampler (Elder et al., 1991).
Preparation of the plots and deuterium injection took place on
January 21, 2007 on four soil plots (A–D). Plots A and B were covered with dark plastic subsequent to tracer application to prevent
3
evaporative loss of the tracer. Five days later (January 26, 2007)
25 ml 10% 18O solution was sprayed on the top of the snow layer
on plot A to explore the delayed infiltration of later melt water
with an independently observable tracer. Spraying was used on
the snow surface to produce a more homogenous tracer application without any disturbance. 18O was only applied on one plot, because of the higher costs associated with the use of 18O labeled
water. After 12 days (February 2, 2007) we installed electric heating tapes powered by a portable generator to accelerate snowmelt
on plots A and B and applied NaCl (500 g) on plot A to further enhance the snowmelt rate and observe how the tracer behaves under more rapid snowmelt versus slower naturally occurring melt
rates. On two plots (C, D) no further treatment was applied to allow
sampling under natural melting conditions. Uranine was not used
as tracer for the snow experiments because it would potentially alter the snow albedo and it would be degraded by shortwave
radiation.
Because soil sampling is inherently destructive, samples were
collected for tracer analyses in time steps of every few weeks on
one plot. Samples were collected at a vertical resolution of
2.5 cm to a depth of 1 m through the end of the growing season
in mid-July 2007 to allow a temporal assessment of soil water
movement and a calculation of unsaturated zone travel times. In
addition, two replicate samplings were conducted with a soil corer
at plots C and D in July to check for heterogeneities within a plot at
the end of the experiment. The soil samples were sealed in tight
plastic bags and transported to the laboratory for soil water extraction. In addition, three replicates of plant samples were collected
from each of the soil plots and an untreated background site for
water extraction and isotope analyses during the sampling in
May, June and July.
2.4. Soil water and plant water extraction and tracer analyses
Several methods are discussed in the literature for extraction of
soil and plant water for isotope analyses (e.g., Revesz and Woods,
1990; Walker et al., 1994; Araguas-Araguas et al., 1995; Scrimgeour, 1995; Hsieh et al., 1998; Koehler et al., 2000; West et al.,
2006). We used the azeotropic distillation with toluene (Revesz
and Woods, 1990) for the soil samples collected in the rain dominated flood plain site and the cryogenic extraction with an apparatus described by West et al. (2006) for samples collected in
Moscow, USA because these methods were shown to be effective
for extraction of soil water for environmental isotope analyses
and were employed at the laboratories used for these
investigations.
Temperatures can have an influence on the water pools that are
extracted when working with naturally occurring concentrations
and extraction methods can therefore cause higher standard errors
than usually reported for isotope analyses (Ingraham and Shadel,
1992; Walker et al., 1994; Araguas-Araguas et al., 1995). A direct
comparison of environmental isotope concentrations derived with
different extraction methods should be interpreted with care because of the issues mentioned above. This is however expected
to be of minor importance when working with an introduced isotopic label, where a clear tracer signal can be interpreted. Interpretations reported in this study mainly rely on observed isotope
peaks and not on a direct comparison of delta values. Transpired
water from plant samples was collected directly with plastic bags.
Deuterium was analyzed by chrome reduction of water (Gehre
et al., 1996) with a ‘‘Thermo Finnigan H/Device” connected to an
isotope ratio mass spectrometer in dual inlet mode and expressed
as relative concentrations in delta units,
d2 H ¼
RSample
1:000%
RV -SMOW
ð3Þ
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where R is the ratio (2H/1H) of the less abundant to the more abundant isotope in the sample and an international standard (VSMOW). Relative values can be converted to mg L1 after Becker
and Coplen (2001) using Eq. (4):
2
Hconc ¼ 34:82 ð1000 þ d2 HV-SMOW Þ=1000
ð4Þ
or absolute concentrations in parts per million as described by Rodhe (1998) in Eq. (5), which is more suitable for modeling of isotope
concentrations:
"
2
Hconc ¼
!
#
d2 H
þ 1 157:6
1000
using a Perkin Elmer LS50B luminescence spectrometer with a
detection limit of 0.002 mg m3 (or parts per billion ppb) at the
IHF.
3. Results
In the following sections we describe results from labeling
experiments in the laboratory first and then compare field studies
derived for the rain dominated site in Germany and the snow dominated site in the northwestern US.
ð5Þ
3.1. Column studies
2
2
Hconc denotes the deuterium concentration (ppm) and d H denotes the relative concentration (‰ V-SMOW). Isotope analyses
were conducted on a Finnigan Delta S mass spectrometer at the
IHF, Freiburg University in Germany and on a Finnigan Delta Plus
at the Idaho Stable Isotopes Laboratory (ISIL) at the University of
Idaho in Moscow, USA. Uranine concentrations were measured
Concentration (ppb, ppm)
100
flow rate
(ml per min)
Breakthrough curves for deuterium and uranine derived with
varying flow rates, column lengths and varying concentrations
are plotted in Fig. 1a–c, respectively. The concentrations on the
y-axis are given in parts per million (ppm) and parts per billion
(ppb) for deuterium and uranine, respectively.
1.4
2.2
(a)
80
3.5
deuterium (filled symbols)
uranine (open symbols)
best fit deuterium
best fit uranine
60
40
column length 0.5 m
concentrations
0.04 g/l Ur, 0.28 ml D
20
0
Concentration (ppb, ppm)
0
200
400
600
800
1000
column length
80
1200
1400
(b)
0.5 m
60
1.0 m
40
1.5 m
flow rate 3.5 ml per min
concentrations
0.04 g/l Ur, 0.28 ml D
20
0
0
200
400
600
800
1000
1200
1400
Concentration (ppb, ppm)
120
concentrations
100
(c)
0.28 ml
0.04 g/l
80
0.14 ml
0.02 g/l
60
40
0.07 ml
0.01 g/l
20
column length 0.5 m
flow rate 3.5 ml per min
0
0
200
400
600
800
1000
1200
1400
Time (min)
Fig. 1. Measured values of uranine (ppb) in open symbols and deuterium (ppm) in filled symbols for column studies with (a) varying flow rates (1.4, 2.2 and 3.5 ml min1), (b)
varying column length (0.5, 1.0 and 1.5 m), and (c) varying concentrations (uranine 0.01, 0.02 and 0.04 g l1, deuterium 0.07, 0.14 and 0.28 ml of concentrated deuterium) and
dispersion model fit of each breakthrough curve.
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As flow rates decrease (Fig. 1a) peak maxima slightly decrease
for uranine but not for deuterium, and dispersion increases for
both tracers. The trend of deuterium peak concentrations shown
in Fig. 1a is subtle, in part due to experimental noise, but is discernable. As column lengths increase (Fig. 1b) peak maxima decrease
and dispersion increases for both uranine and deuterium. The time
corresponding to the maximum peaks is in the same range for both
tracers in both experiments. Varying tracer concentrations (Fig. 1c)
did not greatly influence the time of the maximum peak but did affect the peak concentration for both tracers in a similar manner, as
expected. The modeled values derived with a one dimensional dispersion model and least squares fit to the measured data are also
plotted for each breakthrough curve in Fig. 1. These curves are normalized to maximum peak concentration and time of maximum
concentration for a better comparison in Fig. 2.
The normalized values shown in Fig. 2 allow a comparison of
modeling results. For varying flow rates (Fig. 2a) and varying column lengths (Fig. 2b), the fit of the curve is similar for both tracers.
For varying concentrations (Fig. 2c), there is an obvious shift in
times of peak concentration. The fitting parameters are summarized in Table 1.
1.0
3.2. Rainfall infiltration – southern Germany
Fig. 3 and Table 2 summarize results from the irrigation experiment at the Rhine River floodplain site. Soil moisture concentrations for a background profile taken in December (open dots),
and soil profiles taken 12 days (gray squares) and 35 days (black
triangles) after tracer application are plotted in Fig. 3a. Uranine
and deuterium concentrations in the soil profile are shown in
Fig. 3b and c, respectively.
Soil moisture conditions changed only slightly within the
experimental period because the plots were shielded with plastic
covers to block precipitation, and evaporative demand and transpiration were low during the mid-winter time period. Uranine and
deuterium indicate infiltration depths of approximately 75 and
70 cm, respectively, after 12 days, and 110 cm for both tracers after
35 days. The depth of tracer penetration was estimated by using its
detection limit. For uranine, the detection limit was 0.002 mg m3
and for deuterium, a value of at least 5‰ enrichment relative to a
background value. Unfortunately the background values were only
measured to a depth of 60 cm, however it is not expected that
there would be higher fluctuations in isotopic composition from
(a)
Uranine 1.4 ml min-1
Uranine 2.2 ml min-1
Uranine 3.5 ml min-1
Deuterium 1.4 ml min-1
Deuterium 2.2 ml min-1
Deuterium 3.5 ml min-1
flow rate
C(t) / Cmax
0.8
0.6
0.4
0.2
0.0
0.0
1.0
0.5
1.0
1.5
(b)
2.5
Uranine 0.5 m
Uranine 1.0 m
Uranine 1.5 m
Deuterium 0.5 m
Deuterium 1.0 m
Deuterium 1.5 m
column length
0.8
C(t) / Cmax
2.0
0.6
0.4
0.2
0.0
0.0
1.0
C(t) / Cmax
0.8
0.5
1.0
1.5
(c)
2.0
2.5
Uranine 0.01 g/l
Uranine 0.02 g/l
Uranine 0.04 g/l
Deuterium 0.07 ml
Deuterium 0.14 ml
Deuterium 0.28 ml
concentration
0.6
0.4
0.2
0.0
0.0
0.5
1.0
1.5
2.0
2.5
t/t 0
Fig. 2. Comparison of modeled curves for (a) varying flow rate, (b) column length, and (c) concentration using a normalized time scale (t/t0) for the x-axis and a normalized
concentration (C/Cmax) on the y-axis.
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Table 1
Comparison of fitting parameters (transit time tt and dispersion parameter PD)
derived with the dispersion model for deuterium (2H) and uranine (Ur) in column
studies.
Flow rate
tt (min)
PD (–)
Column length
tt (min)
PD (–)
Concentration
tt (min)
PD (–)
Ur
2
H
Ur
2
H
3.5 ml min1
510.6
507.1
0.028
0.023
2.2 ml min1
323.6
321.9
0.022
0.021
1.4 ml min1
210.4
210.0
0.024
0.023
Ur
2
H
Ur
2
H
0.5 m
210.4
210.0
0.024
0.023
1.0 m
344.6
336.3
0.028
0.020
1.5 m
506.4
497.2
0.029
0.026
Ur
2
H
Ur
2
H
Ur
2
H
0.01 g l1
0.07 ml
216.5
212.5
0.090
0.082
0.02 1
0.14 ml
197.0
195.1
0.044
0.042
0.04 g l1
0.28 ml
193.5
195.1
0.071
0.065
Table 2
Time of sampling, depths of tracer penetration and calculated transport velocities for
the Rhine flood plain experiment.
December 12, 1999
January 13, 2000
Time after
injection
(days)
Uranine max.
deptha
(m)
Deuterium
max. depth
(m)
Velocity
12
35
0.75
1.10
0.70
1.10
0.06/0.04
0.03
(m day1)
a
Detection limit was 0.002 mg m3 after dissolution from soil samples with
distilled water.
60 to 110 cm relative to the 1–60 cm depth. Peak concentrations
for uranine were found on the surface, but for deuterium they were
at 10–15 cm below the surface.
3.3. Snowmelt infiltration – interior Pacific Northwest, USA
Snow water equivalent was 126 mm on the 21st of January,
143 mm on the 26th of January, 152 mm on the 3rd of February
2007, and snow was almost gone by the end of February 2007 with
only minor additional snowfall events during the late winter and
0
Soil depth (cm)
20
(a)
40
60
background Dec. 9, 1999
profile Dec. 21, 1999
profile Jan. 13, 2000
80
100
Soil moisture (Vol.%)
120
0
10
20
30
40
50
0
Soil depth (cm)
20
40
60
(b)
80
100
Uranine (mg/m³)
120
101
102
103
104
105
0
Soil depth (cm)
20
40
60
(c)
80
100
δ²H (‰ V-SMOW)
120
-60
-30
0
30
60
Fig. 3. (a) Soil moisture, (b) uranine concentrations, and (c) deuterium concentrations of soil plots taken before and after an unsaturated zone sprinkling experiment in the
Rhine River flood plain.
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δ18O (‰ V-SMOW)
-20
-15
-10
2500
-5
Plant water
0
Background
Plot-A
Plot-B
Plot-C
Plot-D
0
2000
δ H (‰ V-SMOW)
10
30
40
50
1500
1000
2
Soil depth (cm)
20
60
500
2
H label
18
O label
2
H background
18
O background
70
80
0
140
150
160
170
Plot A
Feb. 6, 2007
90
100
0
10000
20000
30000
40000
180
190
200
DAY
Fig. 6. Mean deuterium concentrations measured for plant samples collected from
profiles and background with error bars calculated for three replicates.
50000
δ2H (‰ V-SMOW)
values on Jan. 21, 2007 were 16.2‰, and 118‰ for d18O and
d2H, respectively) due to the higher injection amount. The 18O values on the surface layer were similar to the peak concentrations,
and concentrations decline to background levels at a depth of
40 cm, whereas small amounts of deuterium label were evident
at a depth of 60 cm.
In Fig. 5 all soil water profile samples are summarized and plotted as relative concentrations against V-SMOW (dots) and normalized concentrations relative to the maximum concentrations
(lines).
The upper panels in Fig. 5 show the soil water deuterium profiles collected through the end of May 2007. The plots all indicate
that the depth of maximum concentration consistently increased.
The lower panel figures show samples collected during July 2007,
where no water movement is apparent based on peak concentrations. However, deuterium concentrations exhibited a steady de-
Fig. 4. Comparison of d2H (black) and d18O values (gray) used as tracers for a snow
melt infiltration experiment on soil plot A, collected 15 days (deuterium) and
12 days (18O) after injection. Background values (d2H values open black circles and
d18O values open gray circles) were taken prior to the labeling experiment.
early spring period. The mean isotope values for snow samples collected between 21st of January and 3rd of February 2009 were
14.1‰ and 99‰ for d18O and d2H, respectively. The results from
the snow infiltration labeling experiment in Idaho, USA are summarized in Figs. 4–6 and Table 3. Fig. 4 shows a comparison of deuterium and 18O concentrations measured on plot A.
Deuterium and 18O maximum concentrations appeared at a
depth of 11 cm, 15 and 12 days after injection, respectively
(Fig. 4). The deuterium concentrations differed much more from
mean background values than the 18O values (mean background
Soil depth (cm)
0
0
δ2H (‰ V-SMOW)
20000 40000 60000 80000
0
δ2H (‰ V-SMOW)
2000
4000
6000
0
0
0
0
0
20
20
20
20
40
40
40
60
60
60
40
δ2H
C/Cmax
60
80
100
0.0
-80
0
Soil depth (cm)
δ2H (‰ V-SMOW)
20000 40000 60000 80000
80
Plot A
Feb. 6, 2007
0.2
0.4 0.6
C/Cmax
0.8
1.0
100
0.0
δ2H (‰ V-SMOW)
-60
-40
-20
0
Plot B
Feb. 17, 2007
0.2
0.4 0.6
C/Cmax
0.8
δ2H (‰ V-SMOW)
2000
4000
1.0
80
100
0.0
6000
0
80
Plot C
Apr. 8, 2007
0.2
0.4 0.6
C/Cmax
0.8
δ2H (‰ V-SMOW)
2000
4000
1.0
100
0.0
6000
0
0
0
0
20
20
20
20
40
40
40
60
80
100
60
Plot A
Jul. 6, 2007
100
0.0
Plot B
Jul. 6, 2007
δ H replicate plot
C/Cmax replicate plot
0.2
0.4 0.6
C/Cmax
0.8
80
100
1.0
0.0
6000
Plot D
May 31, 2007
0.2
0.4 0.6
C/Cmax
0.8
δ2H (‰ V-SMOW)
2000
4000
1.0
6000
40
2
60
80
δ2H (‰ V-SMOW)
2000
4000
60
Plot C
Jul. 15, 2007
0.2
0.4 0.6
C/Cmax
0.8
80
100
1.0
0.0
Plot D
Jul. 15, 2007
0.2
0.4 0.6
C/Cmax
0.8
1.0
Fig. 5. Deuterium concentrations derived from all profiles taken from plot A to D (black dots) and normalized (C/Cmax) curves (lines). Gray symbols are for replicate profiles on
a same plot.
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Table 3
Time after tracer application (days), depth of deuterium peak concentrations (m) and
calculated velocities of deuterium peaks for the snow infiltration experiments in
Moscow, USA.
Profile: sampled (plot)
Time after injection
(days)
Deuterium peak
(m)
Velocity
(m day1)
1a: February 6, 2007 (A)
2a: February 17, 2007 (B)
3: April 8, 2007 (C)
4: May 31, 2007 (D)
5-I:July 6, 2007 (A)
5-II: July 6, 2007 (B)
6-I: July 15, 2007 (C)
6-II: July 15, 2007 (C)
6-III: July 15, 2007 (D)
6-IV: July 15, 2007 (D)
16
27
77
130
166
166
175
175
175
175
0.11
0.14
0.16
0.34
n. d.
0.19
0.20
0.18
0.38
0.33
0.007
0.005
0.002
0.003
a
0.001
0.001
0.001
0.002
0.002
Cover with plastic, induced melting; n. d. not detectable.
crease in the soil due to transpiration loss and mixing of subsequent snowmelt and rainwater. Soil profiles collected on plots A
and B in February show the highest concentrations because snowmelt was induced on these plots and they were covered with plastic during the melting process.
Infiltration velocities calculated from the depths of maximum
concentration are summarized in Table 3.
Through the end of May, calculated velocities ranged from
0.002–0.007 m day1 and if profiles taken in July are also considered, about 0.001 m day1. The replicate measurements taken on
plots C and D in July (gray symbols) indicate comparable infiltration depth, but vary in absolute concentrations.
Fig. 6 shows deuterium concentrations in plant samples collected in triplicate on each date from the labeled plots during the
growing season. Mean values and standard deviations are plotted
against sampling date (day of the year 2007).
All plant samples except the samples collected as background
samples (dD = 59‰) and those collected on plot A in July 2007
clearly show enriched values on all dates. Highest deuterium concentrations appeared on the May 2007 sampling date (up to
1500‰) and 100–500‰ on the last sampling date at the end of July
2007.
4. Discussion
Use of deuterium and uranine in column studies under saturated water conditions showed comparable results in terms of dispersion and residence time of tracers. Modeling results
summarized in Table 1 indicate a slightly faster breakthrough for
deuterium and smaller dispersion parameters. Other studies indicated a more conservative transport behavior for deuterium in
comparison to uranine (Garca Gutirrez et al., 1997; Maloszewski
et al., 1999) and bromide (Becker and Coplen, 2001). A difference
in molecular properties (e.g. size, molecular mass, diffusion coefficient) might cause these findings. Measured peak concentrations of
deuterium in our column studies exhibited more scatter than the
uranine values (Fig. 1a and c). This indicates a higher error involved
with analyses at high concentration and extrapolation from the
SMOW-SLAP scale of environmental concentrations to more enriched samples. It is comparable to the introduction of errors during dilution in the process of calibration of high concentrations of
artificial tracers.
The normalized curves in Fig. 2c (for variable tracer injection
amounts) indicate a delayed tracer peak concentration and more
pronounced tailing of the breakthrough curve for the lowest tracer
injection amounts. At the same time it is obvious that the high concentration curves fall between the middle and lower curves. An
explanation for the first observation may be higher relative absorp-
tion at low concentrations. This may result from a greater proportion of tracer filling the less transmissive regions during the low
concentration runs, whereas during the higher concentration runs,
a similar amount, and hence a smaller proportion may be retained
in less transmissive regions, with the larger amount passing rapidly through the highly transmissive pores. The second observation
that the higher concentration curves plot between the lower and
medium ones could be explained by small fluctuations in flow rate
caused by small diurnal changes in temperature during the summer months. Flow rates were only observed before and after experimental runs after a three month long conditioning phase was
conducted with continuous monitoring of the flow conditions.
Monitoring of flow rates during the experiments was not possible
because this would have interfered with the tracer collection.
However, the experiments suggest that too low a concentration
may yield slightly different results relative to higher
concentrations.
There are both analytical advantages and disadvantages associated with the analysis of deuterium and uranine. The mass spectrometric analyses for deuterium are more complicated and more
expensive than uranine, especially for laboratories where the analysis of enriched materials is not common. Conversely, isotope ratio
mass spectrometric techniques have improved during recent years,
and higher sample throughput with less sample material is possible (e.g., 0.5 ml water sample for 18O and D analyses are usually
sufficient). It is therefore possible to analyze plant or organic samples that cannot be traced with other tracers (dyes, salt). Uranine is
difficult to use as a tracer in the unsaturated zone because of its
high sorption on organic materials, and for surface applications
due to its instability when exposed to shortwave radiation. To
prove conservativeness of D seems to be an obvious result, but direct comparative studies using this tracer are still rare, especially
in comparison to dye tracers.
The first field example on the rain dominated system in the
Rhine River floodplain indicated unsaturated flow velocities of
0.03–0.04 m day1 for D during an irrigation experiment in a sandy
soil system. Subsequent tracer tests resulted in flow velocities of
0.01–0.02 m day1 (uranine powder deposition) and 0.002–
0.004 m day1 (environmental isotopes) at this site (Koeniger,
2003; data not shown). An explanation for these variations is that
these approaches covered time scales of different lengths. The
labeling experiments were conducted within weeks to months
after simulated precipitation events, whereas the results for the
environmental isotopes were derived from seasonal changes over
a three year period.
Depleted deuterium values near the soil surface in Fig. 3 may be
explained by the condensation of water vapor during the experiment because the plots were covered to prevent the influence of
precipitation but to allow vapor exchanges at the ground surface.
Variations in the soil profile background complicate the interpretation of the tracer penetration. We expect that higher concentrations of tracer would have made the interpretations easier. It is
necessary to analyze the background concentrations and estimate
an appropriate injection amount of tracer above the natural variability. We suggest that a clear isotopic enrichment of +5‰ and
+25‰ above background levels for 18O and D, respectively, would
be approximately five times the analytical error after soil water
extraction and hence ideal for tracer studies.
A comparison of D and 18O used as artificial tracers in the unsaturated zone is possible from the snowmelt infiltration study near
Moscow, Idaho. The samples taken from the plot A soil profile in
February, 2007 (Fig. 4) show relatively high 18O concentrations
near the soil surface in comparison to D. This is due to the fact that
this tracer was applied on top of the snow layer whereas D was applied directly on top of the soil, under the snow layer. The tracer
was not pushed into the soil with the melting snow but infiltrated
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slowly from the top. Another advantage of using both D and 18O in
combination is to track melt infiltration from different layers in the
snowpack. Higher injection concentrations would also have been
more useful in this case.
The soil profile samples taken during the snow melt infiltration
experiment reflect a downward movement and infiltration depths
during the wet spring months (until the end of May). The calculated unsaturated flow velocities (0.003–0.007 m day1) are
roughly an order of magnitude lower than those observed in the
Rhine River flood plain study. This is likely due to a combination
of lower infiltration capacities in the loess type soils and by the
higher simulated rainfall intensity during the irrigation experiment in Germany, which was an equivalent of about 26 mm applied over a period of 20 min. This rainfall amount was reached
as a weekly total several times in the weeks prior to tracer application, whereas the snow melt rates are typically on the order of mm
to cm per day.
In contrast to the linear decrease of the tracer peak and flow
velocity observed for plots A–D through the end of May, the plots
during the dry months of June and July show a constant depth of
maxima and a decrease of concentrations due to dry soil conditions
with higher transpiration loss, diffusive transport by water vapor
and mixing with unlabeled precipitation. Either spatial heterogeneities in the soil (i.e. macropores and preferential pathways) or
spatial heterogeneity in the tracer injection could explain the differences in concentrations observed for profiles 6-I in comparison
to 6-II (plot C) and 6-III in comparison to 6-IV (plot D), which were
taken as replicate profiles from the same plots (plots C and D,
respectively) (gray symbols in Fig. 5). However, velocities and
depths of maximum concentrations observed from normalized values (gray lines Fig. 5) lead to the same interpretations.
Plant samples collected during the profile sampling after the
end of May reflect deuterium concentrations in the soil level of
plant water uptake. The background values (black dots) indicate
relatively stable concentrations (59 ± 5‰, N = 8) within the two
month period in comparison to the samples taken on the labeled
plots. Samples collected on plot A turned out to be very close to
background concentrations (28 ± 13‰, N = 6). This appears to
be due to the application of salt to accelerate snow melt on this
plot since there was only minor vegetation growth and transpiration loss due to the introduced salinity, and therefore higher dilution of soil water by infiltrating precipitation. This is similarly
reflected by the low D concentrations of profile 5-I taken on plot
A on July 6th.
Disadvantages of using stable isotopes as applied tracers in
comparison to other tracers (e.g. dyes) include the necessary
extraction of soil water prior to analyses and disruptive sampling
of the soil profile. However, only a small sample amount is necessary (less than 0.5 ml per sample for injection into the H-Device)
and the water extraction (e.g. cryogenic extraction) is less complicated than for studies of environmental isotope concentrations if
the signal of the label is strong enough. Variations due to substrate
or water content of soils (Walker et al., 1994; Araguas-Araguas
et al., 1995) are of less concern if the signal is distinct. For 18O, direct equilibration techniques for soil or plant materials are possible
after irradiation to eliminate biological activity (Hsieh et al., 1998).
In groundwater, applications are restricted because of the high cost
of spiking large water quantities. Deuterium can be purchased in
concentrations of up to 99% but also lower concentrations of 40%
or 70% are suitable, because it often has to be diluted prior to application depending to the study design and because lower concentrations are more affordable. However, applied stable isotope
tracers may bias the background of environmental isotope studies
in some cases.
We showed that applications of stable isotopes of water as applied tracers are useful for rain and snow infiltration and plant
9
water uptake studies and that they are more advantageous in snow
than other tracers. They are effective conservative tracers due to
their similarity to water. An application of isotopes as applied tracers is advantageous especially in combination with other tracers
and under environmental conditions where dyes cannot be used
because of contamination concerns, acidic pH levels or for ecological protection.
5. Conclusions
Deuterium used as an artificial tracer in column and unsaturated zone field studies indicated flow characteristics comparable
to those derived from uranine applications. Since numerous tracer
tests with uranine are known, the comparison of the two tracers allows a sound assessment of the suitability of D. In column studies,
D showed slightly slower transit times and less dispersion and
therefore more conservative behavior, which agrees with findings
from similar, previous studies. The field studies demonstrate that
D is useful to trace unsaturated zone water movement and plant
water uptake. We believe that stable isotope labeling is a promising tool for various applications, especially where relatively small
amounts of tracer (e.g. unsaturated zone, soil plant atmosphere
interactions) can be used to provide answers to challenging problems. Some examples include nutrient and pollutant transport in
the unsaturated zone, separation of evaporation and transpiration
fluxes, or interaction of carbon and water cycles on the leaf level,
which thereby justifies the necessary costs for the label application
and analysis. Stable isotopes of water are also advantageous because they cause less disturbance and environmental impact than
tritium, salts or dye tracers. Mixing processes (other water sources)
and fractionation (evaporation) might alter stable isotope concentrations during a tracer experiment but also can be used for efficient experimental design, especially where highly dynamic (e.g.
vapor phase processes) are involved.
Acknowledgements
We would like to acknowledge the help and contributions of
Matthias Geiges, Morten Karnuth and Steffen Holzkämper (Freiburg group) as well as Arash Malekian, Nerea Ubierna-Lopez, Kea
Woodruff, and Matt Germino (Moscow group) for assistance during laboratory and field work and useful discussions. Two anonymous reviewers contributed with helpful comments to this work.
This research was supported by LGFG Landesgraduiertenförderung
in Germany, USFS Research Joint Venture Agreement number 04D6-11010000-37 and by the National Research Initiative of the
USDA Cooperative State Research, Education, and Extension Service, grant number 2003-01264.
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Please cite this article in press as: Koeniger, P., et al. Stable isotopes applied as water tracers in column and field studies. Org. Geochem. (2009),
doi:10.1016/j.orggeochem.2009.07.006