-------
fluorescent-light is reabsorbed by other molecules), adsorption, and photo-
chemical and biological decay. These effects will be discussed in more
-detail in reference to specific dyes. Another disadvantage of fluorescent
dyes is their poor performance in tropical climates, due to chemical reac-
tions with dissolved carbon dioxide.
The advantages of using these dyes include their very high detecta-
bility, rapid field analysis, and relatively low cost and low toxicity. The
theory of fluorescence is described by Mclaughlin (1982) and Skoog and West
(1980). As described by Mclaughlin, the process of fluorescence involves
the following steps: (1) energy is absorbed by the molecule from sunlight
or an ultraviolet lamp and a transition to a higher, excited electron state
takes place; (2) the molecule relaxes from the highest to the lowest
vibrational energy of that state, losing energy in the process; and (3) if
the excess energy is'not further dissipated by collisions with other mole-
cules (quenching), the electron returns to the lower ground electron
state. This emission of energy due to the transition from the higher to the
lower state is fluorescence.
Field Methods
The basic equipment necessary for dye tracing is a manual or automatic
sampler and a field or laboratory detection device. Sampling is performed
by adsorption of dye onto packets of activated charcoal suspended in the
water (in karst topography), or by taking grab samples at a discharge point
(for karst, porous media or fractured rock studies). A filter fluorometer
or a spectrofluorometer is generally used for analysis, although visual
detection is sometimes used for qualitative results.
The tracer is introduced at a sink hole or well. The detection limit
for fluorescent dyes is very low, so the quantity of tracer used is
101
-------
is relatively small. The amount of tracer needed has been approximated for
karst systems by Drew and Smith (1969). They recommended using 60 grams of
dye per kilometer of underground travel, per 0.15 cubic meters per second of
^
discharge, at the largest likely rising. Atkinson et al. (1973) also
described a method to calculate dye injection quantities for karst tests.
One sampling technique used in karst tracing is the Dunn method,
developed in 1957. Small packets of fine mesh nylon or window screen are
filled with activated charcoal and suspended in the watercourse at the
sampling point. The dye adsorbs very strongly onto the charcoal, and is
later eluted by placing the bag in a solution of 5% NH OH and ethyl alcohol.
After soaking for one hour, the dye can be analyzed. The charcoal packets
must be changed periodically, depending on flow rates and dilution of the
tracer. Some examples of the use of this method are given in Gann and
Harvey (1975) and Drew and Smith (1969). Cotton strip detectors have been
used in a similar manner. Marston and Schofield (1962) described a tracer
test using rhodamine B and cotton detectors.
Flow-through fluorimeters are sometimes used, which eliminate the need
for sample collection. However, the most common method is collection of
samples in sample bottles. Automatic samplers have been discussed in
Chapter 3. Glass bottles should be used rather than polyethylene to avoid
adsorption (Hubbard et al., 1982). Reznek et al. (1979) described pro-
cedures for sampling and analyzing fluorescein, and many of the procedures
apply to other dyes~ The samples and standards should be buffered to within
a 5 to 11 pH range before analysis. If the samples are turbid, it is pref-
erable to centrifuge the samples rather than filter them, as dye adsorbs
102
-------
onto the filter. The dye to be injected and the samples should both be
t
stored out of sunlight and preferably in light-proof containers. Feuerstein
and Selleck (1963) found that some fluorescent dyes exhibit a 50% photo-
chemical decay in two days, even when stored in light-proof flasks. Ob-
viously, it is advisable to analyze samples as soon as possible after
sampling.
Detection and Analysis
It is possible to visually detect some dyes in water at a concentration
of about 40 ppm (Corey, 1968). This concentration is much higher than 10
ppb, which is the maximum permissible concentration allowed at drinking
water intakes (Wilson, 1968). The visual detection method is qualitative
and rarely used.
Other detectors are the filter fluorometer and the spectrofluorometer.
The filter fluorometer (or fluorimeter) consists basically of an ultraviolet
light source, glass curvets (sample holders), and sets of primary and secon-
dary filters which correspond to the absorption and emission wavelengths of
the dyes used. The filter fluorometer must be calibrated with standard
solutions at the same temperature as the samples to be analyzed. As men-
tioned before, the fluorescence of a sample is affected not only by concen-
tration of the dye, but also by background fluorescence, temperature, pH,
turbidity, photochemical decay, and adsorption. Temperature control appa-
ratus and correction charts are available, and methods to avoid the other
interferences have been briefly discussed. Two U.S. Geological Survey
publications are very useful for planning a fluorescent dye test and avoid-
ing these interferences. "Fluorometric Procedures for Dye Tracing," by
Wilson (1968) 1s a classic report, and in 1982, Hubbard et al. published a
very useful updated report, "Measurement of Time of Travel and Dispersion in
103
-------
Streams by Dye Tracing." These two are excellent references, as is the
*
article by Smart and Laidlaw (1977).
Fluorometers are available with individual sample analysis capability,
or with flow-through sampling (see Figure 4.3). They can be equipped with
strip-chart recorders, and can be powered in the field with a portable
generator (with a transformer) or a car battery (Hubbard et al., 1982).
Some of the most well-known fluorometers are made by American Instrument
Company and by G. K. Turner Associates.
Spectrofluorometers are more expensive and more complex to operate than
filter fluorometers. They are generally not taken into the field. An
example of this type of instrument is the Aminco-Bowman ultraviolet spectro-
photofluorometer made by American Instrument Company. Table 4.5 shows sen-
sitivity and minimum detection for certain dyes.
Additional Information
The effects of temperature, pH, and suspended solids concentration on
fluorescence have been mentioned. Fluorescence intensity is inversely pro-
portional to temperature. Smart and Laidlaw (1977) described the numerical
relationship and provide temperature correction curves. The effect of pH on
rhodamine WT fluorescence is shown in Figure 4.9. An increase in the sus-
pended sediment concentration generally causes a decrease in fluorescence.
Adsorption on kaolinite caused a decrease in the measured fluorescence of
several dyes, as measured by Smart and Laidlaw (see Figure 4.10).
The detected fluorescence may decrease, as in this example, or actually
increase due to adsorption. If dye is adsorbed onto suspended solids, and
the fluorescence measurements are taken without separating the water samples
from the sediment, the dye concentration is a measurement of sediment
104
-------
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105
-------
TABLE 4.5
Sensitivity and Minimum Detectable
Concentrations for the Tracer Dyes
Dye
Ami no 6 Acid
Photine CU
Fluorescein
Lissamine FF
Pyranine
Rhodamine B
Rhodamine WT
Sulfo rhodamine B
Sensitivity*
ug/1 Per Scale
Unit
0.27
0.19
0.11
0.11
0.033
0.010
0.013
0.061
Background
Reading**
Scale Units
0-100
19.0
19.0
26.5
26.5
26.5
1.5
1.5
1.5
Minimum
Detectability***
ug/1
0.51
0.36
0.29
0.29
0.087
0.010
0.013
0.061
For a Turner 111 filter fluorometer with high-sensitivity door and
recommended filters and lamp at 21°C.
*- At a pH of 7.5.
** For distilled water.
*** For a 10% increase over background reading or 1 scale unit,
whichever is larger.
Adapted from Smart and Laidlaw (1977).
106
-------
100
UJ
S 80
I
LJ
O
o:
ID
a.
60
40
20
1.0
HCI Si NoOH
HN03 It NaOH
.-^^ i
3.0
5.0 7.0
PH
9.O
11.0
Figure 4.9. The fluorescence of most dyes is
dependent on pH and the types of dominant ions
present. Results of some experiments on the
fluorescence of rhodamine WT are shown in this
figure adapted from Smart and Laidlaw (1977).
107
-------
5 10 15 20
KAOLINITE CONCENTRATION (g/l)
Figure 4.10. Most dyes will be adsorbed on fine par-
ticulate material, particularly on organic fragments
and clays. Results of experiments with the adsorption
of dyes on kaolinite (a type of clay) as reported by
Smart and Laidlaw (1977) are shown in this illustration.
108
-------
content and not of water flow. As mentioned before, the ideal separation is
with a centrifuge, as the dye can adsorb onto filter paper. Adsorption can
can occur on organic matter, clays (bentonite, kaolinite, etc.), sandstone,
limestone, plants, plankton, and even glass sample bottles. These adsorp-
tion effects are a strong incentive to choose a non-sorptive dye for the
type of medium tested.
Dyes travel slower than water due to adsorption, and are generally not
as conservative as the ionic or radioactive tracers. See Figure 4.6 in the
ion section for a comparison of chloride, dextrose, fluorescein, and I131.
Drew (1968) compared lycopodium, temperature, and fluorescein as karst
tracers and found fluorescein breakthrough to be slowest (Figure 4.11). He
questioned the ability of fluorescein to give accurate data on flow rates.
Field data comparing the more recently developed dyes are not yet available.
Atkinson et al. (1973) stated that an advantage of fluorescent dye measure-
ment over lycopodium analysis is the ability to make deductions about dis-
charges, changes in storage, and the geometry of the system. They suggest
that dyes are more useful than spores for obtaining the maximum amount of
quantitative information about a small karst system.
A final point concerning the interpretation of tracer tests is empha-
sized by Brown and Ford (1971). They obtained some very interesting results
by running three identical dye tracer tests in the same karst system. These
yielded three different flow-through times. One of the values differed by
50% from the original test value. Although only one test is generally run
due to economic considerations, it may be advisable to run several tests to
check reproducebility if accuracy is important.
109
-------
TEMPERATURE
LYCOPODIUM CONCENTRATION
FLUORESCE1N CONCENTRATION (visual)
o
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A comparison of the cost of various fluorescent dyes is given in Table
4.6. Prices are given in British pounds per kilogram for bulk dye. Volume
labeled per unit cost is also listed, and rhodamine B appears to be the most
cost effective. However, problems with its use will be discussed in a sub-
sequent section.
Some of the available toxicity data will be mentioned in regard to
specific dyes in the following section. Smart and Laidlaw (1977) discussed
the toxicity of dye tracers, but regulations may change rapidly and should
be researched before conducting a test. Current World Health Organization,
Environmental Protection Agency, and state health standards should be con-
sulted.
* Discussion of Specific Dye Tracers
Green Dyes
Fluorescein
Fluorescein, also known as uranin, sodium fluorescein, and pthalien,
has been one of the most widely used dyes. Like all green dyes, its use is
commonly complicated by high natural background fluorescence, which lowers
sensitivity of analyses and makes interpretation of results more difficult.
It has a very high photochemical decay rate compared to other dyes
(Feuerstein and Selleck, 1963), but this is generally of little concern in
ground-water tracing.
Feuerstein and Selleck (1963) recommended that fluorescein be restricted
to short-term studies of only the highest"quality water. Because this dye is
affected strongly by pH (it becomes colorless in acidic conditions), they
suggested that the sample pH be adjusted to greater than 6 before analysis.
Fluorescein also exhibits an appreciable decrease in fluorescence with
111
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112
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increasing salinity, and is similarly affected by oxidizing agents and sus-
pended solids (Reznek et al., 1979).
.Some examples of fluorescein use include a fractured rock study by
Lewis (1966). Borehole dilution tests resulted in hydraulic conductivity
values similar to pump test values. Another example is a mining subsidence
investigation in South Wales, where more than one ton of fluorescein was used
in a sand-stone tracer test (Mather et al., 1969). A distance of 1,100 feet
was traversed. Tester et al. (1982) used fluorescein to determine fracture
volumes and diagnose flow behavior in a fractured granitic geothermal reser-
voir. He found no measurable adsorption or decomposition of the dye during
the 24-hour exposures to rocks at 392°F. Omoti and Wild (1979) stated that
fluorescein is one of the best tracers for soil studies, but Rahe et al.
(1978) did not recover any injected dye in his hi 11 si ope studies, even at a
distance of 2.5 meters downslope from the injection point. The same experi-
ment used bacterial tracers successfully. Figure 4.11 compares fluorescein,
lycopodium, and temperature as karst tracers.
An advantage of using fluorescein (or any of the green dyes) is its
emission in the green band of the visible spectrum. Fluorescein can be
visually detected at a concentration of about 40 ppm, but other means of
detection are preferred since this is a relatively high concentration. The
approximate sensitivity and minimum detection limit for fluorescein are
given in Table 4.5.
Fluorescein is less costly in bulk than many of the dyes (see Table
4.6, but due to its high photochemical decay rate and high amount of adsorp-
tion, 1t increases in relative cost as the length of the test increases (more
dye must be added to compensate for loss).
113
-------
Pyranine
Another green fluorescent dye, pyranine, has a stronger fluorescent
signal than does fluorescein, but is much more expensive. It has been used
in several soil studies, and Reynolds (1966) found it to be the most stable
dye used in an acidic, sandy soil. Omoti and Wild (1979) recommended pyra-
nine and fluorescein as the best tracers for soil tests, although pyranine
is relatively unstable if the organic matter content of the soil is high.
Drew and Smith (1969) stated that pyranine is not as easily detectable as
fluorescein, but is more resistant to decolonization and adsorption. Pyra-
nine has a very high photochemical decay rate, and is strongly affected by
pH in the range found in most natural waters (Mclaughlin, 1982).
Lissamine FF
This green dye has been used primarily for aerosol tracing (Yates and
Akisson, 1963), and hasn't been used extensively in ground-water tests.
Little information is available on the performance of lissamine FF; however,
Smart and Laidlaw (1977) recommended it as the best quantitative tracer of
the three green dyes discussed. The dye is extremely stable and resistant
to adsorption losses, but is much more expensive than most dyes.
Orange Dyes
Rhodamine B
Rhodamine B was the first of the three orange (or red) dyes to be used in
water tracing. Its high adsorption losses make it a less suitable tracer
for ground-water work than rhodamine WT or sulfo rhodamine B, although it
has been used more frequently. Aulenbach et al. (1978) concluded that rho-
damine B should not be used as a ground-water tracer due to sorption losses,
and Feuerstein and Selleck (1963) reported significant adsorption. They
also found that the fluorescence of rhodamine B is affected by large
114
-------
salinity "changes. Knuttson (1968) reported that the dye is relatively
unaffected by pH in the range found in most natural waters (5-10). The dye
is sensitive to temperature (Omoti, 1977) and exhibits optical quenching by
suspended solids. Like fluorescein, rhodamine B suffers from interference
from high background fluorescence in tropical areas. It is less affected
than the other rhodamine dyes by bacteria and light, but it adsorbs readily
on bentonite, sand and gravel, till, and karst channels, pure quartz sand,
and even plastic and glass laboratory columns (see Table 4.7). Hubbard et
al. (1982) compared rhodamine B and rhodamine WT, and found high adsorption
of rhodamine B on aquatic plants, suspended clays, and glass and plastic
sample bottles. He found rhodamine WT easier to handle and more economical
than rhodamine B. Although the unit cost of rhodamine B is lower, its loss
rate is much higher than that of rhodamine WT.
Rhodamine B was decertified for use in cosmetics by the U.S. Food and
Drug Administration in the 1960's. In 1968, it was illegal for use as a
water tracer in the U.S. (Drew, 1968). Both rhodamine B and.fluorescein
were placed on toxicological classification Clll by the Food and Agriculture
Organization/World Health Organization. Of the dyes discussed in this
article, rhodamine B is generally recognized as the most toxic to man, as it
is readily adsorbed on body tissue. Currently, the U.S. Geological Survey
recommends that tracer tests should result in a final concentration less
than 10yg/l. Numerous studies related to toxicity tests for various aquatic
organisms are reported by Smart and Laidlaw (1977), and they recommend that
the dye not be used as a water tracer.
Rhodamine WT
This dye has been considered one of the most useful tracers for quanti-
tative studies, based on minimum detectability, photochemical and biological
115
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decay rates, and adsorption (Smart and Laidlaw, 1977; Wilson, 1968; and
Knuttson, 1968). Hubbard et al. (1982) stated that it is the most conserva-
tive of dyes available for stream or karst tracing.
Some recent uses of rhodamine WT include projects by Burden (1981),
Aulenbach et al. (1978), Brown and Ford (1971), Gann (1975), and Aulenbach
and Clesceri (1980). Burden successfully used the dye in a water contami-
nation study in New Zealand in an alluvial aquifer. Aulenbach and Clesceri
also found rhodamine WT very successful in a sandy medium.
Gann (1975) used rhodamine WT for karst tracing in a limestone and
dolomite system in Missouri. He used grab samples and activated charcoal
packets, and traced a 14 km (8.7 mile) path. Three fluorescent dyes (rhoda-
mine B, rhodamine WT, and fluorescein) were used by Brown and Ford (1971) in
a karst test in the Maligne Basin in Canada. The highest recovery of dye
(98%) was obtained for rhodamine WT. The fluorescein was not recovered at
all. The horizontal flow path was 1.3 miles, and a Turner III fluorometer
was used for analysis.
Aulenbach et al. (1978) compared rhodamine B, rhodamine WT, and tritium
as tracers in a delta sand. The project involved tracing effluent from a
sewage treatment plant. Sampling was performed with drive points, pumped
wells, and lysimeters. The rhodamine B was highly adsorbed, while the
rhodamine WT and tritium yielded similar break-through curves (see Figure
4.12). Rhodamine WT seems to be adsorbed less than rhodamine B or sulfo
rhodamine B (see Table 4.7). Wilson (1971) found that in column and field
studies, rhodamine WT did show sorptive tendencies.
Rhodamine WT is thought to be slightly less toxic than rhodamine B and
sulfo rhodamine B (Smart and Laidlaw, 1977). This source notes that rhoda-
mine WT and fluorescein are of comparable toxicity, but Aley and Fletcher
116
-------
300
c
£
k_
o
Figure 4.12. Although many researchers have found that
rhodamine WT is sorbed on aquifer material, data presented
by Aulenbach et al. (1978) suggest that this dye can be
used in coarse, permeable sand. Comparative data from the
study by Aulenbach et al. (1978) using tritium and rhoda-
mine WT indicate little difference between the two tracers
as shown in this figure adapted from their study.
117
-------
TABLE 4.7
Measured Sorption of Dyes on Bentonite Clay
Losses Due to
Dye Adsorption on Clay
Rhodamine WT 28%
Rhodamine B 96%
Sulfo Rhodamine B 65%
Source: Repogle et al. (1966)
118
-------
(1976) stated that rhodamine WT is not as "biologically safe" as fluo-
rescein.
Sulfo rhodamine B
Sulfo rhodamine B, also known as pontacyl brilliant pink, has not been
used extensively as a ground-water tracer. Its fluorescence is affected
slightly by high salinity, and it exhibits low adsorption on suspended
sediment (Feuerstein and Selleck, 1963). Table 4.7 compares the adsorption
of the rhodamine dyes onto bentonite. This dye is more expensive than the
other rhodamine dyes, and its toxicity appears to be slightly higher than
that of rhodamine WT.
Blue Dyes
The optical brighteners are blue fluorescent dyes which have been used
in increasing amounts in the past decade in textiles, paper, and other
materials to enhance their white appearance. Water which has been contam-
inated by domestic waste can be used as a "natural" tracer, if it contains
detectable amounts of the brighteners. Glover (1972) described the use of
optical brighteners in karst environments. Examples of the brighteners are
amino G acid and photine CU. These two are the least sensitive of the dyes
reviewed (see Table 4.5), but the blue dyes have much lower background
levels in uncontaminated water than do the green or orange dyes.
Photine CU is significantly affected by temperature variations, and
both dyes are affected by pH below a pH of 6.0. The dyes have high photo-
chemical decay rates, similar to those of pyranine and fluorescein. Amino G
acid 1s fairly resistant to adsorption.
Toxicity studies on optical brighteners were reviewed by Akamatsu and
Matsuo (1973). They concluded that the brighteners do not present any toxic
hazard to man, even at excessive dosage levels.
119
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Some Common Nonionized and Poorly Ionized Compounds -'"
A number of chemical compounds will dissolve in water but will not
ionize or will ionize only slightly under normal conditions of pH and Eh
found in ground waters. Some of these compounds are relatively difficult to
detect in small concentrations, others present a health hazard, and still
others are present in moderate to large concentrations in natural waters
thus making the background effects difficult to deal with in most settings.
A list of a few of these compounds is given in Table 4.8.
The use of these and similar compounds as injected tracers in ground
water is limited to rather special cases. Of those listed, boric acid would
probably act most conservatively over long distances of ground-water flow.
Boric acid has been used successfully as a tracer in a geothermal system
(Downs et al., 1983). Large concentrations, 1,000 mg/1 or more, would need
to be used for injected tracers which, unfortunately, would pose difficult
environmental questions if tracing were attempted in aquifers with potable
water. From the standpoint of health concerns, sugars would be the most
acceptable; however, they decompose rapidly in the subsurface and also tend
to be sorbed on some materials. Results of an experiment using dextrose are
shown in Figure 4.6. Alcohols such as ethanol would also tend to be sorbed
on any solid organic matter which might be present. Another problem with
the use of most of these compounds as tracers is that they would need to be
introduced in moderately large concentrations which in turn would materially
change the density and viscosity (particularly for glycerin) of the injected
tracer mixture.
120
-------
TABLE 4.8
A List of Some Simple Compounds Which are Soluble in Water
Name
Formula
Remarks
Silicic Acid
Boric Acid
Acetic Acid
(After combination
with water)
H3B03
Phosphoric Acid H PO
3 i
Present in normal ground
water in non-ionized form in
concentrations of between 4
and 100 mg/Ji. Low toxicity.
Present in normal ground
water in nonionized form in
concentrations of 0.05 to- 2.0
mg/Ji. Toxic to plants above 1
to 5 mg/A. Toxic to humans in
higher concentrations.
Ionizes above pH of 6.0. Will
form complexes with other dis-
solved constituents. Sorbs on
or reacts with most aquifer
materials. Natural concentra-
tions mostly between 0.05 mg/2.
and 0.5 mg/i.
Moderately toxic in high con-
centrations. Water soluble.
Natural concentrations are less
than 0.1 mg/2 in ground water.
Ethyl Alcohol
(Ethanol)
Sugars
Sucrose
Maltose
Lactose
Glucose
Glycerol
(Glycerin)
C2H60
C12H22°11
C12H22°11
C12H22°11
C6 H12°6
Major component of alcoholic
drinks. Water soluble.
Natural concentrations are less
than 0.05 mq/t in ground water.
Major components of human and
animal foods. Water soluble.
Probably less than 0.2 mg/£ in
most ground water.
Water soluble. Low toxicity.
Probably absent in natural
ground water.
121
-------
Some of these compounds such as sugars, nevertheless, may be useful for
simulating the movement of other compounds which are also subject to rapid
decomposition but which are too hazardous to inject directly into aquifers.
Detection: Silica and phosphates can be determined by rather simple
colorometric methods using standard solutions and.photometric detectors.
Boron is also detected by colorometric methods but the chemical procedure is
*
more complicated than for silica and phosphate. The organic compounds
listed in Table 4.8 are probably best detected by chromatographic methods.
Also, high concentrations of glycerin and sugars are detected easily by
optical refraction techniques.
Gases
Introduction: Numerous natural as well as artificially produced gases
have been found in ground water. Some of these gases can serve as tracers
which are already introduced, generally by natural processes, into the
ground-water system. In addition, gas can be injected into ground water
and the gas which is consequently dissolved can then serve as an injected
tracer. Only a few examples of injected gases used for ground-water tracers
are found in the literature.
The amount of gas which is dissolved in water increases with the gas
pressure, decreases with an increase of temperature, and decreases with an
increase of the salinity of the water. In most situations, once gas is dis-
solved in ground water at near-atmospheric pressures, the gas will tend to
stay in solution as the water enters the ground-water system. This is due
to the fact that fluid pressure increases rapidly as water moves downward
into an aquifer and the gas will effectively be under a pressure far above
122
-------
the original pressure. If gas;such as methane (CH^) is being generated in
the subsurface in large quantities, however, this gas may work its way as
undissolved bubbles of gas through the aquifer and will remove much of the
preexisting dissolved gases from the ground water.
Gases of potential use in hydrogeologic studies are listed in Table
4.9.
Inert Radioactive Gases: Chemically inert but radioactive 133Xe and
85Kr appear to be suitable for many injected tracer applications (Robertson,
1969; and Wagner, 1977) provided legal restrictions can be overcome. Of the
natural inert radioactive gases, 222Rn is the most abundant. It is one of
the daughter products from the spontaneous fission of 238L). Radon is pre-
sent in the subsurface, but owing to the short half-life (3.8 days) of its
principal isotope, 222Rn, and the absence of parent uranium nuclides in the
atmosphere, radon is virtually absent in surface water which has reached
equilibrium with the atmosphere. Surveys of radon in surface streams and
lakes have, therefore, been useful in detecting the locations of places
where diffuse ground water enters surface waters (Rogers, 1958).
Inert Natural Gases: Because of their nonreactive and nontoxic nature,
noble gases are potentially useful tracers. Helium is used widely as a tra-
cer in industrial processes. It also has been used to a limited extent as a
ground-water tracer (Carter et al., 1959). Neon, krypton, and xenon are
other possible candidates for injected tracers because their natural concen-
trations are very low (Table 4.9). Although the gaSes do not undergo chem-
ical reactions and do not participate in ion exchange, the heavier noble
gases (krypton and xenon) do sorb to some extent on clay and organic
material.
123
-------
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The very low natural concentrations of noble gases in ground water make
them useful as tracers, particularly in determining ground-water velocities
in regional aquifers. The solubility of the noble gases decreases with an
increase in temperature. The natural .concentrations of'these gases in
ground water are, therefore, an indication of surface temperatures at the
time of infiltration of the water. This fact has been used to reconstruct
the past movement of water in several aquifers (Sugisaki, 1969; Mazor, 1972;
Andrews and Lee, 1979).
Fluorocarbons: Numerous artificial gases have been manufactured during
the past decade and several of these gases have been released in sufficient
volumes to produce measurable concentrations in the atmosphere on a world-
wide scale. One of the most interesting groups of these gases are the
fluorocarbons (Table 4.10). The gases generally pose a very low biological
hazard, they are generally stable for periods measured in years, they do not
react chemically with other materials, they can be detected in very low con-
centrations, and they sorb only slightly on most minerals. They do sorb
strongly, however, on organic matter.
Fluorocarbons have two primary applications. First, as an environmen-
tal tracer, they can be used in the same way tritium is used. Because large
amounts of fluorocarbons were not released into the atmosphere until the
late 1940's and early 1950's, the presence of fluorocarbons in ground water
indicates that the water was in contact with the atmosphere within the past
30 to 40 years and that the ground water is very young (Thompson and Hayes,
1978). The second application of fluorocarbon compounds is for injected
tracers (Thompson, Hayes, and Davis, 1974). Because detection limits are
so low, large volumes of water can be labeled with the tracers at a rather
125
-------
TABLE 4.10
Properties of Fluorocarbon Compounds
Common -Name
Freon-11
Freon-12
Freon-113
Chemical
Formula
CC13F
CC12F2
CC12F-CC1F2
CBrClF2
CBr.F,
C BrI-CBrF0
Boiling Point
at 1 atm (°C)
23.8
-29.8
47.6
-4.0
24.5
47.3
Solubility in Water
at 25°C (weight %)
0.11
0.028
0.017
unknown
unknown
unknown
126
-------
modest cost. Despite the problem of sorption on natural material and espe-
cially on orgam'cs (Figure 4.13), initial tests have been quite encouraging.
Pi eld Methods
Because the tracers are gases, it is most convenient to transport them
to the field in pressurized containers for tracer injection. The gas is
then bubbled into the water which is used for the tracer. For qualitative
work, this is a simple task. If the initial tracer concentrations are to be
established quantitatively, the gas injection should be made first into a
container where the gas and injected water are turbently mixed and brought
into equilibrium at a known temperature and pressure. Provision should be
made to sample this labeled water just before it is injected into the aqui-
fer to insure that the initial concentrations are constant during the test.
For most fluorocarbons, the tracers are dissolved first in the labora-
tory in methanol or some other solvent which is then injected as a liquid
into the water which is used for the tracer test.
The most critical aspect of the field work is the sample collection and
preservation. All gas tracers will be lost rapidly to the atmosphere unless
samples are sealed in metal or glass containers. Most plastic containers
are somewhat permeable to gas. Even certain types of glass are slightly
permeable to light gases. Furthermore, all seals and caps should be metal
or glass if fluorocarbons are being used because these compounds are sorbed
strongly on many greases and plastic sealers.
The problem of the storage and shipping of water with fluorocarbon tra-
cers 1s one of the major limitations of this method. Glenn Thompson, who
has worked extensively with these tracers, has developed an analytical sys-
tem for field use which largely eliminates the problem of sample integrity
(Thompson and Hayes, 1978).
127
-------
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0
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tNaCI
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i i i i
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PORE VOLUME
NaCI
CC13F
1234
PORE VOLUME
Figure 4.13(b). Tracer elution curves for laboratory experi-
ments with NaCI (common salt) and CC1,F (Freon-11 of" trichloro-
fluoromethane) using (A) Ottowa sand (no fine material) (B)
Yolo sandy loam (small amount of clay and'some silt), and (C)
crushed coal. Note that NaCI curves are similar for all experi-
ments but that fine inorganic material reduces the peak concen-
tration and delays the breakthrough curve for CC13F. Crushed
coal, like most solid natural organic material, will adsorb most
of the CC13F and will release it very slowly to the water as it
passes through the test column. Data from Brown (1980).
129
-------
Analysis
Although a number of inexpensive gas-analysis kits are available, these
are generally unsatisfactory for tracer studies. Quantitative analyses of
gas should be done either with a gas chromatograph (GC) or a mass spectrom-
eter (MS). Commonly a combined instrument, the GCMS, is used. The use of
these analytical instruments is standard and within the training of all good
analytical chemists. The difficult or nonstandard part of the analyses for
most chemists, however, is in the method by which the tracer gases are
removed quantitatively from the sample and fed into the analytical system.
For most laboratories, the development of a gas stripping system for the
samples is not a trivial task unless the chemists have had previous ex-
perience with the analysis of gas from water samples.
The measurement of fluorocarbon compounds is generally accomplished
with an electron-capture detector used in conjunction with a gas chromato-
graph. Special care should be taken that no plastic connectors and valves
are in contact with the sample being analyzed.
Stable Isotopes
Introduction: In this short section, we will look briefly at the use
of natural stable isotopes for water tracers. A detailed treatment of the
topic, however, is beyond the scope of this manual. The reader is referred
to several excellent summaries of the topic (Gat, 1971; Fritz and Fontes,
1980; and Ferronsky and Palyakov, 1982).
An Isotope is a variation of an element produced by differences in the
number of neutrons in the nucleus of that element. Thus, hydrogen has two
stable isotopes. One isotope (*H) has only a proton and no neutron in the
130
-------
nucleus; the other (2H) has a proton plus a neutron in the nucleus. In
addition, hydrogen has an unstable, or radioactive, isotope (3H) which has
two neutrons in addition to the proton in the nucleus. An important char-
acteristic of isotopes is that isotopes of an element, for all practical
purposes, will react chemically in an identical way. For example, varia-
tions of sulfur isotopes (as 32S, 3I*S, and 36S) in the sulfate ion will not
affect the way in which the ion moves with the water. Thus, the water can
be labeled with the isotope without affecting significantly the movement of
the constituent.
In general, the uncertain ability to detect small artificial variations
of most isotopes against the natural background, the high cost of their
analysis, and the expense of preparing isotopically enriched tracers, means
that stable isotopes are rarely used for artificially injected tracer stud-
ies in the field. They are, however, quite widely used to detect sources of
pollution and to help determine areas of natural recharge..
Research into the topic of stable isotopes of various elements in natu-
ral waters is progressing rapidly, and the potential usefulness of these
isotopes to ground-water tracing will undoubtedly increase markedly in the
near future.
Hydrogen and Oxygen: The two stable isotopes of hydrogen (XH and 2H)
and the three stable isotopes of oxygen (160, 170, and 180) form part of the
water molecule, and analyses of their natural concentrations have been used
widely to help understand the movement of ground waters. Natural variations
In shallow ground water are generally related to variations within the orig-
inal recharge water coming from the surface. Because of the large differ-
ences in mass between the two hydrogen isotopes, they tend to fractionate
131
-------
whenever evaporation or condensation of water takes place. Other factors
being equal, waters with a higher 2H (commonly called deuterium) content
will be found near the coastlines, at low elevations, in warm rains, and in
water which has undergone partial evaporation such as in soil moisture dur-
ing periods of little rain or in saline lakes. Although mass differences
among oxygen isotopes are not as large as those of hydrogen, natural frac-
tionation of those isotopes also takes place. The variations in 180 and 170
contents of shallow ground water generally follow those of deuterium. That
is, if the water has a larger than normal 2H/1H ratio, it will generally
have also a larger than normal 180/160 ratio (because 170 is much less
abundant than either 180 or 160, it is rarely reported in routine isotopic
studies). This general relationship is defined by Craig's line and is shown
in Figure 4.14. Possible departures from this line can be caused by exces-
sive evaporation, by reactions between minerals and hot water, and other
less important effects.
The most common use of studies of 2H and 180 has been to trace the
large-scale movement of ground-water and to locate areas of recharge
(Figure 4.15).
Nitrogen: The two abundant isotopes of nitrogen (14*N and 15N) can vary
significantly in nature. Ammonia escaping as vapor from decomposing animal
wastes, for example, will tend "to remove the lighter (1!*N) nitrogen and will
leave behind a residue rich in heavy nitrogen. In contrast, many fertili-
zers with an ammonia base will be isotopically light. Natural soil nitrate
will be somewhat between these two extremes. As a consequence, nitrogen
isotopes have been useful in helping to determine the origin of unusually
high amounts of nitrate in ground water.
132
-------
8 D %o
+ 100 -
-100
-200
-300
Water
from
coastal
areas
/Ocean water
Water
in mountains
and inland
Snow frorn
high mountains
in Arctic
and Anarctic
Direction of
shift due to
.intense evaporation
C>
Direction of 6 ~0 shift
due to high-temoerature
interaction with minerals
Snow at
South
r- / Pole
+ 20
Figure 4.14. Relationship between deuterium and oxygen-18 for
natural waters. Large arrow shows the direction of compositional
change found in geothermal waters where heavy oxygen found in
rock-forming minerals will exchange with the lighter oxygen in
normal ground water (data from Ferronsky and Folyakov, 1982).
133
-------
Santa Catalina Mountains
N
Alluvial basin
LEGEND
Summer runoff from
large basin to south. -9 to -7
Ground water recharged
from Santa Cruz River. -8
Ground water recharged
from summer runoff
from small and basins
in low mountains. -7
Winter runoff from
high mountains. -10 to -12
Ground water recharged
from winter runoff
mixed with some
summer recharge. -IO
Figure 4.15. Differences in the stable isotope of oxygen (18o) in
ground water of the Tucson basin in Arizona reflect different sources
of water. Because all values are negative, the larger number repre-
sents isotopically lighter water. Although the chemical character-
istics of the ground water are quite similar throughout most of the
basin, distinctive isotopic differences help to determine the origin
of recharge for the basin. Data are from several unpublished M.S.
theses at the University of Arizona. Diagram is not to scale.
134
-------
Most nitrogen in ground water will be in the form of the nitrate anion
(NO" ) or dissolved nitrogen gas (N ) from the atmosphere. Locally in zones
devoid of dissolved oxygen, the chemically reduced form (NH^*) may predomi-
nate. In general, nitrate will move as a conservative tracer and is an
important indicator of pollution. If nitrate concentrations exceed about
10 mg/1, the health of infant mammals including humans may be adversely
affected. Also, the presence of more than about 5 mg/1 of nitrate commonly
is an indirect indication of other forms of contamination including those
from chemical fertilizers and sewage.
Sulfur: Most dissolved sulfur within shallow ground water is bound
within the sulfate ion (SO=J. The stable sulfur isotopes (32S, 3"S, and
36S) found in the sulfate ion will vary quite widely and, under certain
circumstances, be useful indicators of the origin of the sulfate. This is
particularly true if, for example, one wishes to distinguish sulfate orig-
inating from natural dissolution of gypsum (CaSO^'ZH^) from sulfate orig-
inating from an industrial spill of sulfuric acid (H2SO^).
Carbon: Two stable isotopes of carbon (12C and 13C) and one unstable
isotope (^C) are used in hydrogeologic studies. Most of the carbon dis-
solved in normal potable ground water is within the bicarbonate ion (HCO~ ).
Contaminated water may also have large amounts of organic materials which
contain carbon. Other forms of carbon dissolved in natural water are car-
bonate (C0=3) and carbonic acid (H2C03), the concentrations of which are
pH-dependent, and the gases carbon dioxide (C02) and methane (CH^).
Most isotopic studies of carbon in water have been centered on 1(*C
which will be discussed in a later portion of this chapter. Although not
as commonly studied as l**C, the ratio of the stable isotopes, 13C/12C, are
135
-------
potentially useful in sorting out the origins of certain contaminants found
in water. For example, methane (CH ) originating from some deep geologic
deposits is isotopically heavier than methane originating from near surface
sources (Figure 4.16). This contrast forms the basis for identifying aqui-
fers contaminated with methane from pipelines or from subsurface storage.
Isotopes of Other Elements: The potential exists for the use of stable
isotopes of a number of other elements as natural tracers of water. Some of
these are chlorine (37C1 and 35C1), strontium (86Sr and 87Sr), boron (10B
and nB), and the isotopes of the noble gases. In general, studies of these
isotopes are related more to the determination of regional directions of
ground-water flow than to problems of the identification of sources of con-
tamination.
Field Methods
Collection of field samples must take into consideration problems of
obtaining a representative sample as discussed in Chapter 3. Also, the
sample must be preserved so that isotopic fractionation does not take place
prior to analysis. For oxygen-deuterium samples, small glass bottles with
vapor-proof caps which hold about 20 to 50 ml are sufficient for most
purposes. For boron, nitrogen, carbon, and sulfur, a larger sample should
be taken. The size of the sample will depend on the water chemistry and the
analytical methods used. Generally, sample sizes are from 1 to 10 liters
for normal potable water. Samples should be stored in the dark and a growth
inhibitor should be added to water samples taken for boron, nitrogen, car-
bon, and sulfur analyses, because biological activity within the sample can
cause significant isotopic fractionation. Analyses of stable chlorine will
generally require samples of 1 to 2 liters of potable water and much less
136
-------
CO 15
IK
_J
Q.
10 h
u_
o
rr
UJ
i s
D
z
n
~ !
,"
1
_ ^ _ ^ «
!*.«
r*1 Bedrock '"
- I.
i ! i i ! !
"L
n
Lr
Glacial |
i drift i | j~~]
-40 -50 -60 -70 -80
$C13 FROM CH4
-90 %,
Figure 4.16. Histogram showing composition of carbon
isotopes from methane from bedrock and from glacial
drift. The contrast in isotopic composition allows
the identification of methane from storage and from
pipelines which may leak out and contaminate ground
water. Natural methane generated in shallow aquifers
is much different isotopically than bedrock methane
that is distributed commercially. (Redrawn from Cole-
man et al., 1977).
137
-------
for saline water. Changes in the isotopic ratios of chlorine will not take
place under normal conditions of storage.
Analyses: Analyses of stable isotopes are made with expensive mass
>
spectrometers which require highly-trained technicians to run. Further
details are given in Appendix E.
Radionuclides
Introduction: This section includes a description of some of the
hydrogeologic applications of radioactive isotopes of various elements,
which are called collectively radionuclides.
In the early 1950's, great enthusiasm was evident for the use of radio-
nuclides both as natural, "environmental" tracers and as injected artificial
tracers. The environmental use has been expanded greatly until it is a
major component of many hydrochemical studies of today. In contrast, the
use of artificially injected radionuclides has all but ceased today in many
countries including the United States. Most use of artificially introduced
radioactive tracers in these countries is confined to carefully controlled
laboratory experiments or to deep petroleum production zones which are de-
void of potable water.
A brief explanation of some aspects of radioactivity is necessary be-
fore discussing isotopes of specific elements. Although for any radioactive
element the radiation given off is in short, almost instantaneous, pulses
which are-randomly distributed in time, if enough individual nuclei are con-
sidered, the process of radioactive decay can be expressed as
«-Xx (11)
138
-------
in which x is the number of nuclei present, t is time, and X is the decay
constant which is unique to each radionuclide. If x0 is the number of
nuclei at zero time and x^ is the number of nuclei at time t, then
xt = x0e-u (12)
The half-life of a particular radionuclide is the time which is taken for
one-half the original number of nuclei" to decay, or
t1/2 - *₯- (13)
The foregoing equations apply to all types of radioactive reactions even
though some reactions produce alpha particles C*He ions), others produce
beta particles (electrons, both negatrons and positrons), and still others
produce gamma rays (an electromagnetic radiation similar to X-rays). A
number of other types of radiation may also be produced but they will not
be discussed in this brief summary.
Injected tracers: For a number of reasons, the detection and counting
of y-radiation is much easier than either 6 or d radiation. Radionuclides
which have a strong gamma emission are, therefore, commonly chosen for
tracers. A number of these radionuclides as well as others are listed in
Table 4.11. In addition, tracers are selected which can be injected into
ground water in a form which is highly mobile in the water phase. This
usually is either in a neutral or anionic form.
Most radioactive tracers are superior to other tracers because they can
be detected easily by field equipment 1n very small concentrations which are
far below levels that would alter the flow characteristics of the ground
water. Also, tracers can be selected which have half-lives so short that
they are essentially decayed after a few hours to a few days. Despite the
demonstrated safety of many of the techniques and tracers, the complexity of
139
-------
TABLE 4.11
Commonly Used Radioactive
Tracers for Ground-Water Studies
Radionuclide Radiation
*H g-
32p (j-
siCr y
'"Co 8-.Y
*2Br B-.Y
85Kr B",Y
1311 B-§T
1 9oAi i Q~ v
i»U P 5 T
Half -Life
y=year,
d=day ,
h=hour)
12. 3y
14.3d
27. 8d
5.25y
35. 4h
10. 7y
8.1d
2.7d
Chemical Compound
H20
Na^PO^
EDTA-Cr and CrCl3
EDTA-Co and K,Co (CNC)
o b
NH^Br, NaBr, LiBr
Kr (gas)
I and KI
AuCl,
140
-------
local and federal regulations makes their field use impractical in many
countries, including the United States.
Radioactive tracers, besides being used for tracers which move from one
well to another, have been used for studies of the local hydraulics near and
within a well. Radioactive gold (198Au) when mixed with water in a well
will plate out on the downstream side of the well as the water moves through
the well. A directional counter will detect this concentrated radioactivity
and thus indicate the direction of water movement in the vicinity of the
well (Figure 4.17). Also, the rate of removal of the radioactivity from the
well water will be a function of the volume of water moving through the well
per unit time. Although giving only conditions near the well, this dilution
technique is useful in obtaining estimates of hydraulic conductivity accord-
ing to the following equations:
co
= e-Bt
in which
C-f. = concentration of tracer in the well of time t;
C0 = original concentration of tracer in the well at
time « 0; and
B * a factor which is constant for simple, steady-state
conditions.
If B is constant, then
B. 2
V
in which
Q * the volume of water per unit of time flowing through
the well and V is the volume of water in the well.
141
-------
DIRECTION OF FLOW
.60
860 900 340
concentrotion in
com
Figure 4.17. The local direction of ground-water movement
as determined by the movement of a radioactive tracer within
a borehole. The hole was not pumped during the test. The
ground water is flowing under natural conditions and enters
the well from the west and leaves the well towards the east.
After release of the radioactive tracer, the gamma radiation
is measured in different directions by rotating a shielded
counter within the well. Although the surveys may be highly
useful, it must be remembered that- flow directions within
the well are influenced by well-construction methods and by
local heterogeneities in the aquifer. The measured direc-
tions, therefore, may not give a reliable indication of
regional directions of ground-water flow. Diagram is re-
drawn from Rodriguez (1977).
142
-------
For fully penetrating wells in isotropic and homogeneous aquifers,
Q = 2dmne7 (15)
d = the effective diameter of the well;
m = the saturated thickness of the aquifer;
ne = the effective porosity of the aquifer; and
V = the average velocity of the ground water outside
of the well (in the aquifer).
If the hydraulic gradient, i, of the ground water is known, then the
hydraulic conductivity, K, is given by
v ne (16)
K =
i
If the experiment has a duration which is 5% or more of the length of
the half-life of the radioactive tracer, then Equation (12) should be used to
correct for radioactive decay during the experiment. Thus in Equation (14),
Ct is the calculated concentration at'time t assuming no radioactive decay
has tak.en place. It is not the actual observed concentration of radio-
activity.
In summary, the progressive dilution of a tracer in a well can be used
to obtain the hydraulic conductivity of an aquifer near the well provided
dimensions of the well are known and estimates can be made of the effective
porosity of the aquifer and the hydraulic gradient near the well.
Atmospherically Distributed Radionuclides: A number of radionuclides
are present in the atmosphere from natural and artificial sources. Many of
these radionuclides will be carried into the subsurface by rain water. The
radionuclides of greatest interest are listed in Table 4.12. The most common
143
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hydrogeologic use of these radionuclides is to obtain some estimate of the
average length of time ground water has been isolated from the atmosphere.
Because of dispersion in the aquifer and mixing in wells that sample several
hydrologic zones, a unique age of the ground water does not exist. Neverthe-
less, it can be commonly established that most or virtually all the ground
water is older than some given limiting value. In many situations we can
say, based on atmospheric radionuclides, that the ground water was recharged
more than 1,000 years ago or that, in another region, all the ground water in
a given shallow aquifer is younger than 30 years.
Tritium, the radioactive isotope of hydrogen (3H) with a 12.4-year half-
life, was produced at low levels by natural processes prior to the detonation
of thermonuclear devices in the early 1950's. Since that time, atmospheric
tritium has been dominated by tritium from man-made sources. Most commonly,
tritium concentrations are measured in tritium units (TU) which is the number
of tritium nuclei per 1018 stable hydrogen nuclei. Prior to the 1950's,
natural levels in rain ranged from 5 to 15 TU, the exact number depending on
several local and regional factors. Owing to the decay of the tritium, water
recharged during the early 1950's will only have 0.8 to 2.5 TU today if the
water has been isolated from the atmosphere since that time. Thermonuclear
explosions increased local rainfall to more than 1,000 TU in the Northern
Hemipshere by the early 1960's (Figure 4.18). Tritium analyses of ground
water are used widely to determine the "age" of young ground water. In
general, ground water in the Northern Hemisphere which has more than about 5
TU 1s more than 30 years. Very small amounts, 0.05 to 0.5 TU, can be pro-
duced by natural subsurface processes, so the presence of these low levels
145
-------
4000
3500
3000
«»
D
b 2500
2000
1500
1000
500
56 58 60 62 64 66 68 7O 72 74 76 78 8O 82
YEAR
Figure 4.18. Average annual tritium concentration of rain-
fall and snow for the states of Arizona, Colorado, New
Mexico, and Utah. During any single year, however, tritium
concentrations may vary by more than 300% with the maximum
concentrations in rainfall during the summer. In northcen-
tral United States and central Canada, concentrations have
been higher than those shown for the western states. Con-
centrations in precipitation along coastlines, in the trop-
ics, and in the Southern Hemisphere are generally much lower
than those shown here. (Diagram redrawn from Vuataz et al.,
1984).
146
-------
does not necessarily indicate water 40 to 60 years old or smalT amounts of
more recent water mixed with very old water.
The radioactive isotope of carbon, ^C, is also widely studied in ground
water. Most 1<+C in potable ground water is contained in the HCO " ion in the
water. Other carbon-bearing material dissolved in water such as C02, C03=,
CH^, H.CO., and organic acids may also contain variable amounts of 1I+C. As
a first approximation, the initial number of llfC nuclei per total carbon
nuclei, or X0 in Equation (2), in a water sample is considered to have been
constant due to the almost constant natural production of 1I+C in the atmo-
sphere by cosmic radiation interacting with the atmosphere. If the only
source of 11+C in the water is originally from the active biosphere, then the
1(*C which is measured in carbon from the water sample can be considered to be
Xt in Equation (2). Because X is known from experimental work, the "age" of
the sample, or t, in Equation (2) can be determined directly..
In practice, however, the use of llfC i's rarely as simple as just de-
scribed. Sources of old carbon, primarily from limestone and dolomite, will
dilute the sample. A number of processes, such as the formation of CH^ gas
or the precipitation of carbonate minerals, will fractionate the isotopes and
alter the apparent age. The complexity of the interpretation of UC "ages"
of water is so great that it should be attempted only by hydrochemists spe-
cializing in isotope hydrology.
Despite the complicated nature of ll*C studies, they are highly useful in
determining the approximate residence time of old water (500 to 30,000 years)
in aquifers. For certain practical problems, this information is essential
and cannot be obtained in any other way.
147
-------
Other radionuclides listed in Table 4.12 are not used routinely in
t
hydrogeologic work owing either to problems of sampling or to problems of
analyses. Of those listed, 36C1 will probably be .used routinely in another
decade after the present analytical bottleneck is solved. The major advan-
tages of 36C1 are the ease of sampling, the stability of the sample in stor-
age, and the fact that 36C1 can give information concerning extremely old
water.
Field Methods
Injected radioactive tracers are handled with great care to avoid radia-
tion exposure and to avoid sample contamination. Otherwise, they are gen-
erally treated as normal chemical tracers. Special down-hole devices to mea-
sure in-place tracer dilution for the application of Equations (14), (15),
and (16) are fabricated by the Institut fur Radiohydrometrie, Gesellschaft
fur Strahlenund Umweltforschung MBH, Neuherberg, Ingolstadter Landstrasse 1,
D-8042 Oberschleissheim, West Germany.-
Field collection of samples for the determination of environmental
levels of tritium must be done with great care to avoid contamination from
the atmosphere, from local sources of tritium such as watch dials, and from
high levels of tritium commonly present in laboratories. From two to four
liters of water are needed if anticipated tritium levels are below 15 TU.
Sample containers should be metal or high-quality glass. Some plastic con-
tainers are permeable to gases, so plastic containers are to be avoided un.-
less the properties of the plastic are known.
Field collection of samples for UC is highly specialized and should be
done by individuals experienced with this type of sampling. For routine 1
-------
the carbon is extracted either by large batch or by flow-through systems.
The use of the tandem accelerator mass spectrometric (TAMS) method for llfC
analysis has greatly reduced the amount of carbon required ,so that one liter
of water or less can be used. Access to the TAMS system, however, is not
routine.
Samples for 36C1 analyses are relatively simple to obtain. About 30 mg
of chlorine should be available for the analysis. Most potable water con-
tains between 10 and 100 mg/1 of chloride, so a sample of a few liters of
water generally is enough. Silver nitrate, AgNO , is mixed with the water
sample, and AgCl is formed. The AgCl precipitate is placed in an amber bot-
tle and stored out of sunlight and excessive heat until analyses can be com-
pleted.
Analysis
The analysis of radioactive materials is a highly specialized branch of
chemistry and is not easy to accomplish except where the field determination
of gamma radiation can be related directly to the concentration of injected
tracers. Scintillation counting using special liquid scintillation fluids is
normally required for beta emitters.
Environmental radionuclides such as tritium, 1<+C, and 36C1 require very
special equipment for their determination. Low-level tritium is concentrated
by electrolysis and counted by liquid scintillation. A number of methods are
used to determine llfC. All processes are complicated. Many end with the
carbon in a gaseous "form which is placed into counters designed to receive
gas. The TAMS method can be used for both llfC and 36C1 analyses. The accel-
erator used is a multi-million dollar instrument and only a few of these are
presently in operation.
149
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-.APPENDIX A
ADDITIONAL USES OF WATER TRACERS
The purpose of this manual is to describe ground-water tracing tech-
niques. However, tracers are widely used in other areas of hydro!ogic
study, such as surface water, the unsaturated (vadose) zone, and the atmo-
sphere. Numerous engineering applications also involve tracer use, includ-
ing petroleum exploration, leak detection, sewer flow, and biological and
medical research. A brief description of these uses is given with reference
articles.
Ground Water
Tracers have been used to determine the flow path, velocity, and resi-
dence time of solutes, and aquifer characteristics such as hydraulic con-
ductivity, disper'sivity, and effective porosity. Ground-water velocity and
aquifer characterization studies have been described in the text.
Examples of flow path measurements are most numerous in karst studies.
The Water Tracer's Cookbook (Aley and Fletcher, 1976), published by the
Missouri Speleological Survey, is an excellent introduction to karst mapping
and characterization through use of a wide variety of tracers. Another
application of karst flow tracing is described by Caspar and Oncescu (1972),
and deals with water exchange between karst mines, depressed regions, and
ground water. Karst tracing has also been used to delineate catchment
boundaries {Smart, 1975). Flow path studies in non-karst regions include
evaluation of the movement of sewage in ground water (Sinton, 1980), and the
determination of the potential for chemical or bacterial pollution of a New
150
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Zealand aquifer (Thorpe, 1979). Vecchioli et al. (1972) studied the travel
of indicator bacteria through the Magothy aquifer in New York.
.Residence time studies include the determination of ground-water re-
charge using environmental isotopes (Vogel et al., 1974, and Fontes and
Fritz, 1975). Ground-water dating, involving the use of cosmic-ray and
bomb-induced radioisotopes, is a growing field of study (Davis and Bentley,
(1982). Environmental isotopes have recently been used to demonstrate the
effect of ground water on storm runoff hydrographs (Skiash and Farvolden,
1979).
Surface Water
Tracers have been widely used in surface water studies to determine
flow patterns (dispersion), flow volume, and time-of-travel (velocity).
Kilpatri.ck et-al. (1967) described flow measurements with fluorescent
tracers. A more recent, general work on the subject is "Measurement of Time
of Travel and Dispersion in Streams by Dye Tracing" (Hubbard et al., 1982),
a handbook published by the U.S. Geological Survey.
Determination of flow patterns yields information concerning movement
of contaminants (such as factory effluents, radioactive waste, and sewage)
in streams (Caspar and Oncescu, 1972). Study of dispersion under turbulent
flow results in determination of eddy-diffusion coefficients. White (1981)
discussed estuary mixing through the use of environmental radionuclides.
Caspar and Oncescu (1972) reviewed the use of tracers in measuring flow
rates 1n natural streams, closed conduits, and reservoirs. Dilution studies
are used to find the time required for inflowing contaminants to be reduced
to acceptable levels.
151
-------
Storm runoff studies employ tracers to obtain travel time measurements
*
in order to help establish flood hydrographs. Smith (1973) noted the use of
environmental tritium in river recharge investigations.
Sediment transport is another aspect of surface water systems which has
been studied with tracers. El rick and Lawson (1969) looked at sediment
movement in rivers, irrigation canals, estuaries, harbors, and the open
ocean. River bank and bed erosion have also been investigated (Caspar and
Oncescu, 1972). White (1981) discussed the dating of sediments and surface
water with environmental radionuclides.
Soil
In the unsaturated zone, soils have been investigated through the use
of various tracers. Infiltration, drainage, and evapotranspiration are
fields of interest. Recent research includes: the use of bromide as a
tracer in the root zone of soils (Tennyson and Settergren, 1980); the use of
radioactive tracers to determine the impact of deforestation on the soil
profile (Ryckborst, 1981); and a general study of water distribution and
movement in the unsaturated soil profile (Ligon, 1980).
Atmosphere
Environmental and injected tracers are utilized in estimating the
travel of pollutants, studying precipitation and evaporation, and tracking
air motion on a global scale through the use of nuclear debris (Elrick and
Lawson, 1969).
152
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Petroleum Industry
The oil and'gas industry has developed tracers for a number of oilfield
applications. Wagner (1977) described the use of chemical and radioactive
tracers for waterfloods and gas drives in the tertiary oil recovery process.
Some of the information to be obtained from diagnosis of interwell hetero-
geneities includes: identification of problem injection wells; directional
flow trends and fluid velocity; and delineation of flow barriers. Preferred
water and gas tracers are listed by Wagner (1977). Greenkorn (1962) also
compared waterflood tracers.
Additional Engineering Applications
Leak detection in water and sewer pipes, embankments, and dams is
another branch of tracer use (Caspar and Oncescu, 1972). Zuber et al.
(1979) discussed tracing of water leakage into salt mines, and Alburger
(1977) described leak testing with dyes as a non-destructive technique for
soils, sewers, electronics components, boilers, tanks, pipelines, etc.
Koerner et al. (1979) reported non-destructive tracer testing methods for
detecting dam seepage.
Sea-water intrusion around the foundation of a nuclear power plant was
modeled by Myer (1981), using I131 as a tracer. Sewage system tracing has
been performed by Renard (1982), and Aulenbach and Clesceri (1980) used
tracers in monitoring the land application of waste water. Finally, sani-
tary landfill leachate has been traced by Ellis (1980) and Murray et al.
(1981), using potassium (from the leachate) and injected lithium bromide,
respectively.
153
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Potential Uses
Radioactive, hazardous waste, and sanitary landfill disposal site
evaluations are likely to employ tracer test results. In addition, soluble
tracers can be mixed in dry form with wastes which are buried so that any
water percolating later through the waste will carry the tracer which in
turn could provide an early warning for the arrival of the bulk of the
slower moving and hazardous leachate from the waste.
154
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APPENDIX B
A DISCUSSION OF DISPERSION AND DIFFUSION
One of the purposes of many tracer tests is to obtain a value of the
aquifer parameter, dispersivity (a). The intent of this Appendix is to dis-
cuss briefly the theoretical background of the parameter, and to present
some current attitudes concerning dispersion.
The transport of a tracer or contaminant in a porous medium is analyzed
by some form of the convection-dispersion equation, introduced by Ogata and
Banks (1961), and discussed by Bear (1961a, 1969). Convection is the bulk
movement of water at the mean velocity of the flow system, u (where u equals
specific discharge divided by porosity, as defined in Chapter 2). Convec-
tion may be caused by differences in density of the water (natural convec-
tion), regional movement in the aquifer (advection), and the pumping of
wells (forced convection) (Sauty, 1980).
Dispersion is the mechanism which causes a solute to mix and spread to
positions which would not be expected by convection alone. Dispersion in
ground water is a combination of mechanical dispersion (mixing) and molecu-
lar diffusion, and it causes a dilution of the solute. Mechanical disper-
sion is due to variations in fluid velocity, and the tortuous flow paths in
the voids of the porous medium at the microscopic scale (Sudicky and Cherry,
1979). On a larger scale, mixing is due to the presence of zones of dif-
ferent permeabilities.
Molecular diffusion is caused by Brownian motion, and is often consid-
ered Insignificant in magnitude in comparison with mechanical dispersion,
for rapidly flowing ground water. In most tracer tests in porous media,
diffusion is neglected because the rate of ground-water flow is too high for
155
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pore-to-pore equalization of concentration (Perkins and Johnson, 1963). A
reasonable value for the diffusion coefficient for non-adsorbed species in
porous media is 1 x 10"10 m2/s (Freeze and Cherry, 1979), while the disper-
sion coefficient is generally orders of magnitude larger.
Derivation of the Convection-Dispersion Equation
The convection-dispersion equation used in contaminant transport model-
ing is based on Pick's first and second laws. Formulated by analogy to heat
conduction, the first law states that the flux of a diffusion or dispersing
substance in a given direction is directly proportional to the concentration
gradient in that direction. The negative sign indicates that flux is posi-
tive in the direction of decreasing concentration. In the following text,
dimensions are given in brackets.
F - - D 3C-
rx x 3x'
where Fv = "^ss flux [-] in the x direction;
* L2T
D - coefficient of proportionality [=];
M
c * concentration [r].
Pick's second law is derived from the law of conservation of mass, as
applied to the first law. It states that:
156
-------
where 72 = ii- 1 + ii- j + lL k
3X2 3y2 322
This assumes that D is constant, while it is actually a function of tempera-
ture, concentration, and other factors.
The convection-dispersion equation for a non-reactive solute is stated,
in one-dimensional form, as:
where
D = coefficient of dispersion tip];
u = average linear flow velocity.
This assumes that flow is parallel to the x direction, with steady-state
velocity, u. It also assumes that the fluid is incompressible.
The coefficient of dispersion, D, may be thought of as a correction
factor which describes the variation of solute distribution about the mean.
The coefficient, D, is a combination of the effects of hydrodynamic disper-
sion and molecular diffusion.
D = oLu = DM
Here, aj_ = longitudinal dispersivity (in the x direction) [L];
DM a molecular diffusion coefficient CT~]«
The term "dispersivity" was introduced by Scheidegger (1954). This
parameter has components in three orthogonal directions. The longitudinal
dispersivity is in the direction of flux. Horizontal transverse dispersivity
157
-------
may be called "lateral dispersivity", and vertical transverse dispersivity
may be referred to as "vertical dispersivity." In laboratory experiments,
the transverse dispersivities are generally 5 to 20 times smaller in magni-
tude than the longitudinal dispersivity (Freeze and Cherry, 1979).
Solution of the Convection-Dispersion Equation
The one-dimensional solution of the convection-dispersion equation for
a step-function input of tracer into a semi-infinite aquifer with natural
flow velocity (Ogata and Banks, 1961) is:
c/c0(x,t) - - [erfc (2l£- + exp (ux/D) erfc (£!£-)] (21)
L. £Ul uUu
where
c/c0 = normalized concentration (relative to source):
x = distance from the measuring point to the source;
u = average linear velocity;
t = time;
D = dispersion coefficient;
erfc = the complimentary error function.
The boundary conditions are:
c(x<0, t) = 0, for all t
c(0, t) = C0, for all t > 0
c(«, t) * 0, for all t
The solution above can be approximated, after a short period of time,
by:
158
-------
c/cn=Ierfclg^-l (22)
This equation can be solved for various boundary conditions, flow re-
gimes, and types of injection (e.g., uniform flow, radial flow, continuous
injection, slug injection). Fried (1975) provided a number of solutions, and
Sauty (1977) developed type curves for uniform or radial flow to characterize
response to continuous or instantaneous pulse input at a point. Lenda and
Zuber (1970) developed analytical solutions in normalized form for different
measurement geometries. They presented type curves for point injection and
line injection in an infinite aquifer. Sudicky and Cherry (1979) developed
type curves for a finite-width pulse injection.
Hoopes and Hareleman (1967a) presented a general equation describing
the nonsteady-state concentration of a tracer during plane radial flow.
Analytical solutions to this equation for a constant input concentration have
been given in that paper and by Gelhar and Collins (1971). These solutions
can be used for single-well injection/withdrawal tests.
For a two-well tracer test, Webster et al. (1970) and Grove and Beetem
(1971) provided solutions. The tracer addition can be continuous or a pulse,
and recirculation can be accounted for.
Measuring Dispersivity
The error function is related to the normal distribution1 (0), as:
£ (z) * [1 + erfc (--)] (23)
1 This holds for tables of the normal distribution with negative infinity
as the lower limit.
159
-------
Then,
This states that the normalized concentration distribution can be de-
scribed by a cumulative normal distribution with a mean of zero and a vari-
ance equal to 2Dt. The variance is also equal to 2a\l^. This is true because
D = au, and x = u t. Here, x and t are average distance and average time.
Then, by plotting c/c0 versus x on normal probability paper, the value of ct^
is obtained.
The Scale Effect
It has generally been assumed in the past that dispersivity (a) is an
aquifer property which is constant. In the past ten years, research has
indicated that dispersivity is scale-dependent (Fried, 1975). Laboratory
breakthrough curves in packed granular columns yield longitudinal dispers-
ivity values of 0.01 to 1 cm (Pickens and Grisak, 1981). Values of OL ob-
tained by field tracer tests range from 1 to 134 meters (see Table 1), gen-
erally increasing with increasing distance between injection and observation
wells. Dispersivity values have also been obtained by calibration of com-
puter models. The longitudinal dispersivity values in Table B.I range from
12 to 91 meters.
Several ideas have been offered to explain these results. Pickens and
Grisak (1981) have suggested that field tracer tests which are-analyzed
using a one-dimensional flow field may produce a scale effect which is par-
tially a consequence of streamline effects (converging or diverging stream-
lines).
160
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TABLE B.I
VALUES OF DISPERSIVITIES
MEASURED BY VARIOUS METHODS
Type of
Aquifer
Alluvial
Single-Well Injection Withdrawal Test
°L
Location (meters) Reference
Lyons, France
0.1-0.5
Fried, 1975
Multiple-Well Tracer Test (including two-well tracer tests)
.» Distance Between
Injection and
Type of Observation Wells 01
Aquifer Location
Chalk Dorset, England
Alluvial
Alluvial
Fractured
dolomite
Fractured
carbonate
Lyons, France
Eastern France
Carlsbad, MM
So. Nevada
Fractured Savannah River
crystalline Plant, S.C.
(meters)
8
6 & 12'
6 & 12
55
121
538
(meters)
3,1
4.3
11.0
38.0
15.0
134.0
Reference
Ivanovich and
Smith, 1978
Fried, 1975
Fried, 1975
Grove and
Beet em, 1971
Classen and
Cordes, 1975
Webster et al
1970
Type of
Aquifer
Alluvial
Single-Well Tracer Test with Surface Geophysics
Location
Lyons, France
Distance Traveled OL
by Tracer (meters) (meters)
- 80 m 5-12
°T
(meters)
Refere
0.009-14.5 Fried,
161
-------
TABLE B.I (continued)
Dispersivities Measured on a Regional Scale By Model Calibration
Approximate
Type of
Aquifer
Alluvial
Limestone
Alluvial
Alluvial
Glacial
deposit
Basalt
Distance Traveled aj_
Location by Solute (meters) (meters)
Lyons, France
Brunswick, GA
Rocky Mtn.
Arsenal , CO
.»
Arkansa's River
Valley, CO
Long Island, NY
Snake River
Plain, ID
1,000
1,500
4,000
5,000
1,000
4,000 '
12
61
30
30
21.3
91
QT
(meters) Reference
4
18
30
9
4.3
137
Fried, 1975
Bredehoeft &
Pinder, 1973
Konikow, 1977
Konikow &
Bredehoeft,
1974
Pinder, 1973
Robertson, 19
162
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Pickens et al., (1976) suggested that large dispersivities obtained from
analysis of two-well tracer tests are a result of mixing of water from dif-
ferent levels, which occurs at the well bore.
"Most researchers feel that the primary cause of the scale effect is the
heterogeneity of an aquifer (Warren and Skiba, 1964; Matheson and de Marsily,
1980; and Gelhar et al., 1979). Recent research indicates that, for certain
hydraulic conductivity distributions, the longitudinal dispersivity approaches
a constant at large time or large mean travel distance. Gelhar et al. (1979)
suggested an improved form of the convective-dispersive transport equation
which incorporates the statistical properties of the hydraulic conductivity
distribution. However, the traditional convection-dispersion equation and its
solutions continue to be used to obtain values of dispersivity until a better
alternative is found.
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. APPENDIX C
FACTORS TO CONSIDER IN TRACER SELECTION
Determination of:
PURPOSE OF STUDY
flow path
velocity (solute)
velocity (water)
porosity
dispersion coefficient
distribution coefficient
Delineation of contaminant plume
Recharge
Dating
Tracer Type to be Used
Nonconservative
Conservative
Conservative
Conservative
Nonconservative
Constituent of plume
Environmental isotope
or anthropogenic
compound
Radioactive isotopes
AVAILABLE FUNDS
Manpower and equipment to run tests to completion (e.g., drilling, tracer
cost, sampling, analysis).
TYPE OF MEDIUM
Karst
Porous media (alluvium, sandstone,
soil)
Fractured rock
Tracer Type
Fluorescent dyes, spores,
tritium, as well as
other tracers
Wide range of choices.
Dyes and particulate
material are rarely
useful.
Wide range of choices.
Dyes and particulate
only occasionally
useful.
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STABILITY OF TRACER
Distance from injection to sampling Must be stable for length
point - of test and analysis
Approximate velocity of water and
approximate estimate of time re-
quired for test, given: distance
from injection to sampling point,
porosity, thickness of aquifer
DETECTARILITY OF TRACER
Background level
Dilution expected in test (function
of distance, dispersion, porosity, and
hydraulic conductivity)
Detection limit of tracer (ppm, ppb,
ppt)
Interference due to other tracers,
water chemistry
- DIFFICULTY OF SAMPLING AND ANALYSIS
Factors to Consider Example of Difficult Tracer
Availability of tracer Radioactive (must have special
permits)
Ease of sampling Gases (will escape easily from
poorly sealed container)
Availability of technology for Cl-36 (only one or two labora-
and ease of analysis tories in the world can do
analyses)
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PHYSICAL/CHEHICAL/BIOLOGICAL PROPERTIES OF TRACER
Density, viscosity May affect flow (e.g., high
concentrations of CT)
Solubility in water , Affects mobility
Sorptive properties Affects mobility
Stability in water Affects mobility
Physical Chemical Biological
radioactive decomposition degradation
decay and precipi-
tation
PUBLIC HEALTH CONSIDERATIONS
Toxicity
Dilution expected
Maximum permissible level ~ determined by federal, state, provincial,
and county agencies.
Proximity to drinking water
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Summary of Most Important Tracers
Tracer
Characteristics
Participates
Spores
Bacteria
Viruses
Used in karst tracing; inexpensive
Detection: high, multiple tests possible
by dying spores different colors
Low background
Moderately difficult sampling and analysis
(trapping on plankton, then microscopic
identification and counting)
No chemical sorption
May float on water, travels faster than
mean flow rate
of
Most useful for studying transport
microorganisms
Detection: highly sensitive
Sampling: filtration, then incubation
colony counting
No diffusion, slight sorption
Detection: highly sensitive
Sampling: culturing, colony counting
Some sorption
Smallest particulate
and
Ions (Non-radioactive,
excludes dyes)
Chloride
Bromide
Conservative
Inexpensive
Stable
Detection: 1 ppm by titration, electrical
conductivity, or selective ion electrode
High background may be problematic
In large quantities, affects density which
distorts flow
No sorption
Inexpensive
Stable
Detection: 0.5 ppm by selective ion
electrode
Low background
No sorption
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Tracer
Characteristics
C. Dyes
Rhodamine WT
Fluorescein
Used in karst and highly permeable sands
and gravels
Inexpensive
Moderate stability
Detection: 0.1 ppb by fluorimetry
Low background fluorescence
Moderate sorption
Properties similar to Rhodamine WT, except:
Degraded by sun
"Chiore!!a" bacteria interferes
High sorption
D. Radioactive Tracers
Tritium
131!
EDTA-51Cr
High stability.
Detection: > 1 ppt by weak 8 radiation
Varying background
Complex analysis (expensive field and
lab equipment)
Half-life « 12.3 years
Radiation hazard
Handling and administrative problems
No sorption
High stability
Detection: high sensitivity by measuring
8 and a emission
Background negligible
Complex analysis
Half-life 8.2 days
Radiation hazard
Sorption on organic material
Moderately stable (affected by cations)
Detection: highly sensitive, by radiation
or post-sampling neutron activation
analysis
No background
Half-life = 28 days
Radiation hazard
Little sorption
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Tracer
Characteristics
E. Other Tracers
Fluorocarbons
Organic anions
High stability
Detection: high sensitivity by measuring
8 emission
No background
Half-life = 35 hours
Radiation hazard
No sorption
Expensive
High stability
Detection: 1 ppt by gas chromatography
with electron capture detection
Low background
Difficult to maintain integrity of
samples
Non-degradahle, volatile, low solubility,
strong sorption by organic materials
Low toxicity
Detection: few ppb by HPLC
Low background
Expensive analysis
Very low sorption
Low toxicity
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APPENDIX D
CHEMICAL SUPPLY COMPANIES
A list of general chemical suppliers is provided, followed by a more
specific list according to type of tracer. It is recommended that several
companies be contacted, as prices can be quite variable. Prices are not
quoted here because they are subject to change. Current prices can be ob-
tained from the supplier by requesting a catalogue and price list, or by
telephone inquiry.
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General Chemical Supplies
Company Telephone
J.T. Baker Chemical Company* (201) 859-2121
222.Red School Lane
Phillipsburg, New Jersey 08865
Eastman Kodak Company* (716) 722-2915
343 State. Street
Rochester, New York 14650
Fisher Scientitle Company* (412) 562-8300
711 Forbes Avenue
Pittsburgh, Pennsylvania 15219
Hach Company* (303) 669-3050
P.O. Box 389
Love!and, Colorado 80537
LaMotte Chemical Products Company* (301) 778-3100
P.O. Box 329
Chestertown, Missouri 21620
Union Carbide Corporation* (212) 551-3763
270 Park Avenue
New York, New York 10017
Bacteriophage
American Type Culture Collection**
12301 Parklawn Drive
Rockville, Maryland 20852
Dyes and Biological Stains
Eastman Kodak Company (716) 722-2915
343 State Street
Rochester, New York 14650
Hach Company (303) 669-3050
P.O. Box 389
Love!and, Colorado 80537
E.I. du Pont de Nemours and Company, Inc.* (302) 774-2421
1007 Market Street
Wilmington, Delaware 19898
Sources: * Analytical Chemistry Lab Guide, 1982
** Water Tracer's Cookbook (Aley, 1976)
*** Personal Communication (Thompson and Bentley, 1983)
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Fluorescent Dyes
Company
Aldrich Chemical Company, Inc.*
940 W. St. Paul Avenue
Milwaukee, Wisconsin 53233
Pylam Products Company, Inc.**
95-10 218th Street
Queens Village, New York 11429
E.I. du Pont de Nemours and Company, Inc.
1007 Market Street
Wilmington, Delaware 19898
Telephone
(414) 273-3850
(302) 774-2421
Gases
Allied Chemical Corporation*
Specialty Chemicals Division
P.O. Box 2064 R
Morristown, New Jersey 07960
Union Carbide Corporation
270 Park Avenue
New York, New York 10017
AIRCO Industrial Gases*
575 Mountain Avenue
Murray Hill, New Jersey
07974
Matheson
P.O. Box 85
932 Paterson Plank Road
East Rutherford, New Jersey
(201) 455-4400
(212) 551-3763
(201) 464-8100
(201) 933-2400
07073
Halogens
Alfa Products*
Thiokol/Ventron Division
152 Andover Street
Danvers, Mississippi 01923
Edmund Scientific Company*
7082 Edscorp Building
Harrington, New Jersey 08007
(617) 777-1970
(609) 547-3488
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Isotopes (Stable and Radioactive)
Company Telephone
Monsanto Company ' (314) 694-1000
800 N. Lindbergh Boulevard
St. Louis, Missouri 63166
Alfa Products (617) 777-1970
Thiokol/Ventron Division
152 Andover Street
Danvers, Mississippi 01923
Edmund Scientific Company (609) 547-3488
7082 Edscorp Building
Barrington, New Jersey 08007
Lycopodium Spores
Carolina Biological Supply Company**
Burlington, North Carolina 27215
Lithium
Foote Mineral Company*** . (215) 363-6500
Rt. 100
Exton, Pennsylvania 19341
Lithium Corporation*** (213) 728-6658
Fluorinated Benzoic Acids
Saber Laboratory, Inc.*** (312) 998-5950
Box 232
Morton Grove, Illinois 80039
Aldrich Chemical Company, Inc. (414) 273-3850
940 W. St. Paul Avenue
Milwaukee, Wisconsin 53233
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APPENDIX E
' ANALYTICAL METHODS FOR THE DETECTION OF TRACERS
Electrical Conductance
An indication of the total dissolved ionic constituents can be obtained
by determining the capability of the water to conduct an applied electrical
current. The relative change in the ability of the ground water to conduct
an electrical current (above the background resistivity prior to injection
during an ion tracer test) will allow the determination of breakthrough time
(travel time) of the tracer in the flow field. The ability of a solution to
conduct an electrical current is a function of the concentration and charge
of the ions in solution and of the rate at which the ions can move under the
influence of an electrical potential. Conductivity or velocity of the ions
is also a function of temperature; thus, it is important to adjust the con-
ductivity readings for any change in temperature.
The device most commonly used for measuring electrical conductivity is
a conductivity meter, read in micromhos. An alternating current is estab-
lished between two points in the flow field and the conductivity (inverse of
resistivity) is measured. A plot of the time versus resistivity or conduc-
tivity readings will indicate the breakthrough time of the tracer. This
technique is very inexpensive and simple to use with various ionic species.
The concentration of the tracer passing through a system at the breakthrough
point cannot be determined by this method. It will, however, provide a
quick method to determine when to sample so that concentration of tracer at
the inflection point (peak conductivity) can be determined analytically.
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Specific Ion Electrode
Specific ion electrode analysis is similar to pH measurement with a pH
meter. Like the pH meter which measures the H+ ion, this technique is ion-
specific and thus, given data from an ionic tracer test, the concentration
of the tracer can be determined using a calibration curve (millivolts versus
mg/1). The reading is a function of temperature, type of ions present, and
concentration of various ions particularly the ion being measured. Specific
ion electrodes can be used in the field or samples can be taken and analyzed
by this method in the laboratory.
Many pH meters used in the field can also read millivolts from specific
ion electrodes. The electrode should be checked using a standard before
initial use and should be checked daily during regular use. This method is
a fast and inexpensive technique for ionic tracers which has a lower limit
of detection of about 0.05 mg/liter for many constituents. Commonly, ions
different than those being measured will produce part of the measured volt-
age, so the electrodes should be used with standard solutions having a com-
position similar to the water sample being measured.
Titration
Titration is the procedure by which a solution of known concentration
(standard solution) is added to a water sample of unknown tracer concentra-
tion until the chemical reaction between the two solutes is complete. The
point at which stoichiometrically equivalent quantities of substance have
been brought together is known as the equivalence point of the titration,
which is usually indicated by a change in color produced by an added dye.
In acid-base titrations, organic dyes known as acid-base indicators are used
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for this purpose. A pH meter can be used instead of a color-metric pH indi-
cator if greater precision is needed. The titration method of analysis
varies in complexity based on the type of chemical tracer involved, and is
very time-consuming if a large number of samples require analysis. Examples
of tracers which can be analyzed by titrimetric techniques include Cl", I",
SCN-, N03-, and
Laboratory Cultuning
The analysis of various bacteria, bacteriophage, and yeast as ground-
water tracers requires sample collection in sterile containers (in order to
minimize the potential of sample contamination by normal soil and water
microorganisms) and the preparation of specific media on which to assay or
culture the desired species. These microbial tracers are usually selected
because of their ease of identification by a microscope on prepared media,
or because they are "marked" by such characteristics as antibiotic resis-
tance.
Once samples are collected, known volumes obtained from serial dilu-
tions of the samples are filtered through membrane filters. These filters
are then placed on prepared nutrient media plates (i.e., agar-agar or mold
broth for yeast) and maintained at the optimum growth temperatures either in
an incubator or at room temperature for the appropriate species-specific
time period. The plates are then analyzed under a microscope for the char-
acteristic markers such as pigmented colonies or other traits. In the case
of bacteriophage, samples can be frozen at the study site and analyzed at a
later date.
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Microscopic Inspection of Spores
%
Various species of spores (i.e., Lycopodium) used as ground-water
tracers are injected into the flow system at locations such as sink holes
and are trapped with plankton nets at potential resurgencies. The spores
(typically marked by dyes) are then examined and counted under a microscope.
Colorimetric Techniques
Analysis by colorimetric methods consists of comparing the extent of
absorption of radiant energy at a particular wavelength by a solution of the
test material with a series of standard solutions. Work with visual compar-
ators requires simple equipment, but is subject to the vagaries of the human
eye; in particular, fatigue and unavoidable low sensitivity under 450 nm and
above 675 nm. The precision of measurement by unaided visual observation is
always less than .that' attainable with photoelectric instruments. Such
instruments, including filter photometers, are suitable for many routine
methods that do not involve complex spectra. Precise work is done with a
spectrophotometer which is able to employ narrow band-widths of radiant
energy and which can handle absorption spectra in the ultraviolet region if
equipped with fused silica optics.
The limitations of many colorimetric procedures lie in the chemical
reactions upon which these procedures are based. Although very few reac-
tions are specific for a particular substance, many reactions are quite
selective, or can be rendered selective through the introduction of masking
agents, control by pH, use of solvent extraction techniques, adjustment of
oxidation state, or by prior removal of interferents (Dean, 1969). Both the
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color-developing reagent and the absorbing product must be stable for a
reasonable period of time.
Numerous ground-water tracers can be analyzed by colorimetric tech-
niques, specifically, the large class of organic dyes (see Chapter 4).
Fluorometry
Fluorometric analysis is a photoluminescent method in which the elec-
tronic state of a molecule is elevated by absorption of electromagnetic
radiation, and as a consequence, the molecule emits light in order to reduce
its energy and return to the ground electronic state. With the exception of
X-ray fluorescence, most of the work lies in the wavelength region between
2000 and 8000 angstroms. Fluorescence provides two kinds of spectra for
identification, the excitation and emission spectra.
Instruments used for fluorometric analysis range from simple filter
fluorometers to highly sophisticated spectrophotofluorometers. Each will
contain four principal components: (1) a source of excitation energy; (2) a
sample cuvette; (3) a detector to measure the photoluminescence; and (4) a
pair of filters or monochromators for selecting the excitation and emission
wavelengths (Willard, 1965).
Fluorescence measurements usually are made by reference to some arbi-
trary chosen standard. The standard is placed in the instrument and the
circuit balanced with the reading scale at any chosen setting. Without
readjusting any circuit components, the standard is replaced by standard
solutions of the test material and the fluorescence of each recorded.
Finally, the fluorescence of the solvent and cuvette alone is measured to
establish the true zero concentration.
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Measurement of fluorescent intensity permits the quantitative determi-
nation of inorganic and organic species in trace amounts. Such ground-water
tracers as dyes can be analyzed by this method. The technique is also very
%
sensitive; the lower limits for the method frequently are less than those
for the absorption method by a factor of ten or better, and are in the range
of a few thousandths to one-tenth of a part per million.
Coulometric Techniques
Coulometric methods of analysis measure the quantity of electricity
(in coulombs) required to carry out a chemical reaction. The coulomb is
that amount of electricity which flows during the passage of a constant
current of one ampere for one second. Reactions may be carried out either
directly by oxidation or by reduction at the proper electrode (primary
Coulometric analysis), or indirectly by quantitative reaction in the solu-
tion with a primary reactant produced at one of the electrodes (secondary
Coulometric analysis). In either case, the fundamental requirement of
Coulometric analysis is that only one overall reaction must occur, and that
the electrode reaction used for the determination proceeds with 100% current
efficiency.
There are two general techniques used in coulometry. One method, the
control!ed-potential method, maintains a constant electrode potential by
continuously monitoring the potential of the working electrode as compared
to a reference electrode. The current is adjusted continuously to maintain
the desired potential. The second method, known as constant-current coulom-
etry, maintains a constant current throughout the reaction period. In this
method, an excess of a redox buffer substance must be added so that the
179
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potential does not rise to a value which will cause some unwanted reaction
to occur. The product of the electrolysis of the redox buffer serves as an
intermediate in the reaction, and must react quantitatively with the
substance to be determined.
Coulometric techniques are particularly useful in trace analyses, being
accurate in the range from milligram down to microgram quantities. This
technique can be used for various ionic tracers .such as Cl~, Br", I", or
SCN-.
Liquid Chromatography
Chromatography encompasses a diverse group of separation methods used
to separate, isolate, and identify components of mixtures which might other-
wise be resolved with great difficulty. In its broadest sense, Chromatog-
raphy refers to processes that are based on differences in rates at which
individual components of a mixture migrate through a stationary medium under
the influence of a moving phase. This rate of movement of a specific com-
ponent is referred to as its retention time. Liquid Chromatography is a
specific class of Chromatography where the mobile phase (injected sample) is
a liquid and, depending on the specific method, the stationary phase is
either liquid or solid.
In order to employ chromatographic techniques, the components to be
separated must be soluble in the mobile phase. They must also be capable
of interacting with the stationary phase either by dissolving in it, by
being absorbed by it, or by reacting with it chemically. Thus, during the
separations, the components become distributed between the two phases.
The most widely used chromatographic method is elution analysis. In
the elution method, a small portion of sample is injected and introducpd at
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the head of the separation column. A differential migration process occurs
in which each component of the sample interacts with the stationary phase,
retarding its flow at a rate characteristic of that specific component down
the length of the column. The time required for a specific component to
reach the end of the column, which is referred to as the retention time, is
a function of the distribution coefficient of the component. The concentra-
tion of each component present is then determined based on the comparison of
its retention time to that of a known concentration standard.
There are numerous chromatographic methods employing a liquid mobile
phase. These include partition, adsorption, ion exchange, paper, and thin-
layer chromatography. All are based on the same chromatographic principles
of separation and isolation as described previously, with variation in the
constituents of the mobile and stationary phases.
Liquid chromatography can be used for the analysis of a wide range of
tracers at very low detection levels. Fluorinated organic acids can be
detected down to concentrations from 1 ppm to 0.01 ppb using reverse phase
and ion exchange high-pressure liquid chromatography (Stetzenbach, 1982).
Halide tracers including Cl", Br~, and I" can be analyzed using liquid (ion
exchange) chromatography.
Gas Chromatography
In gas chromatography, the components of a vaporized sample are frac-
tionated as a consequence of partition between a mobile gaseous phase and a
stationary phase which is either a liquid held on a solid support (gas-
liquid chromatography) or a solid (gas-solid chromatography). In principle,
gas and liquid chromatography techniques differ only in that the mobile phase
in the former is a carrier gas rather than a liquid.
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In gas chromatography, the sample containing the solutes is injected
into a heating block where it is immediately vaporized and swept as a plug of
vapor by the carrier gas stream into the column inlet. The solute components
having a finite solubility in the stationary phase distribute themselves
between that phase and the gas according to the equilibrium law. This par-
titioning process occurs repeatedly as the sample is moved 'toward the outlet
by the carrier gas. Each component (solute) will travel at its own rate
through the column, and consequently, a band corresponding to each solute
will form. The bands will separate to a degree which is determined by the
partition ratios of the solutes and the extent of band spreading. The sol-
utes are eluted, one after another, in the increasing order of their parti-
tion ratios and enter a detector attached to the column exit. If a recorder
is used, the signals appear on the chart as a plot of time versus the compo-
sition of the carrier gas stream. The retention time or time of emergence of
a peak identifies the component, and the peak area reveals the concentration
of the component in the sample. Although the gas chromatographic method is
limited to volatile materials (about 15% of all organic compounds), the
availability of gas chromatographs working at temperatures up to 450°C, pyro-
lytic techniques, and the possibility of converting many materials into a
volatile derivative extend the applicability of the methods (Willard, 1965).
Gaseous tracers such as fluorocarhons (i.e., CC13F and CC1 F ) are
easily detectable in low concentration of between 1 and 100 parts per tril-
lion by gas chromatographic methods.
Mass Spectrometry
Mass spectrometry techniques involve converting the compounds of a
sample into charged ionic particles consisting of the parent ion and ionic
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fragments of the original molecule, and resolving them according to their
mass/charge ratio. A mass spectrometer consists generally of four units:
(1) the inlet system; (2) the ion source; (3) the electrostatic accelerating
system; and (4) the detector and readout system. This ionization process
results in a mass spectrum -which is a record of the numbers of different
kinds of ions. The relative numbers of each type of ion are characteristic
for every compound, including isomers.
Sample size requirements for solids and liquids range from a few milli-
grams to submicrogram quantities as long as the material can exist in the
gaseous state at the temperature and pressure existing in the ion source.
The average sample size for routine gas analysis is about 0.1 ml at standard
conditions, but with special instrumentation, samples of 10"8 ml can be
analyzed (Skoog, 1980). Information useful for elucidating chemical struc-
tures and for accurate determination of molecular weight can be obtained from
the mass spectra literature. Mass spectra can also be employed for the quan-
titative analysis of complex mixtures. In such cases, the magnitude of ion
currents at various mass settings is related to concentration.
Mass spectrometry is often used in conjunction with gas chromatography
techniques. Such is the case for the analysis of fluorinated organic acids
used as ground-water tracers. Lithium salts used for tracing are often
analyzed by mass spectrometry. Stable isotopes (deuterium, tritium, 1(*C,
sulfur, etc.) are also analyzed using mass spectrometry.
Gamma-Ray Emission
Gamma emission is one type of radiation encountered in radiochemical
analysis of both natural and artificial radioactive isotopes which have been
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used as tracers in hydrologic systems. There are three general types of
radiochemical methods: (1) activation analysis; (2) isotope dilution; and
(3) radiometric analysis. In "activation analysis, activity is induced in
one or more elements of the sample by irradiation with suitable particles
and the resulting radioactivity is measured. In isotope dilution, a pure but
radioactive form of the substance to be determined is mixed with the sample
in -a known amount. After equilibrium, a fraction of the component is iso-
lated and its activity analyzed. In a radiometric analysis, a radioactive
reagent is employed to separate completely the component from the bulk of the
sample. The activity of the isolated portion is then measured.
Gamma rays (high-energy photons) are monoenergetic and have a penetrat-
ing power which is much greater than that of either alpha or beta particles,
but a lower ionizing power. The gamma-ray emission spectrum, in contrast to
the alpha and beta emission spectra, is characteristic for each nucleus and
is thus useful for identifying radioisotopes (Skoog, 1980).
One type of detection method for gamma-ray emission is photon counting.
This is a signal processing method where the individual pulse of electricity
produced as a quantum of radiation is absorbed by the transducer and counted.
The power of the beam is then recorded digitally in terms of counts per unit
of time. This operation requires rapid response times for the detector and
signal processor with respect to the rate at which quanta are absorbed by the
transducer. Thus, photon counting is only applicable to beams of relatively
low intensity.
Other types of detectors include gas-filled detectors, the geiger tube,
proportional counters, ionization chambers, and semiconductor detectors. In
most techniques, interference from alpha and beta radiation is readily
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avoided by filtering the beam with a thin window of aluminum or mylar.
Radioactive ground-water tracers such as 131I can be analyzed by gamma-ray
emission.
Beta Particle Emission
Beta particle emission is another type of radiochemical analysis. Beta
particles interact primarily with the electrons in the material penetrated by
the particle. The molecules may be dissociated, excited, or ionized. Beta
particles are produced within a nucleus by the spontaneous transformation of
a neutron to a proton or a proton to a neutron.
Beta particle decay is characterized by production of particles with a
continuous spectrum of energies which is characteristic of each decay pro-
cess. Beta-energy ranges in air are difficult to evaluate. Thus, they are
based upon the thickness of an absorber, such as aluminum, required to stop
the particle. Thin-windowed geiger or proportional tube counters are used to
count a uniform layer of the sample for beta sources having energies greater
than 0.2 Mev. For low-energy beta emitters, such as carbon-14, sulfur-35,
and tritium, a liquid scintillation counter is used. For the liquid scin-
tillation counter method, the sample is dissolved in a solution of the
scintillation compound. A vial containing the solution is then placed be-
tween two photomultiplier tubes housed in a light-tight container. The
output from the two tubes is fed into a counter which records a count only
when pulses from the two detectors arrive at the same time.
Beta particle emission techniques are used for analysis of radioactive
tracers.
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Neutron Activation Analysis
Neutron activation analysis involves the production of a radioactive
isotope by .the capture of neutrons by the nuclei of the substance to be
analyzed. Irradiation is accomplished by placing the sample to be analyzed
in an intense flux of either thermal or fast neutrons for a length of time
sufficient to produce a measurable amount of the desired radioisotope. Radi-
ation detectors are used to analyze the radiation emitted by each sample and
the unique radiation characteristics of the sample are sought.
The method known as post-sampling activation analysis has been described
by Schmotzer (1973) as a tracer technique using low concentrations of Br~.
Although this method of tracer analysis reduces the amount and subsequently
the cost of the chemical tracer, it is a very expensive technique.
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Aley, T., and Fletcher, M. W., 1976, Water tracer's cookbook: Journal of
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