United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada OK 74820
Research and Development
EPA/600/S2-85/022 Feb. 1986
Project Summary
An Introduction to
Ground-Water Tracers
Stanley N. Davis, Darcy J. Campbell, Harold W. Bentley, and Timothy J. Flynn
An ideal tracer does not exist. There-
fore, the selection and use of tracers is
almost as much an art as it is a science.
The full report (manual) provides a guide
for the use of ground-water tracers to
practicing engineers, hydrologists, and
ground-water hydrologists. The manual
is specifically concerned with the selec-
tion of tracers, their field application,
collection of samples containing trac-
ers, sample analysis, and interpretation
of the results.
This Project Summary was developed
by EPA's Robert S. Kerr Environmental
Research Laboratory. Ada, OK, to an-
nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
This research project was designed to
develop a manual that could be used as a
guide for the use of tracers in ground
water. Specifically, the manual is con-
cerned with the selection of tracers, their
field application, collection of samples
containing tracers, sample analysis, and
interpretation of the results.
Certain hydrogeologic principles must
be understood in order to design success-
ful tracer tests, i.e., Darcy's Law, hydraul-
ic conductivity, the direction of water
movement, and the velocity of ground-
water flow. Hydrodynamicdispersion and
molecular diffusion also play an i mportant
role in the area of ground-water tracers.
Results
The purpose and practical constraints
of a potential tracer test must be clearly
understood prior to the actual planning of
a test. Table 1 indicates some factors to
consider in tracer selection. Choice of a
tracer will depend partially on which
analytical techniques are easily available
and which background constituents might
interfere with these analyses. Various
analytical techniques incorporate differ-
ent interferences, and consultation with
the chemist who will analyze the samples
is necessary.
The variety of tracer tests is almost
infinite when one considers the various
combinations of tracer types, local hydro-
logic conditions, injection methods, sam-
pling methods, and the geological setting
of the site. There are five common
problems encountered with tracer tests:
site selection for monitoring and injection
wells, choice of drilling equipment, choice
of casing diameter for monitoring wells,
type of casing particularly if tracers are
organic compounds or metallic cations,
and choice of well construction (screens,
perforation, slots, etc.).
The measured quantity which is fund-
amental for most tracer tests is the first
arrival time of the tracer as it goes from an
injection point to a sampling point. This
conveys at least two items of information.
First, it indicates that a connection for
ground-water flow actually exists be-
tween two points. For many tracer tests,
particularly in karst regions, this is all the
information that is desired. Second, an
approximation of the maximum velocity
of ground-water flow between the two
points may be obtained if the tracer used
is conservative. I nterpretations more elab-
orate than this depend very much on the
type of aquifer being tested, the velocity
of ground-water flow, the configuration
of the tracer injection and sampling
systems, and the type of tracer or mixture
of tracers used in the test.
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Table 1. Factors to Consider in Tracer Selection
Purpose of Study
Determination of: 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
A vailable Funds
Manpower and equipmentto run tests to completion (e.g., drilling, tracer cost, sampling, analysis).
Type of Medium
Tracer Type
Karst
Porous media (alluvium, sandstone, soil)
Fractured rock
Fluorescent dyes, spores.
tritium, as well as other
tracers
Wide range of choices
Dyes and paniculate material
are rarely useful
Wide range of choices
Dyes and paniculate material
only occasionally are useful
Stability of Tracer
Distance from injection to samp/ing point
Approximate velocity of water and approximate estimate of time
required for test, given: distance from injection to sampling
point, porosity, thickness of aquifer
Detectability of Tracer
Must be stable for length of
test and analysis
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
Conclusions and
Recommendations
As used in hydrogeology, a tracer is
matter or energy carried by ground water
which will give information concerning
the direction of movement and/or velocity
of the water and potential contaminants
which might be transported by the water.
A tracer should have a number of
properties in order to be generally useful.
The most important criterion is that the
potential chemical and physical behavior
of the tracer in ground water must be
understood. The objective is commonly to
use a tracer that travels with the same
velocity and direction as the water and
does not interact with solid material. A
tracer should be nontoxic, relatively inex-
pensive to use, and for most practical
problems, easily detected with widely
available and simple technology.
Table 2 provides a summary of some of
the most important tracers presently
available.
Factors to Consider
Example of Difficult Tracer
Availability of tracer
Ease of sampling
A vailability of technology for and ease of analysis
Radioactive (must have special
permits)
Gases (will escape easily from
poorly sealed container)
CI-36 (only one or two
laboratories in the
world can do analyses)
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Table 1. (Continued)
Physical/Chemical/Biological Properties of Tracer
Density, viscosity
May affect flow (e.g., high
concentrations of CT)
Solubility in water
Sorptive properties
Stability in water
Physical
radioactive
decay
Chemical
Biological
Affects mobility
Affects mobility
Affects mobility
decomposition degradation
and precipitation
Public Health Considerations
Toxicity
Dilution expected
Maximum permissible level—determined by federal, state, provincial, and county agencies
Proximity to drinking water
Table 2. Summary of Most Important Tracers
Tracer
Characteristics
A. 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
Most useful for studying transport of microorganisms
Detection: highly sensitive
Sampling: filtration, then incubation and colony counting
No diffusion, slight sorption
Detection: highly sensitive
Sampling: culturing, colony counting
Some sorption
Smallest paniculate
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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Table 2. (Continued)
Tracer
Characteristics
C. Dyes
Rhodamine WT
Fluorescein
D. Radioactive Tracers
Tritium
!3tt
EDTA-5!Cr
*2Br
E. Other Tracers
Fluorocarbons
Organic anions
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
"Chlorella" bacteria interferes
High sorption
High stability
Detection: > 1 ppt by weak fl 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 p 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
High stability
Detection: high sensitivity by measuring /} 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-degradable, volatile, low solubility, strong sorption by
organic materials
Low toxicity
Detection: few ppb by HPLC
Low background
Expensive analysis
Very low sorption
Low toxicity
U. S. GOVERNMENT PRINTING OFFICE:1986/646-116/20769
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Stanley N. Davis, DarcyJ. Campbell, Harold W. Bentfey, and Timothy J. Flynn are
with the University of Arizona, Tucson, AZ 85721.
Jerry T. Thornhill is the EPA Project Officer (see below).
The complete report, entitled "An Introduction to Ground-Water Tracers," (Order
No. PB 86-100 591/AS; Cost: $22.95, subject to change) will be available only
from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Robert S. Kerr Environmental Research Laboratory
U.S. Environmental Protection Agency
P.O. Box 1198
Ada, OK 74820
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-85/022
0000329 PS
AGENCr
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