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Direct Counting: $175; turn around time is 1 to
2 months.
 (See
UA
Laboratory of Isotope Geochemistry for more
information)
Mass Spectrometry: $300
(See
University
of Miami Rosenstiel School of Marine and Atmospheric
Science for more information)
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Cosmogenic
Natural tritium is created in the upper atmosphere
from the cosmic bombardment of nitrogen with neutrons.
Tritium then combines with oxygen to produce tritiated
water (H3HO)
and enters the hydrologic cycle.
Tritium decays to a rare, stable isotope of helium
(3He)
by beta emission.
Lithogenic
Lithogenic tritium is produced by the showering
of lithium present in rocks by neutrons produced
during the spontaneous fission of uranium and
thorium.
This process is limited by the amount of lithium
in rocks. In most cases, lithogenic production
is negligible compared to other sources. The lithogenic
tritium enters the groundwater directly.
Anthropogenic
The figure above illustrates the monthly levels
of tritium in precipitation at Ottawa, Canada,
the longest existing record. (Reprinted from Clark
and Fritz 1997, p. 178; monitoring record established
by R.M. Brown, AECL, Canada).
Natural production of tritium in the atmosphere
is very low. Due to the advent of thermonuclear
technology, this production in the atmosphere
has been supplemented by anthropogenic production.
Beginning in the 1950's, large amounts were produced
from the atmospheric testing of thermonuclear
bombs,. Concentrations of tritium decreased after
1963 because the United States, the USSR, and
the United Kingdom signed the Soviet-American
treaty banning above-ground testing. Because anthropogenic
tritium is also currently produced by releases
from nuclear power plants, present day concentrations
of tritium in the atmosphere have not returned
to natural concentrations, but levels have gradually
decreased since the 1960's as the tritium continues
to decay.
All atmospheric tritium, whether cosmogenic or
anthropogenic, is rapidly incorporated into water
molecules and falls in meteoric precipitation
to enter the hydrologic cycle.
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Tritium concentrations are represented in tritium
units (TU). One tritium unit is equal to one molecule
of 3H
per 1018
molecules of 1H
and has an activity of 0.118 Bq/kg (3.19 pCi/kg).
Decay-counting
Tritium is typically measured by a liquid scintillation
counter. The sample is mixed with a scintillation
mixture of a solvent, emulsifier, and solute.
Tritium emits beta decay electrons, which excites
the solvent. The solvent transfers its energy
to the solute, which emits light photons. The
light pulses are detected and counted.
Mass spectrometry
Tritium and its daughter 3He
can be measured using mass spectrometry, but other
dissolved gases (H2O,
CO2,
O2,
N2,
etc.) must be removed first. Lower detection limits
can be achieved by mass spectrometric measurements
of 3He
produced by the "ingrowth method," where
the tritium sample is stripped of all gases and
then stored for a sufficient time for enough daughter
product 3He
to be produced and measured. The mass spectrometer
method has the advantage over the liquid scintillation
method because both 3He
ingrowth (3H)
and the 3He
of a sample can be measured. This provides quantitative
age determinations discussed in the section below.
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The large pulse of tritium that entered the hydrologic
cycle in the 1960's can be used to establish the
age of recent groundwater recharge. High levels
of tritium (>~30 TU) indicate water that was
recharged during the late 1950's or early 1960's;
moderate concentrations indicate modern recharge;
levels close to detection (~1 TU) are likely submodern
or paleogroundwaters that have mixed with shallow
modern groundwaters. (Clark and Fritz 1997). General
guidelines are:
| <0.8 TU |
submodern (prior to
1950s) |
| .8 - 4 TU |
mix of submodern and
modern |
| 5 - 15 TU |
modern (<5 to 10
years) |
| 15 - 30 TU |
some bomb tritium |
| >30 TU |
recharge in the 1960's
to 1970's |
| >50 TU |
recharge in the 1960's |
Bomb-produced tritium can be used as a tracer
in studying young groundwaters to help determine
flow rates and directions, mean residence times,
and hydraulic parameters such as conductivity,
and can also be helpful in observing preferential
flow paths and in investigating the mixing of
waters. Its use, however, is somewhat limited
by a number of factors, including uneven global
distribution and local variations due to continued
nuclear releases. Some approaches that may be
used to counter the problems using 3H
in dating groundwater include using time series
analysis to monitor the bomb spike for 3H
in an aquifer to provide an indication of mean
residence time.
Groundwater can also be aged quantitatively using
tritium and its daughter, 3He.
Age is determined by:
where t is the time since isolation from contact
with the atmosphere after decay (estimated age),
and 3Het/3Ht
is the concentration ratio of the two isotopes
expressed in TUs.
Aging by the 3H-3He
method also presents problems, since the total
3He
in groundwater comes from a variety of sources:
the atmosphere, 3H
decay, subsurface nuclear reactions, and the earth's
mantle. The measured concentration of 3He
must be corrected for these other sources. 3He
is also not a routinely sampled or measured isotope.
Other concerns are the fractionation of 3He
if a gas phase is present and the fact that the
solubility of He is temperature dependent.
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- Faure, G., Principles of Isotope Geology,
2nd ed., John Wiley and Sons, New York, 1986.
- Clark, I., and P. Fritz, Environmental
Isotopes in Hydrogeology, Lewis Publishers,
Boca Raton, 1997.
- Gonfiantini, R, Environmental isotopes in
lake studies, in Handbook of Environmental
Isotope Geochemistry, vol. 2, edited by
P. Fritz and J.-Ch. Fontes, pp. 113-168, Elsevier,
Amsterdam, 1986.
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International
Atomic Energy Agency, Isotope Hydrology Information
System (ISOHIS)
International
Atomic Energy Agency, The Development of GNIP
(Global Network of Isotopes in Precipitation)
Lawrence
Livermore National Laboratory
USGS
Periodic Table - Hydrogen
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