Tritium (3H or T) is probably the most commonly employed radioisotope used to identify the presence of modern recharge. It is a short-lived isotope of hydrogen with a half-life of 12.43 years (Unterweger et al., 1980). It is directly incorporated into the water molecule (1H3HO or 1HTO) and so is the only radioisotope that actually dates groundwater. For most groundwaters, the strongest evidence that at least some active recharge occurs is the presence of measurable 3H.
Tritium is produced naturally by solar radiation, although much greater
production accompanied the atmospheric testing of thermonuclear bombs between
1951 and 1980. Tritium’s short half-life and recent anthropogenic production
makes it an excellent indicator of modern groundwater recharge. It is easily
sampled. Analysis is routine, and the interpretation is not necessarily
Natural tritium is formed in the upper atmosphere from the bombardment of nitrogen by the flux of neutrons in cosmic radiation, following the reaction:
Natural tritium levels in
precipitation are very low and represent a secular equilibrium between
natural production and the combination of decay in the atmosphere plus
loss to the hydrosphere and oceans. Production rates and concentrations
in precipitation are a function of geomagnetic latitude, with greater production
at higher latitudes. Very few measurements exist of natural, pre-bomb tritium
in precipitation. Precipitation near Ottawa had about 15 TU, as measured
in laboratory reagents mixed with Ottawa River water before 1951 (Brown,
1961). Kaufman and Libby (1954) used vintage wines to determined pre-bomb
concentrations of 3.4 to 6.6 TU for precipitation at the lower latitudes
of the Naples NY, Bordeaux and Rhône regions. In Fig. 7-2 we see
the concentrations of cosmogenic tritium measured in meteoric water prior
to 1951, and the increases observed during the early tests of hydrogen-fusion
Fig. 7-2 Tritium levels in precipitation prior to and during
the earliest atmospheric tests of thermonuclear devices. Atmospheric tests:
1 — George, 75 kilotons, U.S.A.; 2 — Ivy-Mike, 10 megatons, U.S.A.; 3 —
RDS-6s, 400 kilotons, U.S.S.R.; 4 — Castle series, total 47 megatons, U.S.A.;
5 — second Soviet thermonuclear test, 2 megatons.
Thermonuclear (bomb) tritium
On May 9, 1951, the world’s
first thermonuclear flame was ignited by a 2H-3H-235U
device code-named George. The
Hagiwara-Fermi-Teller concept of hydrogen fusion had been successfully
tested. On the first day of November, 1952, The Ivy-Mike shot demonstrated
that megaton energy releases could be achieved by hydrogen fusion and the
thermonuclear superbomb was born. The atmospheric detonation of these devices
began a period of anthropogenic 3H
production which raised concentrations in the stratosphere by several
orders of magnitude.
Early configurations of the hydrogen bomb used the radiation of 235U-fission to compress the deuterium-tritium fuel to initiate fusion. Subsequent designs use the high neutron production of 235U-fission to split lithium deuteride, and the heat of fission to then ignite the deuterium and tritium products:
5He is not very stable (t½ = 6.7 ·
10–22 s) and decays by neutron emission to stable 4He.
The tremendous neutron flux at the end of this reaction chain activates
atmospheric nitrogen, producing 3H.
Although fission from detonation of uranium and plutonium bombs began on
July 16, 1945, the neutron flux from these comparatively small and low-level
fission tests, had no impact on meteoric 3H.
Atmospheric testing of nuclear
devices between 1952 and 1962 generated a tremendous quantity of atmospheric
tritium (Fig. 7-3). This substantial input created a tritium reservoir
in the stratosphere which contaminated global precipitation systems for
over four decades. A 1963 Soviet-American treaty banned the atmospheric
testing of thermonuclear devices, although minor Chinese and French tests
continued until 1980. The final year of megaton tests (1962) generated
a huge peak, which appeared in precipitation in the spring of 1963. This
1963 peak became a marker used in many hydrological studies. Concentrations
in precipitation are now largely back to natural, cosmogenic levels.
Even at the peak, anthropogenic tritium fallout was not a radiological threat to health. Canadian drinking water standards impose a limit on 3H of 7,000 Bq·kg–1, or about 60,000 TU. Minor amounts of 3H are presently released to the environment by nuclear power plants, nuclear fuel reprocessing facilities, preparation of weapons-grade nuclear material and by the manufacturers of tritiated paints and gas used in illuminating dials and emergency lighting.
Fig. 7-3 Tritium in precipitation from thermonuclear bomb tests since 1952. Tritium data for selected stations in North America and Europe, from the IAEA GNIP data base. Bomb test data summarized from various sources by Rath (1988) and Gonfiantini (1996).
The longest record of atmospheric tritium concentrations was initiated by R.M. Brown for precipitation at Ottawa, Canada, and begins in 1953 (Fig. 7-4). The IAEA has established several long-term records from its monitoring stations throughout the world. Numerous short-term records at American stations were generated during the peak input years between 1960 and 1980. All data are available at <www.iaea.or.at:80/programs/ri/gnip/gnipmain.htm>.
Two features should be observed in this figure; the consistent annual
fluctuation in 3H, and the rapid decline from the 1963 peak.
The greatest transfer of tritium from the stratosphere to the troposphere
occurs during the spring in mid-latitude zones. This is due to seasonal
disturbances in this boundary by displacement of the jet stream as upper
level air circulation reorganizes in the spring. This "spring leak" annually
recharges the hydrosphere (Fig. 7-4).
The decline of tritium in precipitation is not due only to decay, which
decreases tritium levels by only 5.5% per year. A major factor is attenuation
by the oceans and groundwaters, which have since become major reservoirs
of thermonuclear 3H.