Contents of Session 6
• Fundamentals of tritium in hydrology
– Physical properties of tritium
– Origin and distribution of tritium
• Examples of applications
– Ocean circulation
– Determination of groundwater recharge
– Separation of young and old water, identification of mixing
Physics of Aquatic Systems II
–
6. Tritium
• Literature: Mook (2001), Vol. 1, ch. 8; V. 2(5); V. 3(2); V. 4(5)
Werner Aeschbach-Hertig
Institute of Environmental Physics
University of Heidelberg
1
Institut für Umweltphysik
Universität Heidelberg
Physics of Aquatic Systems II, 6. Tritium
2
Institut für Umweltphysik
Basic properties of tritium
Half-life of tritium
Tritium in the table of nuclides:
β-
14
N(n,3 H )12 C
Tritium decay:
β--decay, half-life 12.32 yr, λ = ln2/T1/2 = 0.0563 yr-1 = 1.79.10-9 s-1
Energy: 18.6 keV (very low)
Year
Authors
Half-life (yr)
1936
McMillan
>10
1940
O'Neil and Goldhaber
31 ± 8
1947
Novick
12.1 ± 0.5
1951
Jones
12.41 ± 0.05
1955
Jones
12.262 ± 0.004
1966
Merritt and Taylor
12.31 ± 0.13
1977
Rudy and Jordan
12.323 ± 0.004
1980
Unterweger et al.
12.43 ± 0.05
1987
Oliver et al.
12.38 ± 0.03
2000
Lucas and Unterweger
12.32 ± 0.02
(4500 ± 8 d)
3
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Physics of Aquatic Systems II, 6. Tritium
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Physics of Aquatic Systems II, 6. Tritium
Universität Heidelberg
Institut für Umweltphysik
Physics of Aquatic Systems II, 6. Tritium
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Universität Heidelberg
Cosmogenic Production (Latitude Dependence)
Origin and abundance of tritium
Natural tritium production:
Cosmic rays in atmosphere:
14
N(n,3 H )12 C
Charged particles in cosmic radiation are
deflected by the geomagnetic dipole field:
Fluxes and production largest near poles
Rapid reaction to HTO → precipitation → water cycle
Subsurface production:
6
Li ( n,α ) 3 H
Production very low ⇒ concentration very small
Activity concentrations in water ~ 1 Bq/l ~ 10-11 Ci/l ~ 10-15 mol/l
Tritium units:
1 tritium unit (TU) ⇔ 3H/1H = 10-18 (also called tritium ratio, TR)
1 TU = 0.118 Bq/kg = 3.19 pCi/kg = 1.11.10-16 mol/kg = 6.68.107 at/kg
from Masarik & Beer, 1999, J. Geophys. Res., D104: 12,099-13,012
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Physics of Aquatic Systems II, 6. Tritium
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Physics of Aquatic Systems II, 6. Tritium
6
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1
Natural global tritium inventory
Steady state: production = decay
Pre-bomb tritium in precipitation
PA = λ N
Mean production rate: P = 0.28 at cm-2 s-1
Area of Earth:
A = 5.1.1018 cm2
Decay constant:
λ = 0.0563 yr-1 = 1.78.10-9 s-1
⇒ Natural amount of tritium:
N = 8.0.1026 atoms = 1.3 kmol ≈ 4 kg !
Compare: Global water mass ≈ 1.4.1021 kg H2O ≈ 1.6.1020 kg H
from Clark & Fritz, 1997 8
7
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Physics of Aquatic Systems II, 6. Tritium
Institut für Umweltphysik
Tritium production in thermonuclear (hydrogen) bombs
Physics of Aquatic Systems II, 6. Tritium
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Bomb tritium in precipitation
EBq
Total production:
~ 240 EBq = 6.5 GCi
= 230 kmol = 690 kg
EBq = Exa-Becquerels (1018 Bq)
Institut für Umweltphysik
from Clark & Fritz, 1997
9
Universität Heidelberg
Physics of Aquatic Systems II, 6. Tritium
10
Institut für Umweltphysik
Tritium input: Seasonal variation
Physics of Aquatic Systems II, 6. Tritium
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Tritium input: Latitudinal variation
6000
50
5000
Tritium [TU]
40
4000
30
3000
20
2000
10
0
1000
0
1965
1984
1970
1986
1988
1975
1980
Year
Institut für Umweltphysik
Physics of Aquatic Systems II, 6. Tritium
1990
1985
1992
1990
GNIP data from Vienna
Universität Heidelberg
from Clark & Fritz, 1997
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Institut für Umweltphysik
Physics of Aquatic Systems II, 6. Tritium
12
Universität Heidelberg
2
Continental effect for tritium
Continental effect for tritium
from Ferronsky & Polyakov, 1982, Environmental Isotopes in the Hydrosphere
Institut für Umweltphysik
Physics of Aquatic Systems II, 6. Tritium
Institut für Umweltphysik
Tritium in precipitation
•
•
•
•
•
•
•
•
from Weiss et al., 1979, In: Behaviour of tritium in the environment, IAEA.
13
Universität Heidelberg
Physics of Aquatic Systems II, 6. Tritium
Local anthropogenic tritium sources
Natural level ~ 5 TU, latitude dependent (increase towards poles)
Release from fusion bombs mainly into stratosphere (reservoir)
Residence time in stratosphere: a few years (rapid decline of peak)
Hemispheric gradient
Release in N-hemisphere, slow interhemispheric mixing (~ 2 yr)
Latitudinal gradient
Location of stratosphere - troposphere exchange (mid-latitudes)
Rapid removal from troposphere (5 – 20 d), slow N-S mixing
Seasonal effect
Seasonality of stratosphere - troposphere exchange ("spring leak")
Continental effect
Ocean as sink for tritium (dilution, vapour exchange)
Re-evaporation of tritium-rich water from continents
Locally elevated tritium concentrations
Tritium release from nuclear power plants and industry
Industrial sources: Nuclear power, watch industry, waste, etc.
15
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Physics of Aquatic Systems II, 6. Tritium
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Institut für Umweltphysik
The Swiss accidental tritium tracer "experiment"
Physics of Aquatic Systems II, 6. Tritium
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Tritium in Oceanography: NADW formation
1984: 500 Ci (~20 TBq) tritium spilled into small river, breakthrough
in nearby (500 m from river) production well observed
high permeability layer
low permeability layer
from Hoehn & Santschi, 1987, Water Resour. Res. 23: 633-640
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Physics of Aquatic Systems II, 6. Tritium
17
Universität Heidelberg
from Östlund & Fine, 1979, In: Behaviour of tritium in the environment, IAEA.
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Physics of Aquatic Systems II, 6. Tritium
18
Universität Heidelberg
3
Tritium distribution in North Atlantic
Tritium peak in soil water: Recharge rate
Tritium penetration to > 3000 m depth in region of
North Atlantic Deep Water (NADW) production
from: L-DEO Envir. Tracer Group, WOCE page (http://www.ldeo.columbia.edu/~etg/text/woce_proj.html)
Institut für Umweltphysik
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Physics of Aquatic Systems II, 6. Tritium
Tritium in unsaturated chalk, England
19
Institut für Umweltphysik
from Mook, 2001
20
Universität Heidelberg
Physics of Aquatic Systems II, 6. Tritium
Tritium in groundwater: age classification
Tritium in groundwater: River infiltration
Tritium in groundwater from Bangladesh:
Determine penetration depth of recent (post-bomb) groundwater
Tritium in groundwater in
1973 vs. distance from
Reno River, Italy.
¾High-tritium river water
penetrates up to 600 m
¾Delineation of potential
contamination area
from Kendall and McDonnell, 1998
Institut für Umweltphysik
from Aggarwal et al., 2000, IAEA report
21
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Physics of Aquatic Systems II, 6. Tritium
Institut für Umweltphysik
Tritium in groundwater: age classification
22
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Physics of Aquatic Systems II, 6. Tritium
Tritium dating
Classification:
Input
Decay
3H
active:
recent, modern (< 50 yr)
Tritium (TU)
dead, 14C high:
young, submodern (~100 yr)
3H
dead, 14C intermediate:
old (~ kyr)
Tritium [TU]
1000
3H
100
3H
dead, 14C dead:
very old, fossil (> 30 kyr)
3H
active, 14C interm. - high:
mixed
10
1950
14C
Institut für Umweltphysik
(pmc)
Physics of Aquatic Systems II, 6. Tritium
from Mook, 2001
Universität Heidelberg
1960
1970
1980
1990
2000
Year
23
Institut für Umweltphysik
Physics of Aquatic Systems II, 6. Tritium
24
Universität Heidelberg
4
Summary
Tritium dating
• Factors influencing tritium in precipitation
– Latitude and seasonality of stratosphere – troposphere exchange
– Exchange with ocean (↓), re-evaporation from continents (↑)
– Local releases from nuclear plants and other industry
• Applications of tritium
– Marker of ~1963 water in ocean, soils, and groundwater
– Qualitative dating: pre-bomb / post-bomb / mixtures
– Quantitative dating difficult: ambiguous, over-simplified
Tritium [TU]
100
10
1975
1980
1985
1990
1995
Year
Institut für Umweltphysik
Physics of Aquatic Systems II, 6. Tritium
25
Universität Heidelberg
26
Institut für Umweltphysik
Physics of Aquatic Systems II, 6. Tritium
Universität Heidelberg
5