1. Tracers for long residence times 36Cl, 81Kr

2. The paradox of gw age
Bethge and Johnson, 2002

3. 36Cl T1/2 = 301,000 years expressed as
R36Cl=atoms36C/Cl (typically in 10-15)
A36Cl=atoms36Cl/L (typically in 10-7/L)
Cosmogenic production (atmosphere):
40Ar+p => 36Cl+n+a (67%)
36Ar+n => 36Cl+p (33%)
Cosmogenic production (surface):
35Cl+n => 36Cl+g
39K+n => 36Cl+n+a
40Ca+n => 36Cl+p+a

4. 36Cl Subsurface production Radioactive decay 35Cl+n => 36Cl+g
3617Cl => 3618Ar (beta decay, 98%)
3617Cl => 3616S (electron capture, 2%)

5. In-growth and decay of 36Cl
secular
equilibrium

6. 36Cl production

8. Case study: Great Artesian Basin
No 14C in
distal parts
of the aquifer

9. Case study: Great Artesian Basin

10. Case study: Great Artesian Basin

11. Global fallout of ‘bomb’ 36Cl

12. 36Cl as vadose zone tracer
Advantage:
36Cl not volatile

13. Long-term secular variations

14. Milk River aquifer
Flow distance (km)

15. Milk River aquifer
Flow distance (km)

16. 81Kr T1/2= 229,000 years, 81Kr/Kr = 5.2 x 10-13 (very low!)
solubility of Kr in ocean water is 9.5x10-5 cm3 STP/L
1 L of ocean water contains ~1,200 81Kr atoms
=> 1 radioactive decay in 300 years!
Production in the atmosphere:
cosmic ray induced spallation and neutron activation of stable krypton
No significant underground production

17. Three methods have been used to date
Low Level Counting
Possible on large samples (~1000 l), done by H.H. Loosli in Bern
Accelerator Mass Spectrometry
several difficulties (DM/M, low concentration, …)
Laser (ATTA)
Argonne National Laboratory – very promising

18. Dating water from the Great Artesian Basin of Australia
Collon et al, 2000

19. Natural Krypton sample
Collon et al, 2000

20. Groundwater sample (Watson Creek)
Collon et al, 2000

21. Experimental results Age (years) Uncertainty Atmosphere
Raspberry Creek
225,000
±42,000
±(12.7% * t)
Oodnadatta
354,000
±50,500
±(15.3% * t)
Duck Hole
287,000
±44,200
±(13.4% * t)
Watson Creek
402,000
±51,000
±(15.4% * t)
Collon et al, 2000

22. 81Kr in Nubian aquifer Sturchio et al, 2004
Figure 1. Map showing sample locations (red circles) in relation to oasis areas (shaded green), Precambrian basement outcrops (patterned), and other regional features. Groundwater flow in Nubian Aquifer is toward northeast.
Sturchio et al, 2004

23. 81Kr in Nubian Aquifer Sturchio et al, 2004
Figure 2. 36Cl/Cl (×10−15) vs. 81Kr age for Nubian Aquifer groundwater samples (±1σ error bars), showing best-fit exponential decay curve of 36Cl. Intercept on y-axis represents [36Cl/Cl]initial, the initial 36Cl/Cl ratio of groundwater, which is 131(±11) × 10−15 when [36Cl/Cl]seq = 8(±3) × 10−15 is assumed for the secular equilibrium value of 36Cl/Cl in the sandstone.
Sturchio et al, 2004