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Tracers for long residence times 36 Cl, 81 Kr

Explore the use of isotopes 36Cl and 81Kr as tracers for studying groundwater flow over long periods in the context of the Great Artesian Basin and Nubian Aquifer.

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Tracers for long residence times 36 Cl, 81 Kr

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  1. Tracers for long residence times36Cl, 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 • 35Cl+n => 36Cl+g • Radioactive decay • 3617Cl => 3618Ar (beta decay, 98%) • 3617Cl => 3616S (electron capture, 2%)

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

  6. 36Cl production

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

  8. Case study: Great Artesian Basin

  9. Case study: Great Artesian Basin

  10. Global fallout of ‘bomb’ 36Cl

  11. 36Cl as vadose zone tracer Advantage: 36Cl not volatile

  12. Long-term secular variations

  13. Milk River aquifer Flow distance (km)

  14. Milk River aquifer Flow distance (km)

  15. 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

  16. 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

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

  18. Natural Krypton sample Collon et al, 2000

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

  20. Experimental results Collon et al, 2000

  21. 81Kr in Nubian aquifer 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

  22. 81Kr in Nubian Aquifer 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

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