Principles behind the use of environmental tracers

• Tracers: trace substances of natural or anthropogenic origin (stable and radioactive isotopes; chemical compounds. Sometimes toxic or otherwise harmful (contaminants)
• Transient tracers: ‘Dyes’ with known delivery rates to the environment (e.g., ³H, ³He, CFCs, SF₆, ⁸⁵Kr,)
• Radioactive clocks (e.g., ¹⁴C, ³⁹Ar, ⁸¹Kr, ³⁶Cl)
• Special sources (e.g., ¹⁸O, Ba, nutrients, Ra isotopes, Rn isotopes)
• Deliberately released tracers (e.g., SF₆, ³He)

³H

• history
• Rutherford (1935-36) performed ecrolysis of 13,000 t of water: ³H/²H < 1*10^-5
• discovery in nature ~ 1950/51: Falting and Hartek
• production
• main reaction producing ³H in the atmosphere: ¹⁴₇N(n, ³H)¹²₆C, also: ¹⁴₇N + n -> 3 ⁴₂He + ³₁H
• production rate: 0.5 +/- 0.3 atoms cm^-2 s^-1
• decay
• b^- decay
• ³H -> ³He + e^- + n +Q
• E_max = 18.6 keV
• T_1/2 = 12.43y
• inventory
• 1 TU = [T]/[H] = 1*10^-18
• 1 TU = 3.2 pCi/kg H₂O = 0.12 Bq/kg H₂O
• natural level in precipitation ~ 5 TU
• 'bomb tritium' max. levels ~1000 to 2000 TU
• in 1973
• natural T: ~2.5%
• nuclear power plant T: 0.02%
• bombs: 97.5%

³H/³He dating of (young) groundwater

• ³H in the environment has a pronounced peak in 1963 (Northern hemisphere).
• Location of this peak in an aquifer provides an absolute age marker
• ³H decay and dispersion make it increasingly difficult to identify the tritium peak in aquifers.
• Addition of He isotope measurements allows us to determine ‘stable tritium’, i.e., the sum of ³H and its decay product ³He.
• The ratio of ³H and ³He provide a radioactive ‘clock’ that can be used to estimate the time that has passed since recharge
• Schematic view of ³H/³He dating method (Fig)
• example: ³H/³He dating, Danube bank infiltration (Fig)
• potential complications of the ³H/³He dating method
• Terrigenic He (crustal; mantle): separation possible by using Neon
• ³He confinement: can be calculated
• ³H contamination: no problem for moderate contamination levels; high contamination sites have to be evaluated
• influence of terrigenic He on the ³H/³He age (Fig)
• gas confinement (Fig)
• influence of temperature and elevation on the ³H/³He age (Fig)
• perspectives
• large data sets can be measured routinely
• bomb peak can be detected through 3Hetrit. Method can be applied for decades to come.
• most complications of tritium/3He method can be overcome by measurement of additional parameters and careful interpretation of the data set
• method ready for application in many GW flow systems using (large) multi-tracer data sets
• more studies needed in complicated situations such as fractured rock, karst, or springs.

Resources

• detailed information on ³H/³He dating
• ³H/³He measurements at LDEO