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Hydrogeology

  • Dating
  • Advection Velocities
  • Paleo Temperatures

3He/T - Dating

Since the early 1960s the fallout of the radioactive hydrogen isotope, tritium, entered the groundwater reservoirs. Tritium decays with a half-life time of 12.3 y to 3Helium. From tritium and 3He concentrations the period since the last contact with the atmosphere can be calculated. Therefore the 3He-tritium pair serves as a radioactiv clock. With this tool one can study a large set of dynamic properties of a groundwater system such as advection velocities and seep rates.
From tritium and 3He-concentrations a quantity called 'stable tritium' can be calculated. By comparison with the tritium concentration in precipitation it is possible to assess the fraction of 'old water'.

please see also the relevant links for groundwater.

3He/4He Isotopes Ratios

The 3He/4He ratio is basically used to separate 3He which results from tritium decay from atmospheric 3He. In addition with a known tritium concentrations in the water one can assess the period since the last contact to the atmosphere (see below). Groundwater systems in geological active regions can show degassing of primordial gases which contain He-isotopes with significant different 3He/4He ratios than the atmosphere.

4He/20Ne

The isotope 20Ne has no sources in the groundwater. That means all neon comes from the atmosphere and therefore one has a tool to separate the atmospheric helium. The non-atmospheric helium results from radiogenic processes in the groundwater layer. If the production rate of the so called radiogenic helium is known, it is possible to estimate the groundwater age of old water. This may range from 100 to 100,000 years.

20Ne/22Ne

Variations of the 20Ne/22Ne-isotope ratios may give informations about variations during seep processes. The ratio is also used to check the quality of sampling, handling and measurement.

4He/20Ne/20Ar/20Kr/131Xe

For the heavy noble gases the solubility changes much more with temperature than for the light. This is used to receive the water temperature during the seep process. This temperature may have changed during climatological cycles or just by a change in vegetation or the altitude of the seep regions.