Groundwater dating techniques
The reason for this observation is due to the high tritium and 3 He concentration gradients near the bomb peak and the related increased transport of both tracers by dispersive processes.
Tritium is produced naturally in the upper atmosphere by interaction of nitrogen, and, to a lesser extent, oxygen with cosmic rays.
This component can be radiogenic helium, mantle helium, or a mixture of these two components. The total 4 He concentration measured in a groundwater sample can be written as: The most practical approach to determining R terr is to measure it in groundwater samples from the same aquifer that are free of tritium.
If the tritium delivery as a function of time can be reconstructed, this penetration process can be used for quantitative studies of water movement through identification of the bomb peak in certain ground water bodies.
It is independent of the initial tritium concentration of the water sample which is one of the advantages of the method because it eliminates the necessity to establish the exact time- dependent tritium delivery to the aquifer.
If both the tritium and 3 He trit concentrations are measured in TU, it can be calculated as. However, there are natural limits to this method because dating techniques decay and dispersion make it increasingly difficult to identify the bomb peak in groundwater.
Therefore, for quantitative studies, mixing has either to be ruled out as a major factor influencing the flow regime or it has to be accounted for in the data evaluation.
In this equation, only 3 He tot and 3 He eq are determined through measurements. The confinement of 3 He trit is mainly determined by the ratio of advection to dispersion in water parcels moving away from the water table.
Whereas the addition of bomb tritium to the environment practically eliminated the use of natural tritium as a tracer, it offered a new tool, i. If there is no tritium-free groundwater in the studied dating techniques, an estimate of R terr can be obtained in most cases by plotting 3 He versus 4 He.
The total 3 He concentration has a variety of sources equation Tritium 3 H or T is the radioactive isotope of hydrogen that decays with a half life of After oxidation to HTO, it takes part in the natural water cycle.
These tests which were mainly performed in the early s, led to an increase of tritium in precipitation over the continents of the northern hemisphere from roughly 5 TU to levels of the order of TU. These problems can be overcome by using tritium in combination with its decay product 3 He 3 He trit as first suggested by Tolstykhin and Kamensky and experimentally confirmed by Torgersen et al.
If no terrigenic helium is contained in the groundwater sample, 3 He trit can be calculated by using equation