Dating methods for young (ground) waters
Several methods have been developed and applied during the past decades in isotope hydrology to determine (ground) water residence times. In particular, methods based on conservative gas tracers allow the determination of subsurface residence times of groundwater in the range from months up to about 40 years. Among these methods are:
the 3H-method, based on the time dependent input ("bomb peak") of the radioactive hydrogen isotope tritium (3H);
the 3H-3He-method, based on the radioactive decay of tritium (3H) to the stable helium isotope 3He (e.g. Schlosser et al., 1988);
the SF6-method, based on the continuing rise of the trace gas sulfur hexafluoride (SF6) in the atmosphere (e.g. Busenberg and Plummer, 2000);
the CFC-method, based on the rise of the atmospheric content of fluorochlorocarbons (CFCs) from about 1950 until the 1990s (e.g. Busenberg and Plummer, 1992).
It is recommended to apply at least two of the mentioned methods together, in order to increase the reliability of the results and possibly to quantify mixing processes. The 3H-3He- and CFC-methods have proven valuable since more than 10 years to determine mean groundwater residence times and recharge rates (e.g. Cook und Solomon, 1997). The SF6-method has been used increasingly only during the past few years. It closes the dating-gap for young waters that that has been created by the end of the atmospheric increase of the CFCs, and it probably will replace the CFC-method over time.
The radioactive hydrogen isotope 3H (half-life 12.32 years), was released in the 50s- and early 60s by tests of thermonuclear bombs in the atmosphere. The resulting "bomb peak" in precipitation rendered tritium an important tracer in hydrology. However, since the temporal dynamics of tritium in precipitation has strongly decreased in the past decades, tritium data are nowadays often not very meaningful.
The 3H-3He-method, based on combined measurements of 3H and its decay product 3He, allows precise dating of water in the range of months up to 40 years, independent of the tritium input curve. This methods was originally developed for oceanography, but soon was applied to study vertical mixing in lakes (e.g. Torgersen et al., 1977; Hohmann et al., 1998) as well as to date shallow groundwaters (e.g. Schlosser et al., 1988; Solomon et al., 1995). The 3H-3He-method can probably be called the most precise and reliable method for the dating of young waters. Nevertheless it is recommended to combine it with at least one additional dating method, whereupon the combination with the SF6-method (see below) is particularly attractive. In groundwater this combination offers among others the advantage that from the measurement of Ne (and possibly heavier noble gases), which is routinely performed for the 3H-3He-dating, the so-called "excess air", an enhancement of gas concentrations relativ to solubility equilibrium typical for groundwater, can be determined. This information is necessary to carry out the respective correction of the SF6-results. In young groundwater, which infiltrated during the past years to decades, environmental tracers such as 3H-3He and SF6 are reliable tools for the age dating and thus for the determination of recharge rates (e.g. Solomon et al., 1993) and flow velocities (e.g. Stute et al., 1997). Therewith groundwater flow patterns and most importantly fluxes can be defined more precisely as solely on the basis of hydraulic head data. In the case of interaction between ground and surface water, a clear difference in the age and hence in the concentrations can be observed between the two components, which allows mixing calculations to be performed.
Sulfur hexaflouride (SF6) is an anthropogenic trace gas, which among other things has been used to study atmospheric exchange processes, e.g. the exchange of air masses between the northern and southern hemispheres. Its high stability and the related long life-time are very advantageous properties for such applications. Via the contact with the atmosphere, SF6 enters the hydrosphere according to its solubility. SF6 has been used as aquatic tracer since the late 1980s in release experiments in the Ocean and in lakes to derive quantitative results on horizontal and vertical flow and mixing processes. SF6 has also been used for investigations on the gas transfer through the water surface. As a result of the continuous increase of the atmospheric mixing ratio of SF6 a time information is imprinted onto ground and surface waters at the time of separation from the atmosphere, i.e., the SF6-concentration in closed water bodies reflects the time of the last contact with the atmosphere. For young groundwaters (<30 yr), SF6 measurements offer - together with CFCs, which behaved in principle similary at least until the 90s (see below) and tritium - a nowadays often used and appropriate dating tool. The accuracy of SF6-age is however limited by uncertainties e.g. about the recharge temperature and the excess air in newly formed groundwater. A combination with the 3H-3He-method is therefore recommendable, because the noble gas data obtained therewith allow corrections of the SF6-data.
Fluorochlorclorocarbons (CFCs) or freons, in particular F-12 (CCl2F2) and F-11 (CCl3F), are anthropogenic trace gases, which behave nearly conservatively in the environment. In analogy to the SF6-method, the increase of the atmospheric mixing ratios of CFCs from about 1950 until the 1990s enables the dating of younger waters (e.g. Busenberg and Plummer, 1992). Since however as a result of the production stop (Montreal treaty) the increase of the CFCs in past 10 years or so has only been very small or even negative, this method is not appropriate anymore for recent waters, as they can be expected e.g. in the vicinity of surface waters. Yet, for water with ages between about 10 and 40 years, the CFCs still offer a good, relatively simple and cheap dating method. However, local contamination or the degradation of CFCs under anoxic conditions can complicate the interpretation. On the other hand, excess air corrections are of minor importance only.
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