HydroTraP Group Tracer & Proxies

A difficulty of the noble gas method (as well as of the dating methods based on gaseous tracers) lies in the occurrence of the so-called "excess air" in groundwater, which has be be taken into account in the calculation of noble gas temperatures by inverse numerical methods (Ballentine and Hall 1999; Aeschbach-Hertig et al. 1999). Excess air in groundwater is however not just an annoyance, but can potentially yield useful indications about the recharge conditions. Especially in semi-arid regions a relationship between excess air and recharge or precipitation intensity has been suggested (Aeschbach-Hertig et al. 2002a, Beyerle et al. 2003, Kulongoski et al. 2004), which could render "excess air" a proxy for paleo-humidity.

Stable Isotopes (δ¹⁸O, δ²H)

The stable isotopes of the water molecule (δ¹⁸O, δ²H) have been applied successfully for decades in hydrology and climatology (e.g. Clark and Fritz, 1997). The serve to identify recharge conditions, origin, and mixing of different waters. In regions with marked topography, the stable isotopes can be used to determine the recharge altitude (by the so-called altitude effect, Gonfinatini et al., 2001), which allows a delineation of the recharge areas and is of high importance for the interpretation of gas tracer data. Particularly important in paleoclimate studies on groundwater is the already mentioned possibility to use stable isotopes as paleotemperature proxies. Compared to the noble gases, the stable isotopes have a much wider field of application as temperature proxy, but their calibration as a thermometer is a difficult and much discussed problem (e.g. Fricke and O'Neil 1999). The combination of noble gas temperatures and stable isotopes in groundwater studies offers a unique possibility to locally calibrate the relationship between temperature and isotope ratios, which is needed as a basis for the interpretation of other continental climate archives (e.g. ice cores, lake sediments, speleothems).

Excess air in groundwater

The experience from many measurements of dissolved conservative gases (mostly noble gases) in groundwater has shown that their concentrations lie almost always above the expected solubility equilibrium with the atmosphere. The composition of the gas excess relative to the solubility equilibrium proves its atmospheric origin, which led to the common term "excess air" (Heaton and Vogel, 1981). The cause of the excess air are certainly small air bubbles that are trapped in the so-called quasi-saturated zone during a rise of the groundwater table (Faybishenko, 1995).

Originally it has been assumed that excess air in groundwater was due to complete dissolution of such entrapped air bubbles (Heaton and Vogel, 1981; Andrews et al., 1979) and therefore should have the exact same composition as air. The detailed investigation of noble gas data sets, especially with inverse numerical methods (Ballentine and Hall 1999; Aeschbach-Hertig et al. 1999) has however shown that this assumption is in many cases not correct. As a solution, more complex models of the formation of excess air have been developed (Stute et al. 1995, Aeschbach-Hertig et al. 2000), which include a fractionation of the composition of excess air with respect to air.

According to our experience, the model of formation and composition of excess air by equilibration between water and entrapped bubbles in a closed system has proven to be very reliable (Aeschbach-Hertig et al. 2000). This model also provides a theoretical basis for the interpretation of the excess air as an indicator of environmental conditions during groundwater recharge (Aeschbach-Hertig et al. 2002a). It shows that the hydrostatic pressure has a dominating influence on the size of excess air. Because the hydrostatic pressure is related to water table fluctuations, eventually a relationship between excess air and recharge or precipitation intensity can be expected (Aeschbach-Hertig et al. 2002a, Beyerle et al. 2003, Kulongoski et al. 2004). These relationships have essentially been corroborated by detailed investigations on excess air under controlled conditions (Holocher et al., 2002, 2003).

Because of the importance of the excess air correction for the ³H-³He and SF₆ methods, we combine whenever possible complete noble gas analyses with these dating methods. Both the recharge temperature and the excess air component can then reliably be determined by the inverse modeling and are correctly accounted for (including a possible fractionation) in the age calculation.

Tritium (³H)

The radioactive hydrogen isotope ³H (half-life 12.32 years), was released in the 1950s 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 on their own are not always very meaningful.
 

³H-³He

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 60 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 relative 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.

SUlphur hexaflouridE (SF₆)

 Sulphur hexaflouride (SF6) is an anthropogenic trace gas that has been used, among other things, to study atmospheric exchange processes, e.g. the exchange of air masses between the northern and southern hemispheres. Its high stability and the associated long service life are very advantageous properties for this purpose. Through contact with the atmosphere, SF6 also enters the hydrosphere in accordance with its solubility. Using SF6 as an aquatic marker tracer, quantitative statements on horizontal and vertical flow and mixing processes have been made in dispersion experiments in the ocean and in lakes since the end of the 1980s. SF6 has also been used to investigate gas transfer through the water surface.

Due to the constant increase in the atmospheric mixing ratio of SF6, groundwater or surface water is given time information when it is separated from the atmosphere, i.e. the SF6 concentration in closed water bodies reflects the time of the last contact with the atmosphere. For young groundwater (<50 years), SF6 measurements - together with CFCs, which behaved in a similar way at least until the 1990s (see below), and tritium - provide a widely used and suitable dating tool today. However, the accuracy of SF6 ages is limited by uncertainties, e.g. about infiltration temperature and excess air in newly formed groundwater. A combination with the 3H-3He method is therefore indicated, as the resulting 
noble gas data allow corrections of the SF6 data.

Fluorochlorocarbons (FCKW)

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 has changed into a decrease in past decades, 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 20 and 50 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.