Age Dating by Tracers

Tracers for Groundwater Age Dating

Tritium content in precipitation in Central Europe since 1950 (Source: IAEA, International Atom Energy Agency).

Tritium (3H) is an unstable isotope of hydrogen and has a half-life of 12.4 years. On 10¹⁸ hydrogen atoms, 1 tritium atom is accounted for, the total amount of tritium on earth is about 1.8 kg. It is formed in the uppermost layers of the atmosphere by cosmic radiation. The atomic bomb tests of the 1960s brought an anthropogenic tritium input into the precipitation, which was about 1000 times higher than the natural background. Even today, this impulse is still often measurable, which is why 3H is suitable for determining the age of relatively young groundwaters. The natural atmospheric tritium background in precipitation in Central Europe was about 6 TU (1 tritium unit = 0.12 Bq 3H per liter of water or 1 x 3H per 1018 H atoms).

Rule of thumb for qualitative groundwater dating based on tritium content (modified according to Clark & Fritz 1997)

<0.5 TU water older than 1952 (pre-modern)
0.5 – ~ 3 TU mostly mixture of premodern and young (<10 a) water
3 – 10 TU mostly younger than 10-15 years or dominant share premodern
10 – 20 TU significant proportion of water from the 60s / 70s
>30 TU water from the 60s / 70s dominate or contamination
>50 TU additional contamination with tritium

By mixing models (e.g., Piston Flow, Binary Mixing, or Dispersion Model) assuming specific recharge periods, flow paths, and mixture constants, the average residence time of the various age components of a groundwater body can be more accurately calculated.

Precipitation line of 2H & 18O in the Southern Bavarian region with a diverging line of evaporation of groundwater samples from a seepage.

²H/¹H und ¹⁸O/¹⁶O are heavy stable isotopes of water that provide information on the origin and conditions of groundwater formation of different aquifers. In the fresh precipitation, the values differ due to temperature and height effects, but there is a highly significant correlation of the two isotope values (so-called precipitation line). Subsequent evaporation processes (for example in lakes or rivers) increase the isotope ratios of both elements, whereby the effect is more pronounced at 2H / 1H (so-called deuterium excess). In the two-isotope plot, groundwaters from the infiltration of lakes or rivers are therefore easily discernable. Stream bank filtration processes can be further investigated, since the two stable isotope values often have a considerable seasonal periodicity (meltwater supply or increased evaporation in the summer), which continues with an infiltration in the groundwater in a weakened form and with a time delay.

Seasonal periodicity of ²H&¹⁸O isotope values in the Danube (Passau).

With a plot of 3He/4He against Ne/He different potential helium entries in the groundwater can be distinguished. The sample (red) contains primarily mantle helium from the earth’s crust.

Helium-3 is enriched as a decay product of tritium in groundwater. From 3He enrichment an admixture of young groundwater to pre-modern groundwater can be calculated. The “clock” of additional helium formation due to tritium decay begins to run with groundwater recharge.

By considering neon (Ne/He ratio), potential artifacts must be checked such as outgassing of mantle helium or supersaturation effects. Due to the respective known ratios of light and heavy helium isotopes or neon concentration, the processes can be well distinguished in a specific plot. Newly formed groundwater has known solution equilibria of 3He/4He as well as Ne/He between atmosphere and precipitation. The conditions may change due to gas excess effects in unsaturated pores (excess air), entries of mantle helium or the breakdown of tritium to tritiogenic 3He. In the case of a significant decay of tritium to 3He, the measured values in the adjacent figure would be above the dashed line in the adjacent figure.

Concentration of CFC and SF6 in the air of Central Europe since 1950.

CFC (Fluorochlorinated Hydrocarbons) and SF6 (Sulfur Hexafluoride) are largely inert chemicals that have been used industrially as refrigerants, propellants, and protection and fillers since the 1960s. Their atmospheric content and the resulting input into groundwater increased exponentially until the 1990s (CFCs) and even today (SF6). In groundwater dating, the various input functions are taken into account and compared with different flow models.

¹⁴C-Content of the atmosphere over the past 30,000 years.

¹⁴C and ¹³C get into the groundwater through precipitation and dissolution processes. The carbon-14 content of the atmosphere of 1950 (100% modern) was set as the current standard. However, this value was around 140% between 20,000 and 30,000 years ago (Clark & Fritz 1997). When interpreting the 14C values in groundwater, carbonate exchange processes must be considered when calculating the age of water. For these exchange processes, various approaches are available for correcting the 14C concentration in the groundwater during recharge (start value). The simplest model is a statistical correction value (q = 0.7). With a chemical mixture model (Vogel 1970), the starting value of 14C is calculated to be 70% modern.

Increase in krypton-85 in the atmosphere since the late 1950s.

Krypton-85 is an unstable noble gas isotope and has a half-life of 10.76 years. It is suitable for determining the age of relatively young groundwaters. Unlike tritium, krypton-85 has been increasing in atmospheric activity since 1955 due to anthropogenic input from nuclear facilities. The noble gas 85Kr is particularly suitable for age dating of groundwater, as it does not undergo chemical reactions and only minimally participates in exchange and sorption processes.

If one determines tritium and krypton-85 of the groundwater, the mixing ratio and age of different groundwater components can be derived by a dual-isotope plot (see adjacent figure).

The measuring point GW has a tritium content of 3TU and a krypton-85 content of 15 dpm/ml. In the dual-isotope plot, a proportion of 35% to 40% young water with a mean residence time of 14 to 18 years is determined.

Further tracers that can be used to identify young groundwater are gadolinium (X-ray contrast media), glyphosate (pesticides), triazoles or acesulfame (sweetener). The latter are detectable at a relatively high concentration in municipal wastewater and are hardly removed when it is being cleaned (Storck et al., 2012). By bank filtration of the receiving water they can reach the groundwater. Wastewater, which seeps directly into the subsoil, also enriches the sweeteners in the groundwater. If groundwater contains saccharin or cyclamate, this is an indication of untreated wastewater, which may have leaked into the ground via leaky sewers.

We are happy to advise you about the application possibilities of these tracers.

Tracers for Groundwater Age Dating

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