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Further information

Age determination of groundwater

There are several isotopic methods and analyses of trace gases available to determine the age of groundwater. A competent estimation of age requires the inclusion of the frame conditions like location and environment of the water resource, hydrogeological/geological cross sections, hydraulic characteristics, hydrochemical composition, supraregional studies, and former investigations of the recharge area.

Fig.1: Dating range of several isotopes.

Depending on the age composition of the groundwater, environmental isotopes cover a wide range of time and scale from days to several millennia.

Very young groundwater recharge in the range of weeks, months or even a few years can be analysed over the compilation of time series of the stable isotopes oxygen-18 and deuterium.

 

TaDa Abb.2: Harp diagram of 3H- and 85Kr-contents for the graphical determination of the young water fraction and mean residence time.

For the determination of the mean residence time of young groundwater recharge within the last 55 years the radioactive isotopes tritium (3H) and krypton-85 (85Kr) are very useful. By simultaneous determination, 3H and 85Kr provide a possibility to quantify the age and fraction of the groundwater younger than 55 years.

 

 

 

 

Fig.3: Time series of 3H- and 85Kr-contents in precipitaion (example of a weather station in the south of germany).

The radioactive environmental isotopes (3H, 85Kr, 14C, 39Ar) mark surface waters on a global scale. Anthropogen activities are the main source for radioactive environmental isotopes besides cosmic radiation in the upper atmosphere. The greater part of 3H originates from nuclear weapon tests whereas 85Kr is discharged from nuclear facilities. The input into the groundwater happens directly via precipitation (3H) or via dissolved soil gas in percolating water (85Kr, 39Ar, 14CO2).

Due to longterm measurements at varous precipitation stations the input of the environmental isotopes via the precipitation is well known.
However, major local differences exist.

Fig.4: 14C-DIC- in dependence of 39Ar-contents for an age estimation and assessment of mixed systems.

Radioactive environmental isotopes can also be used to estimate the age of old groundwaters in the range of hundreds to thousands of years. Primarily, the determination by carbon-14 (14C-DIC) in combination with carbon-13 (δ13C-DIC) has to be mentioned. By a simultaneous determination of 39Ar and 14C a contribution of mixtures can be recognised and quantified.

 

 

 

For the groundwater age range from thousand to ten thousand years isotopes and gas parameters can be used as indirect analysis method indicating the climatic conditions of the recharge. This can be achieved by the determination of the stable isotopes oxygen-18 and deuterium or by measuring the noble gas temperature. This way groundwater recharge during cold periods (Pleistocene) can be differentiated from recharge in warm periods (e.g. Holocene).

In case of groundwaters of high ages the determination of the gas isotopes 3He/4He and 36Ar/40Ar are applied as indirect analytical methods.

Very high groundwater ages can be estimated by the activities of the radioactive isotopes chlorine-36 and krypton-81.

 


Characterisation of groundwater systems of different ages in view of a more valuable usage (mineral water, medicinal water, etc.)

Tab. 1: Characterisation of groundwater systems.

Groundwaters of different ages require a sophisticated handling in terms of their usage. The main characteristics of "old" groundwaters, mixed systems, and "young" groundwaters with regards to costs, efforts, protection, stability, and water abstractionis are summarized in Tab.1.

 

 

 

 


Investigation of evolution, recharge, and flow dynamics of deep groundwaters, mineral and medicinal waters

Fig. 5: Profile sketch of various influence factors on deep groundwaters.

Besides the groundwater age estimation several isotope methods are available to investigate evolution, recharge, and flow dynamics of groundwaters. All these methods use the isotope signature of dissolved compounds:

 

 

 

 

Fig. 6: Sulphate isotopes with range of values of several aquifer rocks and transformation processes.

Isotope signatures of species of sulphur (sulphate, sulphide) provide information about the origin like evaporite solution, pyrite oxidation, fallout sulphur, etc. as well as secondary processes like sulphate reduction.

  • Sulphur-34 und oxygen-18 of sulphate (δ34S-SO4 and δ18O-SO4)
  • Sulphur-34 of sulphide (δ34S-H2S)

 

 

Fig. 7: Nitrate isotopes with range of values of various origin and transformation processes.

Isotope signatures of species of nitrogen (nitrate, nitrite, ammonium, gaseous nitrogen) provide information about the origin such as organic or mineral fertiliser, geogenic origin, etc. as well as secondary processes like nitrate reduction.

  • Nitrogen-15 und oxygen-18 (δ15N-NO3 and δ18O-NO3)
  • Nitrogen-15 of ammonium (δ15N-NH4)
  • Nitrogen-15 of gaseous nitrogen (δ15N-N2)

 

Fig. 8: Strontium isotope ratios in sea water in the course of earth's history.

Isotope signatures of dissolved strontium provide information on the migration ways of groundwaters in different lithologies or over the fallout of nuclear accidents (e.g. Tschernobyl) respectively.

  • Strontium isotope ratio (87Sr/86Sr)
  • Strontium-90 (90Sr)

 

 

 

 
Fig. 9: δ13C-range of values of several natural components (Clark & Fritz, 1997).

Isotope signatures of anorganic and organic carbon components provide information about the origin such as volcanic gases, dissolved carbonates, organic materials, etc. and transformation processes like thermocatalytic, microbial, etc.

  • Carbon-13 of DIC (δ13C-DIC)
  • Carbon-13 and oxygen-18 of carbondioxid (δ13C-CO2 and δ18O-CO2)
  • Carbon-13 of DOC (δ13C-DOC)
  • Carbon-13 and deuterium of gaseous hydrocarbons (δ13C-CH4 and δ2H-CH4 as well as δ13C-C2-C4 and δ2H-C2-C4)
  • Carbon-13 and deuterium of organic contaminations like HCH, BTEX, PH

 

Isotope signatures of radioactive daughter products of the decay series of uranium-238, uranium-235, and thorium-232 provide information not only on the radioactive contamination of groundwaters but also on the origin, lithology, and changes in the flow dynamic.

  • Activity concentrations of the radium isotopes (223Ra, 224Ra, 226Ra and 228Ra)
  • Activity concentration of radon (222Rn)
  • Activity concentrations of uranium isotopes (234U, 235U, 238U )
  • Activity concentrations of radon daughter products (210Pb, 210Po)

 

Further isotope signatures of dissolved compounds like chloride, boron, lead, lithium, etc. are used for specific issues of geogenic origin or anthropogenic contamination (munition, mining, landfills, sewer, etc.).

  • Chlorine-35 (δ35Cl)
  • Boron-11 (δ11B)
  • Lithium-6 (δ6Li)
  • Lead isotopes (206Pb, 207Pb, 208Pb)
  • Iron
  • Calcium
  • Chromium

 

The input of radioactive parameters from nuclear accidents and medical wastewater provide information not only on radioactive contamination but also on the age of soil samples.

  • Caesium isotopes (137Cs and 134Cs)
  • Iodine isotope (129I and 131I)
  • Americium  

 


Stable isotopes oxygen-18 und deuterium

The study of the isotope signatures of δ18O and δ2H in groundwater can help answering several questions:

  • Identification of the origin of waters
  • Dating of young groundwaters
  • Identification of glacial recharged groundwaters
  • Infiltration of river water
  • Infiltration of lake water
  • Deuterium as artificial tracer for ground and pore water tracer tests as well as for biological applications
Fig.10: Hydrological cycle with examples of δ18O-contents.

The hydrological usage of the content analyses of the stable isotopes δ2H- and δ18O- in water molecules is essentially based on the occurence of different concentrations in natural waters. Those differences  are the result of several physical processes. Primarily they trace back to the evaporation depending on temperature. The various isotope effects cause a local and temporal characteristic marking of the precipitation and, consequently, also a local and temporal characteristic marking of the different water bodies of the hydrological cycle.
 

Fig. 11: Example illustration of stable isotopes of water data with the global meteoric water line.

Studying the stable isotopes should be included in every extended survey on groundwater age and origin. Those parameters are very suitable for all kind of monitoring programs to observe changes of the  inflow.

The graph illustrates different groundwaters in a fictional region:

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