Isotope investigation for understanding groundwater flow regime

 

Environmental isotope investigation is a tool for understanding the regional groundwater flow and confirms or refutes inferred relations between the aquifers. Isotope investigation can give information about groundwater sources, ages, travel times and flow paths.

The following text is based on the paper “Arid-Zone Hydrology: Investigation with isotope techniques” written by H. Hötzl [Hötzl, 1980]. 

 

 

Isotope background

 

The isotopes can be defined as atoms of the same element, differing one from each other by the number of neutrons [Mazor, 1991]. Each element has a constant number of protons, but they can have several types of isotopes depending on the neutron number.

For example; Hydrogen has three isotopes:

H – Common hydrogen, 1 proton

D – Deuterium, heavy stable hydrogen, 1 proton + 1 neutron

T – Tritium, radioactive hydrogen, 1 proton + 2 neutrons

And Oxygen has also three isotopes:

16O – common oxygen, 8 protons + 8 neutrons

17O – heavy (very rare) oxygen, 8 protons + 9 neutrons

18O – heavy oxygen, 8 protons + 10 neutrons

Isotopes can be classified in stable and radioactive (unstable).

 

 

Stable groundwater isotopes

 

Stable isotopes are from natural origin, do not appear to decay to other isotopes in geologic time and may be produced by the decay of other isotopes. Stable isotopes analyzed in this research are Deuterium and 18O.

The stable isotopic composition of a low mass element is usually expressed as “delta” (δ) deviations of a standard composition.

δIsotope (in 0/00) = (Rsample/Rstandard – 1).1000

Where: “R” is the ratio of the heavy to light isotope.

The isotopic composition of water in this research is compared with the Standard Mean Ocean Water - SMOW. This composition is expressed by per mil (0/00) deviation from the SMOW standard.

For the case of Deuterium:

δD (in 0/00) = ((D/H)sample / (D/H)SMOW - 1) x 1000

 

 

Isotopic fractionation during evaporation

 

The physical and chemical properties of different isotopes are slightly different because of their mass differences. Light water particles evaporate more efficiently than the heavy ones, as a result, an isotopic fractionation occurs. The air takes the light water molecules with relatively negative δD and δ18O values, while the residual part is richer in heavy isotopes. (More positive δD and δ18O)

The values of δD and δ18O are progressively enriched as the evaporation takes place and develop a unique isotopic composition. When plotted, these values have a linear trend called “evaporation line” and its slope is determined principally by the air temperature and air humidity.

 

 

Global Meteoric Water Line (GMWL)

 

When the isotopic compositions of precipitations of all over the world are plotted in δ2H versus δ18O, they can be described by the equation [Craig, 1961]:

δ2H = 8. δ18O + 10 (in 0/00)

The slope of 8 is due to equilibrium conditions during condensation of precipitation in clouds at 25 - 30 °C. The value of 10 0/00 at the intersection with the Y axis is called Deuterium excess value. This Deuterium excess value applies only to the data “fitted” to a slope of 8; typical d-excess values are in the range from 0 to 20 [Kendall, 1998] and are indicators of moisture conditions in the source region.

 

 

Unstable Groundwater Isotopes

 

The Radioactive (Unstable) isotopes are atoms isotopes that spontaneously decay over time to form other isotopes emitting alpha or beta and even gamma particles [Kendall, 1998].

 

 

Tritium

 

Tritium is a radioactive isotope produced naturally by the interaction of cosmic rays with the atmosphere and artificially by the release of fusion bombs mainly in the stratosphere [Salomon, 2005]. The values of tritium are expressed in absolute concentrations called Tritium Units, TU, where a TU unit corresponds to 1 tritium atom per 10e-18 hydrogen atoms.

Tritium was mainly produced by testing of nuclear weapons in the mid-1960´s. Depending on the TU units observed in water samples, the water can be qualitatively dated between the pre-bomb test period (before 1960), post-bomb test period (from 1960 to 2005) or a mixture of both.

 

 

14C

 

Carbon-14 is a radioactive isotope produced in the atmosphere by cosmic ray neutron interaction with 14N. Carbon-14 has a half life of 5 730 years and decays to stable 14N by the emission of a beta particle.

Carbon-14 may be used to date groundwater for ages up to 60 000 years, although this range can be shorter due to poor preservation and contamination of the old material.

 

 

References

 

Craig, H. -  Isotopic variations in meteoric waters - Science, 133, 1702-1703, 1961

Hötzl,, H. - Arid-Zone Hydrogeology: Investigations with Isotope Techniques – International Atomic Energy Agency, Vienna, 1980

Mazor, Emanuel -Applied chemical and isotopic groundwater hydrology - Open Univ. Press, 1991

 

 

 

Posted on October 3, 2013 and filed under Hydrogeology.