Age Structure of Groundwaters Determined by Isotope Methods

Abstract

For the calibration of flow models, isotope methods play an important role. It is possible to deduce time information from radionuclides and therefore information about groundwater ages or flow velocities. Usually a flow system has not one single age (which is also called piston flow age) but an age distribution. In addition, groundwater may be a mixture of different components. For waste disposal investigations it is especially important to know if a small fast flowing water component is admixed. Therefore, one isotope method alone does not allow to answer all questions and a combination of isotope methods is advantageous. The results of such isotope combinations are presented. Possible isotopes are ³H, ⁸⁵Kr (in combination also with Freon measurements) for a young component; ³⁹Ar allows dating up to about 1000 years and ¹⁴C (together with δ¹³C and carbonate chemistry) reaches back to about 35‘000 years. Qualitative time indicators for very old waters are ⁴He, ⁴⁰Ar/³⁶Ar ratios, stable isotopes δ²H and δ⁸O and special chemical concentrations. In the first part of the poster the input functions of some isotope methods are given for the last decades. A combination is especially successful if the input functions have a totally different shape and therefore the isotope ratio a strong time dependence. In the second part, two to four examples of a successful combination of isotope methods are given. Main area is the Wellenberg site of the NAGRA investigations. Various combinations are demonstrated with time ranges from a few decades to palaeowaters. The conclusions illustrate the possibilities and limits of isotope combinations.