A MULTI TRACER STUDY TO INVESTIGATE THE GROUNDWATER IN THE ODENWALD REGION, GERMANY

The mountaineous Odenwald region in the federal state of Hessia/Germany is one of the main local recharge areas for groundwater of the surrounding depressions, where substantial extraction for public water supply takes place. Therefore we investigate the groundwater in the Odenwald to study residence times and mixing ratios of the groundwater, define regions of groundwater recharge and understand the groundwater inflow from the Odenwald to the surrounding areas.

This study includes different stable and radioactive gas and isotope tracers such as ²H, ¹⁸O, ³H, noble gases, ²²²Rn and SF₆. Based on the stable isotope data it is possible to distinguish between groundwater from different areas of the Odenwald. This will help us to define source regions for the groundwater in the surrounding areas. Additionally the isotopic signatures show that the groundwater was formed by "annual" precipitation and not only in winter- or summertime.

Noble gases (He, Ne, Ar, Kr, Xe) can be used in principle to calculate recharge temperatures or the infiltration altitudes (above sea level) of recharge areas. Furthermore noble gases give important information to correct other gas tracers for so called "excess air" that oversaturates gases in groundwater. Comparing the results of the two independent dating methods – SF₆ and ³H-³He - we see that dating with SF₆ is not possible in the crystalline region of the Odenwald. The results indicate that SF₆ is influenced by a natural source in the subsurface that varies with lithology. ²²²Rn data from part of the wells seem to be related to the natural SF₆, consistent with the idea of radiochemical SF₆ production in rocks. Data from the ³H-³He method give robust groundwater ages in the range of some years to values higher than 40 years. Furthermore, regions where mixing of old and young groundwater occurred can be distinguished.