Factors controlling terrigenic SF6 in young groundwater of the Odenwald region (Germany)

• SF6 and 3H–3He dating were used to investigate groundwater of the Odenwald/Germany.
• The application of SF6 dating is not possible in certain areas due to high SF6 excess.
• High SF6 is clearly linked to the lithology of the sampled area.
• The SF6 excess is of terrigenic origin, radio-chemically produced in the subsurface.

The mountainous Odenwald region in the federal state of Hesse/Germany is one of the main recharge areas for groundwater of the surrounding depressions, where substantial extraction for public water supply takes place. We investigated the groundwater to study residence times and mixing ratios of groundwater of different ages, define regions of groundwater recharge and understand the groundwater inflow from the Odenwald to the surrounding areas. This multi-tracer study included stable and radioactive gas and isotope tracers such as 2H, 18O, 3H, noble gases (He, Ne, Ar, Kr, Xe), 222Rn and SF6. Noble gases were used to calculate recharge temperatures of the groundwater and to correct all gas tracers for so called “excess air”. Comparing the results of the two independent dating methods – SF6 and 3H–3He – reveals that dating with SF6 is not possible in the crystalline region of the Odenwald, while the 3H–3He method gives robust groundwater ages. The results indicate that SF6 is influenced by a terrigenic source in the subsurface that varies with lithology. 222Rn data from part of the wells seem to be related to the terrigenic SF6, consistent with the idea of radiochemical SF6 production in rocks and release from the aquifer matrix.