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

Abstract 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 2 H, 18 O, 3 H, noble gases (He, Ne, Ar, Kr, Xe), 222 Rn and SF 6 . 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 – SF 6 and 3 H– 3 He – reveals that dating with SF 6 is not possible in the crystalline region of the Odenwald, while the 3 H– 3 He method gives robust groundwater ages. The results indicate that SF 6 is influenced by a terrigenic source in the subsurface that varies with lithology. 222 Rn data from part of the wells seem to be related to the terrigenic SF 6 , consistent with the idea of radiochemical SF 6 production in rocks and release from the aquifer matrix.