Radio-sulphur as groundwater residence time tracer – Adapting the 35S extraction approach for water samples rich in sulphate

Cosmogenic radio-sulphur ( 35 S) is applicable as aqueous environmental tracer for investigating sub-yearly groundwater residence times. For the purpose 35 SO 4 has to be extracted from large-volume water samples (ca. 20 l) and measured by liquid scintillation counting (LSC). Publications that discuss sample preparation approaches focus on waters with low or moderate sulphate concentrations based on sulphate extraction using an ion-exchange resin. However, sulphate extraction by ion-exchange is not feasible for water samples that contain total sulphate loads of over about 1500 mg. Our paper presents an approach for the preparation of water samples with sulphate loads of up to 6000 mg based on BaSO 4 precipitation directly from the sample. The key challenge of this approach is the co-precipitation of interfering natural radionuclides ( 226 Ra, short-lived progeny of 222 Rn, 210 Pb + progeny, 32/33 P), which complicates LSC measurement of 35 S. Two options are discussed to allow either pre-precipitating the unwanted radionuclides before, or keeping them in solution during Ba 35 SO 4 precipitation. • 35 S can be used as a tracer for detecting sub-yearly groundwater ages. • Sulphate extraction using an ion-exchange resin becomes infeasible for samples with sulphate loads > ⁓1500 mg. • Sulphate loads between 1500 and 6000 mg can be extracted by BaSO 4 precipitation directly from the sample. • The co-precipitation of interfering radionuclides can be avoided by carbonate precipitation or EDTA complexation.