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

Cosmogenic radio-sulphur (³⁵S) is applicable as aqueous environmental tracer for investigating sub-yearly groundwater residence times. For the purpose ³⁵SO₄ 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 BaSO₄ precipitation directly from the sample. The key challenge of this approach is the co-precipitation of interfering natural radionuclides (²²⁶Ra, short-lived progeny of ²²²Rn, ²¹⁰Pb + progeny, ^32/33P), which complicates LSC measurement of ³⁵S. Two options are discussed to allow either pre-precipitating the unwanted radionuclides before, or keeping them in solution during Ba³⁵SO₄ precipitation.