Radio-sulfur (35S) as short-term water residence time tracer – Step-by-step instruction for sample preparation and LSC setup

Due to its short half-life (87 days), naturally occurring radio-sulfur (³⁵S) is applicable as aqueous environmental tracer for investigating groundwater residence times shorter than one year. Being a pure β-decaying radionuclide, ³⁵S is detected straightforwardly by means of liquid scintillation counting (LSC). The rather low ³⁵S activities in natural waters require (i) a careful sample preparation aiming at extracting ³⁵SO₄^2- from large-volume water samples (ca. 20 L) resulting in samples ready for LSC measurement and (ii) an optimal device-specific setup of the LSC to maximize the ³⁵S signal-to-noise-ratio. A few publications that discuss approaches for sample preparation and device-specific LSC setup optimization are available. This paper presents a summarizing step-by-step instruction for both optimized sample preparation and LSC setup. For practical reasons, two different sample preparation approaches are presented, one for samples with low total sulphate inventories (up to 350 mg) and one for samples with elevated total sulphate inventories (350-1500 mg). LSC setup optimization aiming at the measurement of the two resulting types of samples is described for three LSC devices, namely Quantulus GCT, TriCarb 3170 TR/SL, and Quantulus LB 1220.