Application of 35-S as Aquatic Tracer for Determination of Sub-yearly Groundwater Residence Times

Michael Schubert1, Kay Knöller1, Jürgen Kopitz2
1 Dpt. Catchment Hydrology, Helmholtz Centre for Environmental Research GmbH - UFZ
2 Institute of Pathology, Heidelberg University

11.2 in Isotopenmethoden zur Analyse von Stoffumsatz und Fließprozessen

28.03.2020, 09:15-09:30, Weißer Saal

The knowledge of groundwater residence times is mandatory for (i) recommending groundwater abstraction rates that allow sustainable aquifer management, (ii) assessing groundwater travel times and related matter (and contaminant) transport, and (iii) evaluating the aquifer vulnerability regarding anthropogenic contamination. A suitable tool for the investigation of groundwater residence times is the application of environmental tracers.

Naturally occurring radioisotopes are powerful in this regard. Ideally, their half-lives should be in the same time range as the investigated physical and/or chemical processes. Rather long-lived radionuclides, such as 3H, 14C, 36Cl, 39Ar, 81Kr, and 85Kr, have proven suitable for investigating long term processes in many studies. On the other hand, radiotracers that are applicable for studying shorter groundwater residence times, i.e. time spans of less than one year, are scarcely discussed in the literature.

A novel approach for covering this timescale applies radioactive Sulphur (35S), which is continually produced in the upper atmosphere by cosmic ray spallation of 40Ar. After its production 35S rapidly oxidizes to sulphate, gets dissolved in the meteoric water and is finally transferred with the rain to the groundwater. 35SO42- activities in precipitation range generally between ca. 5 and 100 mBq/l. Since there are no natural sources of 35S in the subsurface the 35S activity concentration in the groundwater starts to decrease by decay with an 87.4 day half-life as soon as the rainwater enters the ground. This makes 35S a tool for investigating sub-yearly groundwater residence times.

35S measurement in aqueous samples is done by means of liquid scintillation counting (LSC). However, the low natural activities require a pre-concentration of 35SO42- from large water samples (e.g., 20 litres). An applicable method entails the precipitation of fine-grained 35S-containing BaSO4 and its homogeneous suspension in a gel-forming scintillation cocktail. The approach was developed (Urióstegui et al., 2015) and improved (Schubert et al., 2019a) and is applicable for the analysis of water samples that contain rather high sulphate loads (up to 1500 mg) with rather low 35S/32SO42- ratios as they are typical for groundwater.

A challenge regarding the application of 35S as residence time tracer is the highly variable 35S activity in rainwater even on short timescales. Since all 35S based groundwater modelling necessitates the setup of a reliable 35S input function, the recording of extended time series of 35S in rainwater is required. The presented study aimed at optimizing water sample preparation and measurement by either improving or even avoiding the labour-intensive BaSO4 precipitation step (Schubert et al., 2019b). The key findings of the study, which do now allow an optimum 35S sample processing and detection of both groundwater and rainwater samples, will be presented in the talk.

Urióstegui, S.H., Bibby, R.K., Esser, B.K., Clark, J.F. (2015): Analytical Method for Measuring Cosmogenic 35S in Natural Waters. Anal. Chem., 87: 6064-6070.

Schubert M., Kopitz J., Knöller K. (2019a): Improved approach for LSC detection of 35S aiming at its application as tracer for short groundwater residence times. J Environ Radioact, 208-209: 106022,

Schubert M., Kopitz J., Knöller K. (2019b): LSC detection of 35S in low-sulfate water samples avoiding BaSO4 precipitation. J Environ Radioact, subm.

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