Low-sulphate water sample preparation for LSC detection of 35S avoiding sulphate precipitation. (March 2020)

• Record Type: Journal Article
• Authors: Schubert, M.; Kopitz, J.; Knöller, K.
• Is Part Of: Journal of environmental radioactivity. Volume 213(2020)
• Publication Date: 2020-03
• Subjects: 35S -- Liquid scintillation counting -- Method improvement -- Aqueous tracer -- Groundwater resources management -- Groundwater residence time; Radioactivity -- Periodicals; Radiation, Background -- Periodicals; Radioecology -- Periodicals; Radioactive pollution -- Periodicals; Environmental Pollutants -- Periodicals; Radioactive Pollutants -- Periodicals; Radioactivity -- Periodicals; Radioécologie -- Périodiques; Pollution radioactive -- Périodiques; Fond de rayonnement -- Périodiques; 539.752
• DOI: 10.1016/j.jenvrad.2019.106153
• Languages: English
• ISSNs: 0265-931X

Abstract

Abstract: Information about groundwater residence times is essential for evaluating appropriate groundwater abstraction rates and aquifer vulnerabilities and hence for sustainable groundwater management in general. Naturally occurring radionuclides are suitable tools for related investigations. While the applicability of several long-lived radionuclides for the investigation of long-term processes has been demonstrated frequently, residence times of less than one year are only scarcely discussed in the literature. That is due to the rather small number of applicable radionuclides that show adequately short half-lives. A promising approach for investigating sub-yearly residence times applies radioactive sulphur. 35 S is continuously produced in the upper atmosphere from where it is transferred with the rain to the groundwater. As soon as the water enters the subsurface its 35 S activity concentration decreases with an 87.4 day half-life. This makes 35 S suitable for investigating sub-yearly groundwater residence times. However, the low 35 S activities in natural waters require sulphate pre-concentration for 35 S detection by means of liquid scintillation counting (LSC). That is usually done by sulphate extraction from large water samples with an anion-exchange resin (Amberlite IRA400, Cl-form), elution from the resin with NaCl, and precipitation as BaSO4 . Our study aimed at optimizing the standard sample preparation procedure by avoiding the laborious precipitation step. We suggest (i) sulphate extraction using the exchange resin Amberlite IRA67 (OH-form), (ii) elution with ammonium hydroxide, (iii) evaporation of the eluate and (iv) dissolving the resulting dry precipitate in 2 ml H2 O. In contrast to the standard approach our method results in a final sample solution of low ionic strength, which allows applying the water miscible scintillation cocktail Hionic-Fluor®. Since Hionic-Fluor accepts only aqueous solutions of low ionic strength the approach is applicable for waters with high 35 S/ 32 SO4 2− ratios, i.e., low total sulphate sample loads (e.g. rainwater). Highlights: Sustainable groundwater resources management requires knowledge of groundwater ages. 35 S has great potential as tracer for detecting groundwater ages of less than one year. State-of-the-art approach for 35 S liquid scintillation counting includes BaSO4 precipitation. An approach is introduced that allows avoiding the labor-intensive BaSO4 precipitation step. The approach is applicable for low-sulphate waters (loads of up to about 100 mg).