Atmospheric washout of 35S during single rain events – Implications for 35S sampling schemes. (October 2021)

• Record Type: Journal Article
• Authors: Schubert, M.; Weise, S.M.; Knöller, K.
• Is Part Of: Journal of environmental radioactivity. Volume 237(2021)
• Publication Date: 2021-10
• Subjects: 35S -- Input function -- Short term variation -- Groundwater residence time -- Tracer; 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.2021.106669
• Languages: English
• ISSNs: 0265-931X

Abstract

Abstract: Cosmogenic radiosulfur ( 35 S; half-life: 87.4 days) is transferred with the rain to the groundwater (as 35 SO4 2− ) and can be used as residence time tracer for the detection of sub-yearly groundwater residence times. Due to the distinct but non-regular annual 35 S pattern in precipitation, related data evaluation requires consideration of a 35 S input function that is based on representative rain samples. While minor rain events can easily be sampled quantitatively and hence representatively, a long-lasting rain event may get documented by a sample that represents only a certain sequence of the event, thus potentially resulting in a 35 S activity concentration that might not be representative. With the aim to examine the magnitude of temporal variations of the 35 S activity concentration in rain during long-lasting rain events, we present and discuss two related exemplary 35 S time series. Furthermore, we evaluate the applicability of the parameters total sulfate and electrical conductivity (EC), both detected in rainwater as easily attainable 35 S proxies. The results of the study show (i) that the 35 S activity concentration may vary substantially during long-lasting rain events due to atmospheric migration processes and aerosol washout and (ii) that neither sulfate nor EC are suitable as 35 S proxies due to the different origin of 35 S on the one hand and sulfate/EC on the other. Hence, for the determination of a 35 S activity concentration that is representative for a long-lasting rain event 35 S analyses of an adequate number of sub-samples cannot be avoided. Highlights: Long-lasting rain events may show an intra-event change in 35 S activity concentration of the rain water. Representative 35 S sampling of such rain events requires considering these f (t) changes. Sulfate and electrical conductivity in rain are not applicable as 35 S proxies due to their different origin. 7 Be in air is not applicable as 35 S proxy due to local occurrence and short-time nature of rain events. Sampling and 35 S measurement of an adequate number of sub-samples can hence not be avoided.