Atmospheric washout of 35S during single rain events – Implications for 35S sampling schemes

Cosmogenic radiosulfur (35S; half-life: 87.4 days) is transferred with the rain to the groundwater (as 35SO42−) 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 35S pattern in precipitation, related data evaluation requires consideration of a35S 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 a35S activity concentration that might not be representative. With the aim to examine the magnitude of temporal variations of the 35S activity concentration in rain during long-lasting rain events, we present and discuss two related exemplary 35S time series. Furthermore, we evaluate the applicability of the parameters total sulfate and electrical conductivity (EC), both detected in rainwater as easily attainable 35S proxies. The results of the study show (i) that the 35S 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 35S proxies due to the different origin of 35S on the one hand and sulfate/EC on the other. Hence, for the determination of a35S activity concentration that is representative for a long-lasting rain event 35S analyses of an adequate number of sub-samples cannot be avoided. •Long-lasting rain events may show an intra-event change in 35S activity concentration of the rain water.•Representative 35S sampling of such rain events requires considering these f(t) changes.•Sulfate and electrical conductivity in rain are not applicable as 35S proxies due to their different origin.•7Be in air is not applicable as 35S proxy due to local occurrence and short-time nature of rain events.•Sampling and 35S measurement of an adequate number of sub-samples can hence not be avoided.