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 ³⁵SO₄^2-) 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 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.