Sulfur 35 is a cosmogenic isotope that has a half life of 87 days. It is produced in the upper
atmosphere by the spallation of argon atoms by cosmic rays (see figure below). The short half life of
35S allows it to be used to examine the influence of recent (~1yr) precipitation.
The presence, or subsequent absense, of 35S indicates the source of at least part of the water
is recent precipitation, and there is a flowpath which delivers this water to the sampled location.
35S is a conservative tracer,
and acts as sulfate as it flows through the system.