Natural 35S as a watershed-scale tracer

The authors interest was in exploring the potential use of 35S as a tracer in watershed-oriented research. Its use as a watershed-scale SO42- tracer requires (1) measurements of the 0.167 MeV 35S beta in the presence of large amounts of stable S, (2) low-background liquid scintillation counting techniques and (3) decontamination from other beta-emitting isotopes. Their progress in resolving these problems will be discussed along with the current measurement limitations. Gram quantities of SO42- have been incorporated into a stable counting matrix. Liquid scintillation counting efficiencies have been improved over past methods. Minimum detectable activities of ≤ 1 pCi have been achieved using a specially-designed detection system which uses an annular NaI guard detector to reduce background events in the liquid scintillator