The use of Th-234/U-238 disequilibrium measurements is well establishe
d in marine systems and is potentially very valuable in determining th
e settling velocity of particles in freshwater systems. However, the m
uch lower uranium concentrations in fresh waters prevent established t
echniques from being used. An alternative approach, based on a combina
tion of Cerenkov and liquid scintillation counting and a-spectrometry
has improved the sensitivity of the technique so that freshwater sampl
es can be measured.