Measuring low concentrations of Th in water and sediment

²³⁴Th/²³⁸U disequilibria have been used extensively in studies of particle dynamics and the fate and transport of particle-reactive matter in marine environments. Similar work in low salinity, estuarine, and freshwater systems has not occurred primarily because the lower concentrations of both parent and daughter nuclides that are typical of these systems often render established methods for the analysis of ²³⁴Th inadequate. The application of this radionuclide tracer technique to these systems, however, has great potential. To this end, we present a method for measuring low activities of ²³⁴Th in relatively small samples (<200 l) using low background gas-flow proportional counters, a ²²⁹Th yield monitor, and empirical corrections for the interferences from real and apparent betas that are emitted by other thorium isotopes and their progeny. For samples with low ²³⁴Th/²²⁸Th activity ratios, we improve upon current beta counting methodologies that rely on immediate sample counting, weak beta absorption, or multiple beta counts so that, using the analytical approach outlined here, it should be possible to measure ²³⁴Th activities (i) as low as 1.5 dpm/total sample, (ii) up to 2 weeks after radiochemical purification of thorium, and (iii) with only one sample count for alpha and beta activity.