Uranium and thorium series radionuclides in rainwater over several tropical storms

The activity concentrations of uranium and thorium series radionuclides were measured in rainwater samples from 16 storms in northern Australia. For six of these storms, sequential intra-storm samples were collected. The results indicate that a substantial fraction of the measured activity concentrations of 238U, 234U, 230Th, 226Ra and, to a lesser extent, 210Po were due to below-cloud washout of dust transported from the nearby Ranger uranium mine. For the six intra-storm studies, the concentrations of these dust-related radionuclides decreased to one half or less of their initial value after the first one to two millimetres of rainfall, although in some cases they later increased when the rainfall intensity reduced. Most of the 210Pb activity appears to have been sourced from in-cloud rainout, although below-cloud washout was probably responsible for significant reductions in concentrations observed over the course of three of the six storms. 210Pb residence times based on the 210Pb-210Po couple varied significantly from storm to storm, but relatively little over the course of each of the six storms studied in detail. This implies that, apart from the initial reduction in below-cloud washout, there was no substantial change in the source air mass for the 210Pb during these storms.