TRANSPORT OF NATURALLY-OCCURRING RADIONUCLIDES BY A SEASONAL TROPICAL RIVER, NORTHERN AUSTRALIA

Natural radionuclide transport is used as an analogue to determine the likely fate of uranium mine effluent in a seasonal tropical river in northern Australia. Concentrations of alpha activity determined at regular intervals in both filtered and unfiltered water are presented from four sites along the sand-bed channel of the creek, and one site at the outfall of the 200 km2 flood plain. Annual mean radium isotope concentrations were also measured. It was found that transport by the creek was conservative, in that concentrations and Ra-226/Ra-228 ratios remained constant. About 50% of the creek load was associated with particulates. No particulate concentrations were detectable at the flood plain outfall. Alpha activity loads were calculated for the creek input and flood plain outfall. It is concluded that all the creek particulates were trapped by the flood plain, probably at the upstream end. A substantial fraction of the solute/colloidal load was also retained. A 50% decrease in the Ra-226/Ra-228 ratio between the input and outfall supports this conclusion. Systematic errors arising from sampling are considered by examining the Ra-226 concentrations in detail through a flood at one creek site, and it is concluded that such errors are negligible. Previous modelling had assumed that only 20% of released activity would be retained on the flood plain. It is now recommended that complete retention should be assumed, with deposition occurring preferentially at the upstream end of the flood plain. This will have the effect of increasing the predicted radiation dose to members of the public by more than a factor of five.