NATURALLY-OCCURRING RADIONUCLIDES IN ACID SALINE GROUNDWATERS AROUND LAKE TYRRELL, VICTORIA, AUSTRALIA

Many natural radionuclides have been measured in regional groundwater, acid spring waters and brines in the vicinity of Lake Tyrrell, Victoria. The concentration of nuclides within the U and Th decay series range over several orders of magnitude. The acid groundwaters display very high Ac, Ra, Rn-222, Pb-210 and Po-210 activities in the springs on the western margin of the lake compared to those generally recorded in less saline and neutral groundwaters. This is an area of the lake margin where Ra-226 and Ra-228 have accumulated in shallow sediments. Measured and estimated distribution coefficients for Ra-226 range from 10 to 800, which combined with flow rates places the source of the Ra-226 within 5 km of the radioactive spring zone. Ac-227 showed the strongest control by pH of all the measured radionuclides and was similar to Al in this behaviour. Pb-210 and Po-210 appear to be quite mobile at high salinity when pH was < 5. U and Th concentrations were not generally very high in any combination of acid/saline waters except for elevated Th-228 activities in the spring waters. These probably relate to high concentrations of its parent Ra-228 in the waters.