Radium isotopes in saline seepages, south-western Yilgarn, Western Australia

Water samples from saline seepages in the south-western Yilgarn Block of Western Australia contain high activities of the four naturally-occurring radium isotopes. Activities of up to 310 pCi/l for 226Ra and 1720 pCi/l for 228Ra were measured and the 228Ra/226Ra ratio averaged 6.1. Activities of the two short-lived radium isotopes were also high. 223Ra activities of up to 94 pCi/l were found with an average 226Ra/ 223Ra ratio of 3.3, considerably lower than the natural abundance ratio of 21.4. Activities of up to 23 pCi/l 227Ac, the long-lived grandparent of 223Ra, were also measured. The analysis of surface granite samples, the probable source rocks of the radium, gave Th/U activity ratios of around 1.5. The higher 228Ra/226Ra ratios of the waters were attributed to readily leached 228Ra in the weathered granites as a result of thorium remaining after weathering. Leach experiments on U-Th ore by NaCl solutions showed that all four radium isotopes were equally leached. Sulphate anions reduced the 226Ra and 228Ra leaching to a greater extent than for 223Ra and 224Ra, suggesting that the latter isotopes were being supported in solution by parent isotopes. (author)