Deposition of trace elements and radionuclides in the spring zone, Lake Tyrrell, Victoria, Australia

Acidic-saline groundwaters in the region of Lake Tyrrell carry unusually high concentrations of trace metals but the brines of Lake Tyrrell do not have the concentrations that would be generated by evaporation of those groundwaters. This study investigated the fate of trace metals and radionuclides in the groundwater-evaporation lake system. Six shallow cores collected across the spring zone on the western shore of Lake Tyrrell, northwest Victoria, where groundwater enters the lake, were studied. The area chosen has anomalously high surface radioactivity. Only minor deposition of many major and trace elements is apparently occurring in the sediments. Most deposition is confined to the upper 15 cm of the sediments and, for the trace elements, concentrations were quite low. X-ray diffraction identified a thin band of alunite at ∼4-8-cm depth which is found to be enriched with Cr, Sc, Ti, La, Y, Zr, Sr, Ba, P and Mn by factors up to 10 over surrounding sediments. Cu and Fe are the only metals enriched below 15 cm. ²²⁶Ra, the source of the anomalous radioactivity, is also enriched within the alunite band. In addition, auto-radiography of an Fe-oxide (with akaganéite)-jarosite layered sample identified jarosite as another mineral phase that sequesters Ra. Analyses of radionuclides such as ²¹⁰Pb and ²²⁸Ra with short (<25 yr) half-lives in the sediments suggest that the deposition of radionuclides at Lake Tyrrell has occurred within a 100-yr time span. A ¹⁴C measurement of organic mud in the center of the zone with Cu enrichment confirmed its recent origin.