Rainfall isotope (3H, δ2H and δ18O) input to groundwater in Australia

Abstract

The stable isotopes of water, δ2H and δ18O, are conservative tracers available for studying mixing of water in the hydrosphere. Radioactive tritium (2H, half-life = 12.3 years), derived from both cosmogenic and anthropogenic sources (nuclear testing), is an important tracer for dating of young groundwater. Measurements of stable water isotopes and tritium in Australian rainfall have been made monthly at six coastal sites and Alice Springs since 1962 as part of the Global Network of Isotopes in Precipitation (GNIP). Since 2006 this network has been expanded to include seven inland sites in New South Wales, Queensland, South Australia and Western Australia (δ2H and δ18O analysed only). In addition, event-based studies of stable water isotopes have been conducted at four locations in the Sydney region since 2005. These data have been analysed to determine local meteoric water lines, weighted averages and to investigate the relationships between rainfall isotopic composition, temperature and precipitation amount. Stable water isotopes are not completely conservative as they undergo fractionation as a result of hydrological processes such as evaporation, precipitation, ice and snow formation and melting, and geothermal activity. The fractionation can be used to understand the provenance and history of groundwater and to define end members for mixing studies. For age dating of groundwater using tritium the rainfall tritium composition is required. In addition to the 50-year tritium record available from GNIP for six sites, data for an additional eleven locations throughout eastern Australia were compiled for varying periods mainly between 1970 and 1991, thereby improving the spatial resolution of the tritium time series in Australia. Unlike δ2H and δ18O, the spatial distribution and seasonal variation of tritium in rainfall is largely controlled by the stratosphere to troposphere exchange of anthropogenic tritium from nuclear testing, with the highest concentrations occurring at Adelaide and Melbourne during the early spring. Modern concentrations appear to be stabilising with average annual concentrations in the range 1–3 TU increasing with latitude. These data have also been used to estimate the tritium composition of rainfall resulting in the January 1974 Queensland floods, which are believed to have resulted in significant recharge to aquifers in Queensland and northern NSW. © Geological Society of Australia In