Mean residence times and sources of Christchurch springs

Springs in the Christchurch region were sampled and geochemically analysed with the aim of understanding their sources and subsurface flowpaths. As well as the chemistry and water stable isotopes, a range of age tracers including tritium, chlorofluorocarbon, sulphur hexafluoride and Halon-1301 concentrations were examined. The chemical compositions and stable isotopes showed that the waters are dominated by seepage from the Waimakariri River with variable additions of infiltrated rainfall. Tritium and the gas concentrations showed that the four springs emerging on the western side of Christchurch are young (2 to 14 years), and likely sourced from the first confined aquifer (Aquifer 1). The two eastern Christchurch springs are very old (>180 and 150 years) and are likely to represent leakage from deeper aquifers.