Use of carbon-14 and tritium to investigate flow and storage of water in the Christchurch groundwater system

Combined ¹⁴C and ³H measurements on groundwater from Christchurch, New Zealand, are proving vitally important for revealing water age distributions, which are key to understanding the diverse flows in the system (including younger potentially polluting water flows from near the surface and much older pristine water flows from depth). The deep gravel aquifers 350-500m thick under Christchurch have been studied since 1970 using ¹⁴C along with ³H, ¹⁸O and chemical concentrations to characterize the residence times, sources and flowpaths of the water. Of note in this long-term study is the successful use of ¹⁴C to determine mean ages of groundwater in the age range from 5 years to 2000 years, which is made possible by the absence of carbonate in the aquifer rocks and the presence of bomb ¹⁴C in some samples (Stewart, 2012). The ¹⁴C mean ages are showing that the groundwater system has changed markedly over time because of exploitation of the system, from young ages (60-70 years) across the system in 1976 to much older ages (i.e. 400 years on the west and 1600 years on the east) in 2017. Increasing amounts of deep stored water are being tapped by the wells, especially on the east (coastal) side.

Wells on the west side of the system have moderate ¹⁴C mean ages (400-600 years) and some of the samples have ³H showing that they also contain fractions of young water. Using binary mixing models allows the proportions and mean ages of the young fractions to be estimated. The mean ages of the young fractions have become younger over time, showing that nitrate contamination is becoming more likely. On the other hand, the fraction of older water is becoming larger and therefore more able to dilute the young fraction. Wells on the east side have much older ¹⁴C ages (1600 years) and are ³H-free showing that there are no such younger contributions. The results are providing valuable information for improved understanding and better management of the resource.