Radiocarbon ages of groundwaters in South Canterbury

GNS Science (GNS) was commissioned by Environment Canterbury (ECan) to analyse and interpret radiocarbon analyses as part of the South Canterbury Deep Groundwater Investigation. This report provides the results of radiocarbon analyses of dissolved inorganic carbon in 33 groundwater samples taken from wells between Timaru and Waimate in the South Canterbury region, Samples were collected by ECan staff in 2014. An interpretation of groundwater age based on these analyses is provided. Two previous radiocarbon measurements taken in 2007 from wells in the same area are included in the interpretation. Groundwater ages derived from radiocarbon measurements range from modern (less than 150 years) to around 22,000 years. Maximum groundwater ages are well defined, but rely on the assumption that dissolved inorganic carbon sourced from oxidation of organic carbon is of modern origin. The chemistry survey of Burbery and Abraham (2016) has shown this may not necessarily be true. Therefore, an alternative method calculating minimum ages which assumes some carbon is sourced from older deposits of organic carbon within the aquifer system has also been implemented. These minimum ages have higher uncertainty, and could be refined by sampling a further selection of wells which are representative of recharge waters. (author).