Age-dating of New Zealand groundwaters using sulphur hexafluoride

Abstract

An analytical system for measuring dissolved sulphur hexafluoride (SF6) concentrations, at the extremely low background levels found in young groundwaters, has been established at the Water-dating Laboratory of the Institute of Geological and Nuclear Sciences, New Zealand. This system has been used to analyse 136 environmental samples from both surface and ground waters. Duplicate analyses have demonstrated the system to be precise, while analysis of modern surface waters has demonstrated the accuracy of the system. SF6 analyses were undertaken on 96 New Zealand groundwater samples taken from a range of hydrogeological settings. Measured SF6 concentrations ranged from 0.01 to 15.0 fmolL-1, with a mean concentration for all samples of 1.54 fmolL-1. Conversion of measured SF6 concentrations into equivalent atmospheric concentrations using estimated recharge temperatures, allowed the measured concentrations to be compared to historical atmospheric concentrations and the estimation of groundwater age using the exponential-piston flow model (EPM). These ages were compared to groundwater ages derived from other well-established age-dating tracers (tritium and chlorofluorocarbons). The results from four different hydrogeological environments were examined in detail. SF6 groundwaters ages observed in the confined alluvial aquifers of the Lower Hutt Groundwater Zone increased uniformly with distance in the confined aquifers, and displayed a reasonable agreement with tritium groundwater ages. Chlorofluorocarbons (CFCs) in this groundwater system appear degraded. SF6 groundwater ages from shallow (<10m) bores in unconfined alluvial aquifers in Southland were much younger than the corresponding CFC groundwater ages. This difference was attributed to excess air. A correction was applied, based on fitting both SF6 and CFC concentrations to the EPM, which resulted in groundwater ages ranging between 15 and 20 years for most of these bores. For the rhyolitic aquifers of the Lake Taupo catchment, there was good agreement between SF6 and tritium groundwater ages for modern groundwaters from shallow bores, while there was some evidence of CFC degradation. Some groundwater from deeper bores appeared to have SF6 concentrations resulting from naturally produced SF6. In the highly variable alluvial aquifers of the Waimakariri-Ashley Plains in Canterbury there was generally poor agreement between all tracer results, reflecting a range of different processes acting upon the tracers. In addition to groundwater age information, the comparison of SF6 to CFCs and tritium enabled the physical processes occurring in the aquifer systems to be more fully understood. Many SF6 results appeared affected by excess air, leading to the calculation of SF6 groundwater ages which were too young. Thus the need for the ability to accurately determine the amount of excess air in groundwater. CFC degradation appeared to occur in several groundwater systems. In these systems, SF6 may have additional benefits over CFCs.