Interpretation of the 2017 groundwater age tracer and isotope results for the Christchurch aquifer system

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Stewart MK, van der Raaij RW. 2019. Interpretation of the 2017 groundwater age tracer and isotope results for the Christchurch aquifer system. Lower Hutt (NZ): GNS Science. 41 p. (GNS Science report; 2019/65). doi:10.21420/Y32E-BC76.

Abstract:
This report interprets the results from groundwater age tracer and isotope measurements in the Christchurch aquifer system in 2017. The study included carbon-14, tritium and trace gases measured at GNS, stable isotopes (oxygen-18 and deuterium), and water chemistry measured at the same time (results given in Appendices 1 and 2). The interpreted mean ages and water sources are compared with results from measurements from each decade between 1976 and 2006 to see how the system is responding to exploitation. The goal is to improve understanding of the hydrogeological processes in the Christchurch aquifer system and allow for better management of the resource. It is hoped that the work will lead to improved conceptual models and support better digital modelling of the system. The 2017 survey showed that the 14C ages of deep water in the Christchurch aquifer system are continuing to become older as groundwater abstraction continues (in particular, the 2017 ages are older than the 2006 ages). In addition, the steep west-east gradient in ages continues to be observed (e.g. from 400 years to 1450 years in the central transect). The system is clearly accessing deep storage, which is much older on the eastern than on the western side. However, in addition to the increasing ages of deep water, there are indications of inflow of increasingly younger water on the western and northern sides of Christchurch. This has implications for long-term risks to the quality of the system, such as risk of microbial contamination, because such contamination would be carried by very young water penetrating from the surface. Shallow layers are more vulnerable than deeper levels, so drinking water should be drawn from deeper levels. Monitoring ages and groundwater sources with tritium and gas age tracers and chemical measurements is recommended. Risk of saline intrusion on the coastal side of the system to deeper aquifers is considered small because artesian pressures recover each winter after summer drawdown, despite continued groundwater abstraction. However, shallow levels of the system are more vulnerable due to possible drawdown by abstraction and sea-level rise due to climate change. Breakthrough of conservative contaminants (such as nitrate) brought in by young water inflow to the deep system may be a possibility in time and will be worth monitoring with nitrate and 14C measurements. The two deep wells near the Waimakariri River gave very young ages, but the inferences to be drawn from this are unclear at present. It is recommended that more emphasis should be given to 14C measurements in the future to monitor the functioning of the deep system. This is because 14C can give the real mean ages of the water, and therefore provide fundamental information on the deep system flow paths and storages. (auth)