Moreau, M.; Coble, M.A.; Clark, G.M.; Morgenstern, U.; Houghton, K.M.; Levy A.; Anderson, M.; Ettema, M. 2024 Groundwater dynamics, source and hydrochemical processes inferred from existing Otago Regional age and chemistry data. Lower Hutt, N.Z.: GNS Science. GNS Science report 2023/54. 67 p.; doi: 10.21420/FAGX-H847
Abstract
This study aims to holistically describe the flow sources, pathways and lag times of water through the rivers and aquifers of the Otago region catchments. It brings together geochemical data from shallow and deep groundwater sources, providing an update to the last regional synthesis undertaken in 2001. This information is required to ground truth and improve groundwater flow models and management tools. This study was funded through the Ministry of Business, Innovation and Employment’s (MBIE) Endeavour programme, Te Whakaheke o Te Wai, the Strategic Science Investment Fund National Groundwater Monitoring Programme and an Envirolink Medium Advice Grant. Groundwater chemistry and age tracer data were assembled from a range of sources:(1) data from the National Groundwater Monitoring Programme, hosted by GNS Science, (2) Otago Regional Council’s (ORC) regional datasets (State of the Environment monitoring and specific site investigations), (3) existing data from the Water Dating laboratory and (4) water samples collected in March 2023 as part of this study. The aggregated dataset consists of 281 sites and spans the period 1985–2024. The environmental tracer data (age tracers, stable isotopes, temperature, gas concentration and chemistry) were combined and interpreted using graphical analysis (e.g. Piper diagrams), spatial analysis and multivariate statistics. These analyses inform the characterisation of groundwater flow and hydrochemical processes for Otago’s aquifers. Otago’s groundwater resources consist of distinct aquifer systems comprising Quaternary sediments, Tertiary sediments, volcanics and pre-Cretaceous fractured hard rock. Groundwater is generally of low total dissolved content and mostly of calcium-carbonate water type with strong distinctive features between aquifers, likely to reflect lithological differences. Short to long residence times (<10 to over 100 years) are typical even at shallow depths, and reducing conditions are prevalent in older waters. Longer residence times lead to increased water-rock interaction and, therefore, higher total dissolved content. Nitrate concentration hotspots in groundwater occur throughout the region and are often associated with high-intensity land use and septic tanks. Some of the highest concentrations occur in older groundwater (>30 years). Groundwater quality long-term trends were detected in multiple aquifer systems (e.g. freshening in Wakatipu; increasing nitrate concentrations in Oamaru). Rivers and streams are hydraulically connected to shallow, unconfined groundwater systems in the region, however, strong local spatial controls affect this connectivity. This is reflected in the wide range of estimated recharge rates from low (0.5 m/yr for rainfall recharge to 1 m/yr for river/lake-recharge). Groundwater recharge rates or storage could not be assessed for the deeper, confined aquifers due to insufficient data. (auths)
