Moreau, M.; van der Raaij, R.W.; Morgenstern, U.; Ferry, J.; Murphy, P. 2021 Tairawhiti Gisborne groundwater dynamics and hydrochemical evolution as inferred from regional water age and chemistry data. Lower Hutt, N.Z.: GNS Science. GNS Science report 2021/44. 91 p.; doi: 10.21420/EAF2-0A70
Abstract
This study aims to holistically describe the flow sources, pathways and lag times of water through the rivers and aquifers of the Tairawhiti Gisborne catchments. It brings together geochemical data from shallow and deep groundwater sources, providing an update from the 2001 regional synthesis. 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 (MBIE) Endeavour Fund programme Te Whakaheke o Te Wai. Groundwater chemistry and age tracer data were assembled from a range of sources, namely: (1) GNS Science’s national and regional datasets (National Groundwater Monitoring Programme, North Island Hikurangi margin forearc fluids composition), (2) 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 2020 and November 2022 as part of this study. The aggregated dataset consists of 238 sites and spans the period from 1963 to 2023. The environmental tracer data (age, isotopes, temperature, gas concentration and chemistry) were combined and interpreted using graphical analysis (e.g. Piper diagram) spatial analysis and multivariate statistics. These analyses inform the characterisation of groundwater flow and hydrochemical processes in the Tairawhiti Gisborne district’s aquifers. Tairawhiti Gisborne groundwaters consist of relatively fresh groundwater held within gravels of the flats, separated from deep brines upwelling from the Hikurangi Subduction Zone (HSZ). The fresh groundwaters have significantly higher solute concentrations in relation to average concentrations of New Zealand groundwaters. At the ground surface, the brines are expressed as springs (both cold or warm) or mud volcanoes at various locations in the district. In the Poverty Bay flats, the aquifer system comprises successive gravel aquifers, with varying thickness and lateral continuity. The hydrochemical signature of this system is generally of calcium-carbonate type without strong distinctive features between aquifers, likely reflecting the same source rocks. Moderate to long residence times (six to over 100 years) are typical even at shallow depths, and reducing conditions are prevalent in older waters. The smaller East Coast alluvial aquifers also generally exhibit moderately old freshwater (10 to 35 years). The deeper brines have a distinct sodium chloride signature and significantly more positive stable isotope signatures than the fresh groundwaters. These brines are mostly disconnected hydraulically from the overlying freshwater systems. Rivers and streams are hydraulically connected to shallow Holocene gravels within the flats (both Poverty Bay and East Coast), however, estimated recharge rates are typically low (25 mm/yr for rainfall recharge, 200 mm/yr for river recharge). Long-term declining water levels occurring in the deepest Poverty Bay aquifers indicate continuing storage depletion. In the Poverty Bay flats, two opposite, long-term trends are observed at multiple locations and depths: freshening and salinisation. Freshening trends likely reflect localised river recharge, leakage from overlying aquifers or possible irrigation at unconfined shallow locations. Salinisation trends indicate limited recharge and suggest that deeper, more saline water is captured. In the Poverty Bay Te Hapara Sand aquifer (shallow and coastal), chemistry changes are rapid and significant, indicating a locally responsive aquifer. Recommendations in the light of long-term trends in groundwater quality are provided to review and consolidate the existing quality and quantity networks (auths)
