- 1GNS Science, Lower Hutt, New Zealand (m.stewart@gns.cri.nz)
- 2GNS Science, Taupo, New Zealand
- 3GNS Science, Lower Hutt, New Zealand
- 4Tasman District Council, Richmond, New Zealand
The objective of this work is to understand the mechanism of seawater intrusion and nature of water components supplying the slightly brackish Te Waikoropupū Springs (TWS) in New Zealand (Stewart and Thomas, 2008; Williams, 2023). Seawater intrusion in karst has special features due to flow being in conduit networks below sea level, very different from seawater inflow to porous aquifers (Fleury et al., 2007). Three mechanisms of seawater intrusion in karst have been identified (venturi suction effect, head balance process, and freshwater dilution of a near-constant brackish water flow, Arfib & Charlier, 2016). This work proposes a fourth mechanism at TWS – near-constant freshwater flow with increasing brackish water contribution as spring discharge increases following rainfall in the catchment.
The salinity of TWS Main Spring ranges between 0.02 and 0.23 g/L varying accurately with discharge, with a mean of 0.18 g/L. It is believed that two distinct water stores, designated Fresh Component (FC) and Brackish Component (BC), combine to produce the springs’ outflow. At low flow (low salinity) the discharge is almost all FC, then flow and salinity increase as BC is added. Chemical data over 50 years shows that the FC contribution has been near-constant and the salinity of BC has not changed in that time. While the positive relationship between salinity and discharge rate might suggest seawater intrusion by venturi suction, the nature of the system suggests otherwise.
Fleury et al., 2007 classed the TWS coastal karst aquifer as Type 3 (i.e. ‘a system with well-developed karstification below sea level, partially or totally closed to the sea’). This is exemplified by relatively slight brackishness of the spring water and no clear offshore outlets, although it is clear that freshwater escapes to the sea. A conceptual model of the system will be presented.
References
Arfib, B., Charlier, J.-B. Journal of Hydrology 540, 148–161, 2016.
Fleury, P., Bakalowicz, M., de Marsily, G. A review. Journal of Hydrology 339, 79–92, 2007.
Stewart, M.K., Thomas, J.T. Hydrology and Earth System Sciences 12(1), 1-19, 2008.
Williams, P.W. Carbonates and Evaporites 38:44, 2023.
How to cite: Stewart, M., Moreau, M., Morgenstern, U., and Thomas, J.: Seawater intrusion mechanism in coastal karst (TWS brackish spring in New Zealand), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7611, https://doi.org/10.5194/egusphere-egu25-7611, 2025.
