Wellington Harbour Bores - Exploratory Drilling Findings
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Supply of water to Wellington City relies on bulk water mains that cross the Wellington Fault line in several locations. These pipelines are expected to break in a significant Wellington Fault rupture event resulting in Wellington being without water for an extended period of time, prior to repairs.
Unexpected failure of bulk supply assets during normal operation may also result in limited supply, water restrictions, or potentially even loss of supply to areas of Wellington City depending on the nature and location of the failure.
To improve the resilience of Wellington City’s water supply, an alternative supply project has been proposed by Wellington Water. The Cross Harbour Pipeline (CHP) project aims to provide an alternative pipeline route that does not cross the Wellington Fault. A borefield located within Wellington Harbour was identified as a possible alternative solution at potentially significantly reduced cost. This would likely be the world’s first offshore fresh water bore supply as whilst fresh water has been identified elsewhere below the seabed, no evidence was found suggesting an offshore borefield has been used for a significant drinking water supply.
This paper outlines the opportunity that the Harbour Bores option presented. This includes the investigations completed and results, the future planned use and applicability of the data obtained for the Wellington region’s water supply, and wider geological implications for the data that has been obtained.
The Waterloo treatment plant currently sources water from the Waiwhetū Aquifer from bores along Knights Road in Lower Hutt. The same aquifer is understood to extend out underneath the Wellington Harbour, discharging at the ‘Falcon Shoals’ area near the harbour entrance. Drilling investigations were completed to determine if a suitable quality and sufficient flow of water could be obtained from the aquifer to provide an alternative water supply to Wellington City through a borefield located in the harbour.
The project involved drilling beneath the harbour floor, through the aquitard and into the Waiwhetū “Upper” and “Lower” aquifers, and the deeper Moera aquifer. Soil and water samples were taken during the drilling to assess the water, geological, and geotechnical properties of the harbour bed and aquifers, and low yield pump testing was carried out to estimate the potential flow a borefield would produce.
No hydrogeological or geological data has previously been obtained within the harbour south of Matiu/Somes Island leaving a significant gap in information and knowledge. The exploratory drilling has provided new data that is assisting the proposed Cross Harbour Pipeline project and has wider application throughout the Wellington region generally. Greater Wellington Regional Council is using the information to update existing hydrogeological models to better manage this critical regional water supply source, particularly with the management of the saline intrusion risk through known springs in the harbour and from other brackish aquifers. The National Institute of Water and Atmospheric Research (NIWA) and GNS Science (GNS) will be using the information to undertake further analysis and update the understanding of the Wellington Region in their respective fields in future studies.