“A lot of work has been done over the years to understand where the aquifers are and where the freshwater is, but until now, we didn’t know where in the seabed the freshwater was leaking through,” National Institute of Water and Atmospheric Research (Niwa) marine geoscientist Dr Joshu Mountjoy said.
“Due to Wellington’s vulnerability to earthquakes, ensuring access to drinking water is a priority in case a source is cut off or damaged, and the Waiwhetū Aquifer is a critical resource for our resilience.”
Natural freshwater springs, which are found in the ocean as well as on land, provide a useful window into the submarine aquifers they’re connected to – but finding them isn’t easy.
Typically, scientists have had to mount costly drilling campaigns to locate and define them – but the Niwa study showed other techniques to track and sample ocean-based springs.
“We used every tool we had available to understand the nature of these natural springs, including acoustic measurements, sea floor samples, remotely operated vehicles and seawater sampling,” he said.
“We wanted to see which techniques worked best, with a mind to apply them to other locations in the future.”
Using a mix of novel methods, Mountjoy and his team recorded video footage and acoustic soundings of freshwater flowing out of various “pockmarks” - or crater-like depressions caused by fluids erupting and streaming through the sediments – in the harbour.
There happened to be several hundred of them, with the largest spanning across more than 100 metres.
“We managed to create a detailed map of freshwater springs in the Harbour, which will be important for managing the aquifer as a drinking water resource for Wellington,” he said.
“We can start applying the techniques we used here to find freshwater leaking out of other aquifers, such as in Marlborough, Canterbury and Hawke’s Bay.”
These were barely understood beyond the coastline, he said, but vitally important nonetheless.
“We are at the beginning of understanding offshore groundwater systems around the world, which might be the most important resource some countries have.
“The implications are huge, especially for drought-prone and water-scarce areas.”
The study comes as a GNS Science-led team have published the first database of the 30,000 known seabed pockmarks lying around New Zealand’s coasts.
Along with having potential links to subsurface hydrocarbon systems, these pockmarks also provided habitats for benthic organisms, and could sometimes indicate a hazardous risk of sea floor instability.
“We know very little about the seabed environment: we’re getting to the point where we’re mapping the sloping parts of it, but a lot of the shallow water is unsurveyed,” Mountjoy said.
“In continuing to fill in the gaps, things like freshwater pockmarks related to aquifers will show up in other places.”
Just a few years ago, Mountjoy and colleagues revealed an aquifer sitting about 60 kilometres off the South Island’s east coast, thought to hold the equivalent of half the volume of groundwater in all of Canterbury.
“Every time that we go out and do another survey, we find something we weren’t expecting, as there’s just so much out there to understand.”
