A new report has identified the possibility of water from Marlborough's main aquifer leaking from ocean fault lines into Cook Strait.
With it came the "unlikely" risk of saltwater entering the "ocean outlet" and contaminating the Wairau Aquifer, which feeds household water to Blenheim, Renwick and Woodbourne.
The Marlborough District Council has seen water levels in the aquifer drop since 1973 - the year the region's first vineyard was planted - at rates unable to be explained by irrigation.
After years researching how the aquifer refilled itself, the council turned its attention to how it emptied, saying there had "always been speculation" about water seeping offshore.
Beca Limited senior geologist Paul Wopereis was asked to uncover whether this was possible, and detailed his findings at the council's environment committee meeting last week.
The gravels that made up the Wairau Aquifer were laid down by the Wairau River about 20,000 years ago, when sea levels were 120 metres lower. These extended out to the Cook Strait Canyon, about 10 kilometres off Wellington's coastline.
Most of the aquifer's water emerged through springs, like at the Grovetown Lagoon, but geological evidence suggested some moved out to sea and then "escaped", Wopereis said.
This could be happening at spots where the aquifer was exposed to the ocean, or along the Wairau Fault, which had displaced the seabed and was proved to be leaking gas.
There could also be freshwater springs under the ocean.
"No detailed work has been done on the seabed for us to know that for certain but, geologically, it stands to reason that ... the aquifer likely has some outlets to the seabed."
A recent underwater mapping project found evidence of freshwater springs in Tōtaranui/Queen Charlotte Sound and would likely find similar telltale "pockmarks" in Cloudy Bay.
Drilling into the seabed would provide scientists with more detailed information, but this came with a high price tag, and Wopereis thought the council would be unable to justify it.
Harvesting the offshore freshwater was "pretty unlikely".
The issue with aquifers that extended beyond the coast was seawater could seep in if the aquifer's water pressure fell too low or if too much water was pumped from on-shore wells.
"It's very important that this aquifer is carefully managed. It's the key to the whole of Marlborough's economy," he said.
National Institute of Water and Atmospheric Research (NIWA) are trialling new technology in Wellington Harbour that detects freshwater springs. If successful, it could be used to find underwater discharges in Cloudy Bay.
Wellington Harbour has gravels similar in age to the Wairau Aquifer's, and was found to be storing freshwater offshore.
An investigation into Wairau Aquifer's decline in water levels found low flows in the Wairau River were a major reason levels dropped, which could be made worse by irrigation.
Councillors approved Wopereis's report at the meeting.
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