Offshore aquifers highly likely in Canterbury

Scientists say there are probably huge offshore aquifers around most of New Zealand, especially off Canterbury.

Scientists say there are probably huge offshore aquifers around most of New Zealand, especially off Canterbury.

They don’t know if these offshore reservoirs hold drinkable freshwater or even their size.

But there was a “high likelihood” that they exist, wrote doctors Leanne Morgan​ of the University of Canterbury and Joshu​ Mountjoy,​ from Niwa, in a new scientific paper.

The pair said in an interview that some offshore aquifers were probably connected to onshore aquifers under cities such as Christchurch.

Morgan said it was “quite likely that we’re already drawing on it”, so we “need to be accounting for that” when drawing up the water supply and use plans.

The offshore reservoirs should not be considered a “new potential freshwater source” that left farmers, industry and residents thinking the water supply was almost unlimited, they stressed.

Offshore aquifers probably date to the last Ice Age when the coastlines were tens of kilometres further out to sea than they are now. Rain fell onto this land and collected in underground reservoirs.

When the ice melted, and sea levels rose, the reservoirs were protected from salt water inundation by impermeable “caps” of mud and sediment. Perhaps some weren’t protected. It’s unclear.

In other cases, rain falling near current coasts or leaking from rivers made its way to offshore aquifers.

Offshore aquifers found more and more

There were known aquifers in Wellington Harbour, and scientists have speculated for years that the rest of New Zealand was surrounded by a “skirt” of offshore reservoirs.

But it wasn’t until a huge one off the coast of South Canterbury was accidentally discovered in 2011 and later investigated in greater detail that they had proof of one.

Drilling beneath Wellington Harbour in 2017 showed the water contained elevated iron and manganese levels. The scientists reported that treatment to make it drinkable was considered too expensive to continue.

The regions where offshore reservoirs were considered highly likely are Canterbury, Marlborough and Hawke’s Bay​. All three regions were all arid and largely reliant on groundwater. Climate change will make them drier.

We need to “understand these systems more if we’re going to keep using them. Otherwise, we’re going to get into trouble”, said Mountjoy.

Seawater has about 35 grams of salt per litre. Drinking water has less than one gram of salt per litre. “Freshened water” – suitable for desalinisation – should have less than 10 grams per litre, Morgan said.

The scientific paper describing the South Canterbury aquifer described its origins as “meteoric”.

But in this context, meteoric means “derived from the earth’s atmosphere” – in other words, rain.

The next step is to drill offshore wells to learn more about the reservoirs, though such drilling is monstrously expensive.

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