Dune erosion on Sable Island caused loss of groundwater along exposed coast

Where sand dunes disappear on Sable Island, so does the underground freshwater supply, according to new research underscoring the importance of dunes to the island’s ecosystem.

The dunes serve as a natural barrier, protecting against the relentless waves and temporary salinization of groundwater caused by ocean flooding. Dunes also help refresh the water table by funnelling water from rain and snow into aquifers that, in some places, are 40 metres deep.

But on the south coast — which is most exposed to storms, erosion and seawater flooding — dunes have disappeared in recent decades.

And so too has a 10-metre layer of groundwater that used to lie underneath them, said Julia Cantelon, a PhD candidate at Dalhousie University in Halifax in the department of civil and resource engineering.

Cantelon co-authored a study on saltwater intrusion on the remote Atlantic island located 290 kilometres southeast of Halifax.

It was published this month in the journal Water Resources Research.

Loss of fresh groundwater

“We don’t see any freshwater right along the beach where that dune was lost. So we really see a trend where the dunes are lost along the south beach, we see a loss of fresh groundwater,” said Cantelon.

“The fresh groundwater resources on the inland portion of the island where the dunes are healthy, we don’t see that same decline.”

Cantelon and other researchers from Dalhousie collected groundwater data between 2019 and 2022 and compared it to measurements taken 50 years ago.

Without the dunes, a cycle of salt water inundation prevented replenishment of the freshwater supply along the south coast.

In hydrology, the supply is called a “lens” which is a convex-shaped layer of fresh water that floats above the denser saltwater underneath the island.

She estimates the south beach lens disappeared sometime between 1996 and 2014.

Still fresh groundwater mid-island

The good news is that, in the middle of the island, there was no loss of fresh groundwater and the north beach has experienced localized loss.

“At present, our findings don’t indicate any sort of dramatic groundwater loss that is going to have deleterious impacts to the ecosystem, but they are really important to show those connections between the land surface and the subsurface and help understand what might go on in the future or predict future losses,” Cantelon said.

The next stage in her research is to look at the impact of climate change which is expected to increase the forces that threaten dunes.

Sable Island National Park reserve is home to birds, plants, insects more than 400 feral horses.