‘Wonky Holes’ Help Coastal Water Planning
The discovery of ℠wonky holes´ — strange freshwater seeps resembling miniature volcanoes on the seafloor — is giving Australian water scientists important new insights into the nation´s groundwater resources, and how they may be used by coastal communities.
Wonky holes — named by fishermen whose nets they snagged, threatening to capsize their boats — were first found in a 60 km-wide strip fringing the Great Barrier Reef (GBR) coastline.
These mysterious holes resemble pockmarks on the seafloor, about 10 to 20 meters across, and up to four meters deep. So far, researchers have identified some 200 holes along the GBR. Thought to be the remnants of fossilized rivers, they may also exist in other places round the 35,000-km Australian coast.
“For years people thought of water as draining from the land into the ocean mainly through surface rivers. We didn´t realize how much groundwater also leaks from shore into the ocean floor itself — or the implications for the supply of fresh water on land,” says Professor Craig Simmons of the National Centre for Groundwater Research and Training (NCGRT) and Flinders University.
“We think that wonky holes are actually undersea fresh water springs — the remains of ancient river channels — that still flow sporadically when water levels in aquifers on land are high,” Dr Stieglitz of NCGRT and James Cook University says.
“Now we think wonky holes could help us assess how much groundwater discharges from our aquifers to the sea — and also its role in sustaining ecosystems on both the seabed and on land.”
Originally greeted with skepticism, wonky holes have prompted researchers to ask whether ecosystems in the marine environment depend on groundwater from the land, how much they use, and whether this draining of fresh water in turn affects coastal environments and human communities.
It turns out that ℠wonky holes´ are in fact environmental flows, and that, like river estuaries, they may act as rich nurseries of sea life, including prawns and fish — one of the reasons the fishers were targeting them. But they also have upstream effects on coastal aquifers, wetlands and ecosystems.
By accounting for the flows from groundwater into the ocean, scientists can better calculate how much water is available in aquifers on land for uses such as growing crops, providing water for coastal communities or natural landscapes.
“It is all a part of understanding the coastal aquifers´ water balance. This helps us to avoid over-extraction, which can be disastrous.” Prof Simmons says.
“If the water levels in an aquifer fall below levels in the connecting ocean, the sea water flows back up the aquifer, turning it saline.
“When this happens, the aquifer can be ruined — coastal communities may lose their water supplies, animals can´t drink the water, and we may also lose the natural coastal vegetation that depends on it.”
Dr Stieglitz explains that wonky holes were formed when the last ice age ended 15,000 years ago: “Previously, the sea water level was 120 meters lower than it is today, and we had rivers flowing right across where the GBR is today.
“Once the ice melted, the sea level rose and inundated these rivers. They were then covered by sand and mud, becoming springs whenever fresh water broke out through the seabed.”
The researchers emphasize that fresh water that flows into the ocean isn´t wasted: “The coastal ocean is one of the richest areas for marine life on the planet,” they say. “This is due to its mixture of fresh and salt water, and a lot of these ecosystems thrive on groundwater that flows into the sea.”
Since the discovery of wonky holes, scientists have been trying to measure how much groundwater seeps into the Great Barrier Reef and other coastlines of Australia. One way to find out is to search for particular fresh water-dependent plants on the beach, Dr Stieglitz says.
“For instance, in one location we saw melaleuca trees growing on the beach, below the high tide mark, which was strange because these trees can´t live on salt water alone,” he says. “We then found out that they were surviving on fresh water that was traveling underground past their root system and heading towards the ocean.”
Researchers are currently using seepage meters to measure groundwater flow. As natural elements such as radium and radon gas are found in groundwater, scientists also use these to calculate how much groundwater is flowing out to the sea.
“We weren´t aware of them until some fishermen told us about these ℠wonky holes´ along the GBR that kept snagging their nets due to rough outcrops around the holes,” Dr Stieglitz says. “This was ten years ago, and we didn´t realize their full significance at first.
“But they have opened our eyes to something that has been overlooked for years on our shorelines. It´s a great example of how ℠citizen scientists´ can help the experts to better understand the environment.”
The National Centre for Groundwater Research and Training is an Australian Government initiative, supported by the Australian Research Council and the National Water Commission.
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