Glacial Gift -Winter 2017
by: Peter Sutters
photography by: Nicole Harnishfeger
Fourteen thousand years ago, Nantucket was a very different place.
Not an island yet, it has been described by geologists as a small rise of hills in an otherwise flat plain of land, covered by a shallow water at times, and ice at others.
The Earth was getting warmer as the end of the Wisconsin Glaciation Period was approaching.
Up until the end of the last ice age, Nantucket was the most southerly location for the glaciers. As they slowly slid south and reached warmer temperatures, they began to melt. Gravel, pebbles and other stones picked up along the way were released from the ice and piled up into what many of us call home today.
Then it started to get warmer.
As these massive glaciers that created Nantucket were receding to the north, the melt-water began to pool in what is now Nantucket Sound, but what at the time was Glacial Lake Nantucket. Over the years, as the glacier grew and retreated three times, the lake was sometimes covered by ice and at others open to the air.
Water from the lake flowed south through those small hills you can stand on top of today and out to the Atlantic Ocean, which was still miles and miles south from where the waves lap our current shoreline.
The remnants of those rivers still scar the island landscape. Look at any map of Nantucket and notice the shape and direction of some of the Great Ponds: Miacomet, Hummock and Long Pond. They are thin and run generally north to south. Other rivers have dried up, but left behind valleys, like in Maddequecham.
Fourteen thousand years ago, one could have stood atop a south-facing hill and seen these deep blue, glacial melt-rivers carving through the sandy deposits that make up Nantucket as they headed to the ocean, doing their part to fill the sea to the levels we are more familiar with today.
The ponds and hills, however, were not the only things left behind. The glacial lake can still be found, if you know where to look.
Underneath the sandy soils of the island sits possibly its most important body of water, a mas-
sive aquifer of melted glacier water, untouched since it melted, percolated through the sand and became trapped between two layers of non-permeable clay some 240 feet below.
You would think water trapped for so long, so deep below the surface of the island would be impossible for a person to access today, but you would be wrong.
Just turn on the faucet of any home connected to Nantucket’s municipal water supply.
You are drinking melted glacial water, showering in melted glacial water, your toilet is filled with ... you get the idea.
But how exactly, does this melted glacial water get from there to here?
The Wannacomet Water Company maintains two wells that tap directly into this aquifer, drawing the pristine waters that last saw the light of day when they melted from glacial ice to running water 14 millennia ago.
There are actually two aquifers beneath Nantucket. In addition to the glacial reserve, the top one is smaller and the result of rain falling on the island and draining through the sand. This is the aquifer that most homes on Nantucket draw from that are on well water, rather than municipal water.
Because Nantucket’s only source of fresh drinking water is from the aquifers, they need to be protected.
Wannacomet Water Company manager Mark Willett, and recently retired Wannacomet manager Bob Gardner believed salt-water intrusion to be the main threat.
They worked with a hydrology expert starting in the early 2000s to see just how much safe drinking water was below Nantucket.
Salt-water intrusion occurs when fresh water is drawn out of the aquifer, allowing seawater to seep in and alter the salinity, possibly turning fresh drinking water into brackish non-potable water.
A widely-known formula, called the GhybenHerzberg Relation, says for every foot above sea level the water table can be found, there will be 40 feet of fresh water under it until the salt water begins to intrude.
Based on Nantucket’s groundwater, you
should expect to see salt-water intrusion at 200 feet in the best-case scenario.
“We started to drill deep holes,” Willett said. “We got down to 240 feet in the first well in Wyer’s Valley between Milestone and Old South roads and we sampled and not only did we not find higher levels of sodium, it wasn’t even brackish. That was very surprising because the one-to40 rule says it has to be there. It has to be within a few meters.”
They kept going.
“We got down to 250, 260 feet and we hit solid clay,” Willett said. “We found the bottom of that lake.”
When a core sample was drawn from the well field, it revealed a clue in the form of a drop stone that assured them they had found the bottom of the lake.
“It’s about as big as a softball, I still have it in my van,” Willett said. “In the middle of all this clay that looks like Play-Doh, there is a stone. These stones were once sitting on top of the ice and when the ice melted, they fell through to the bottom of the lake.”
The decision was made to keep drilling to see if they could find the salt water, Willet said.
“We went about 50 or 60 feet into it and we had to stop because we don’t know what would happen
if we poked through that clay. We sat on it for two days and had a couple of long nights at The Chicken Box talking about what to do and decided we had the rig in place, let’s just keep going.”
The kept drilling and went through 100 feet of clay. The tip of the drill bit was now some 400 feet below the surface.
If there is water on the other side of that clay, according to the Ghyben-Herzberg rule, it has to be salty.
“We punched through that clay, set a screen and started to pull samples,” said Willett. “It wasn’t even brackish. The one to 40 rule is totally thrown out the door.”
The implications of this discovery for Nantucket are good news, in that islanders can rest assured there is an abundance of fresh drinking water beneath the island.
If the water company had enough pumps at its disposal, every faucet on the island could be turned on for 100 years and it wouldn’t make a dent, Gardner said.
“Please don’t do that. We only have two pumps,” he said with a laugh.
The implications for potentially debunking the Ghyben-Herzberg rule, however, could be even greater.
Until now, well-drillers would not even bother to look for water in places beyond the one-to-40 rule.
Now, in certain conditions, like exist on Nantucket, that rule can be broken, and there could be undiscovered aquifers all over the world with parched land and people above.
“The clay is a huge protective layer for us,” Willett said. “So we need to start to find the extent of that clay and where it ends. We are still working on that project today.”
They are still looking for exactly where that clay lens ends and the current best guess is about halfway across the island.
There are places, notably in Monomoy at low tide, where water seeps out of the clay lens. Birds can often be seen drinking from it as it flows into the harbor.
“The bowl is full and spilling out the sides,” Willett said.
Because the drill punched through two layers of clay and reached fresh water on the other side, it is likely there is a final layer of clay on the bottom of that aquifer.
This is because the glacier that created Nantucket grew and receded three times. Each time it grew, it pushed the water down, while silt falling from the melting glacier would form the clay layers that protect our water today.
“Right now, we are drinking the youngest of the Glacial Lakes of Nantucket,” Willett said. ///
Peter Sutters is a staff writer at The Inquirer and Mirror, Nantucket’s newspaper since 1821.