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With everyone on Nantucket sipping from the same aquifer, what happens during a drought?
Each spring, as the weather warms and the bustle of summer can be heard in the distance, people begin to arrive on Nantucket. All winter, the island has been preparing for them, and not just the hardy year-round residents, bracing themselves for the onrush of another fifty thousand people; the land itself has been preparing, hydrologically. In its bones, in its sand, silt, clay and gravel, the island has been recharging its aquifer. As rain and snow soak the island, the water table rises. If you know what to look for, you can watch it happen.
“Every month, I go around and measure wells for the U.S. Geological Survey,” says RJ Turcotte, Nantucket’s Waterkeeper and a staff member at the Nantucket Land & Water Council. The aquifer, the island’s sole source of freshwater, is a lens (or layer) of saturated sediment whose upper boundary is typically a few feet beneath the surface, and Turcotte uses a special tool to find out exactly where. “It's basically a tape measure with a probe on the end. You lower it down a pipe into the ground, and when it buzzes you know you’ve hit groundwater.”
The surface of the island is pierced by hundreds of wells, and their levels, month-by-month, tell a story. “When everybody gets here in the spring, we start drawing up millions of gallons more water, and you can see the aquifer drop,” Turcotte says. “And then Labor Day, everyone starts to leave, and we start to get precipitation that isn't being immediately taken up. You start to see the aquifer rebound.”
Some of the water drawn up from the ground is used for drinking and bathing, but much of it goes to the splashy trappings of summer life on Nantucket — for watering lawns, gardens, and golf courses; filling swimming pools; hosing off salty, sandy cars and boats. As more people arrive, these activities accelerate. When dry conditions combine with high water usage by island residents, the water table sinks further below the surface, and Nantucket officially enters a drought. As the drought becomes more severe, it triggers mandatory water restrictions.
The past few years have seen a ratcheting intensity of drought on Nantucket, along with widespread flaunting of water restrictions. Last summer, as a mild drought in June became a critical drought by October, the town voted to impose fines for violators — up to $500 on the third offense. But for many people on the island, that’s a small price to pay, so the town’s public utility, the Wannacomet Water Company, threatened to cut off water completely for serial offenders.
“It’s really hard, because everyone’s on vacation in the summertime,” says Turcotte, who also sits on the Conservation Commission. “People’s mentality is: I deal with problems all day long back home when I’m at work. I don’t want to hear about how I need to use less water. For the most part, people aren’t here very long, and it's really hard to educate when [they’re] just ripping through for a bachelorette party.”
Even the concept of water scarcity can feel counterintuitive in New England, because we have lots of water, especially compared to the arid landscapes in the western United States. Even in the dog days of summer, we can experience intense, drenching rainstorms. So what does a drought really mean for a place like Nantucket, which is sitting atop on tens of billions of gallons of fresh water? And if the aquifer’s not going to run completely dry, why should anyone care if it drops a couple feet?
What Lies Beneath
“I think a lot of people have the notion that, like, there's some Journey-to-the-Center-of -the-Earth-style caves below the island, full of water,” says Turcotte. I’m not a geology guy, and I confess to harboring exactly that notion, so I turned to Tim McCobb, a hydrologist at the USGS New England Water Science Center, to set me straight.
“What's unique about the islands, obviously, is they're surrounded by saltwater,” McCobb says. “You have this lens of freshwater sitting on top of that saltwater inside this matrix of sands and gravels.”
By lens, he really does mean the shape. The aquifer sits beneath the island more or less like a contact lens balanced on your finger, deepest in the center and curving up near the edges of the island where denser, heavier seawater presses in. Unlike places with abundant surface water — streams, lakes, and reservoirs like the Quabbin — this freshwater lens is the sole water supply on Nantucket.
“And its only source is precipitation,” says McCobb. Keeping the aquifer topped off takes tens of billions of gallons of precipitation per year. “Usually, less than 50 to 60% of that makes it as recharge to the water table. The whole system has to get saturated at the surface before anything trickles down.”
It’s a slow process, and it happens most efficiently in the winter, when plants take up less water at the surface. “We get most of our recharge in the late fall through the spring,” McCobb says.
As rain and snow fall on the island, any fresh water that doesn’t get absorbed by plants, evaporate, or roll off the edges of the island trickles down through the ground, and over decades and centuries it piles up, pushing down on the denser salt water below and giving the aquifer its shape and thickness.
And the aquifer is thick — Turcotte pointed me to some evidence of this near the center of the island. “At the intersection of Milestone Road and Russells Way, if you bushwhack a little bit in the woods there, you'll find this steel pipe sticking out of the ground,” he says. “It's about the diameter of a five-gallon bucket.”
This pipe is what remains of a Stratigraphic Test Well from the 1970s. “Way back when the USGS came out to quantify the aquifer, they took a drill rig truck on the ferry, and they used it to go down and find the bottom of the fresh water,” Turcotte says.
The drill descended more than 1,600 feet, making its way back in time as it went. It pulled up small marine fossils, shrimp-like crustaceans and molluscs from the Pleistocene period giving way to plankton from the Eocene epoch and plant pollen from the Cretaceous, finally coming to a stop against the dense, ancient basalt of the oceanic crust. At this well, freshwater began about twenty feet down, and continued for another five hundred feet before becoming salty, as seawater infiltrated laterally from the edges of the island. (For more on ancient aquifers, see Britt Bowker’s story.)
If you don’t have a drill rig, there are a few other ways to look at Nantucket’s aquifer. In most places it’s several feet down, but at low spots or near the coast it can intersect the surface, trickling out of seeps and springs at the beach or filling shallow depressions like the Doughnut, Wigwam, and the Pout Ponds. Setting aside sediments for a moment, imagine you were a being that could swim in the fresh water beneath Nantucket. Looking up, these ponds would be your windows to the sky, shimmering portals between aquifer and atmosphere. Some hydrologists even refer to these ponds as “outcrops” of the aquifer, as though it’s a mountain of water we can only glimpse when it’s not covered by land.
The ponds are sentinels. During a drought, as the water table drops, they dry up. “Reed Pond was dry over the summer, and the Pout Ponds out in the Moors,” Turcotte says. “Some of these smaller wetland systems are like the canaries in the coal mine, and if the drought stayed severe and kept going, you’d start to see it in areas that aren’t quite as small and isolated.”
Drought can also have an outsized impact near the coast, where groundwater is thin and close to the surface, wells tend to be shallow, and saltwater is lurking just below. If too much fresh water is pumped out, saltwater can intrude, turning your drinking water to brine.
Imagine you were a being that could swim in the fresh water beneath Nantucket. Looking up, these ponds would be your windows to the sky, shimmering portals between aquifer and atmosphere.
In some cases, neighbors blame each other when ponds dry up or wells start pulling in salt, and it’s true that as new houses pop up and water usage increases, the water table can drop locally. But it’s more complicated than that. “It’s so hard, because the neighbors reach out and they're all fired up because they're like, ‘they’re running their sprinklers again,’” Turcotte says. “It’s probably not helping, but forensically, scientifically, it's really hard to point the finger. You have to deal with the porosity of the soil, and which way the groundwater is flowing, and it gets really complicated.”
“You need a three dimensional model to really understand how groundwater flow works,” McCobb says. There isn’t yet a public model for Nantucket, but he’s spent the past few years working on one for Martha’s Vineyard, which has a pretty similar water table setup. A model is helpful because the island isn’t uniform; different parts of it have different groundwater levels, and get recharged by rain and snow at different rates. “That's where geology comes into play,” he says.
McCobb points to a USGS well in the eastern part of the island, off Milestone Road near Tom Nevers swamp. “Like this one here, it’s out in the outwash plain, it's in sand and gravel, it's pretty shallow, and it responds quickly to each precipitation event,” he says.
Like lots of things on the Cape and Islands, we have the Ice Age to thank for the way groundwater moves on Nantucket. The hills that stretch east-west across the island were bulldozed there by a glacier, and the low-lying sand and gravel south of that moraine was deposited by streams flowing off the melting ice. Today, water flows more easily though the outwash plain than it does through the moraine.
“The moraines occur on this northern part of the island as a mix of silts, unsorted sands, and clays in little layers,” says McCobb. “And those clays can keep water from percolating down to the water table, or slow it down. As you move into these wells in the center of the island, they’re less responsive to precipitation.”
I found myself thinking of the island kind of like a pourover coffee, maybe a Chemex. Water percolates down through the coffee grounds to reach the mug — the aquifer; but because whoever ground the beans did a haphazard job of it, some grounds are very fine and some very coarse, and water drips through them at different rates.
This all leads to some bated breath as the island enters and then climbs out of each drought. “There’s a lag, ” says McCobb. Even in periods when precipitation goes up, the water table can continue to drop while the system resaturates, which can make for a frustrating experience if you’re waiting for a drought to be over. McCobb’s advice: “Be patient, it’ll eventually get there.”
More materially, “you can actually look at that lag and figure out what it is,” McCobb says. All you have to do is chart how far above or below normal the precipitation is, do the same for the groundwater level, and compare. On Nantucket, he says, there seems to be a four- to five-month lag. “If you have really normal precipitation in October, November, December, you're not going to see that start to come back until the early spring.”
So — the aquifer is big, complicated, and can be slow to recover from prolonged drought. But there’s still a huge amount of water in it, and most wells are pretty deep, so what is there to be worried about? Why not give the Land Rover another rinse?
For one thing, a small drop in the water table can still have consequences, not just for beloved local ponds. According to the Massachusetts Well Driller Database, there are dozens of private wells on Nantucket that have only a few feet of wiggle room before they go dry or risk saltwater intrusion.
In the long term, repeatedly overpumping during times of drought could end up making the whole aquifer thinner, a problem exacerbated by climate change. “There’s the concern that as the sea level comes up, the whole freshwater lens is going to rise,” McCobb says. “As it comes up, the freshwater zone is going to get thinner as it sheds off the top, to the coast. So you're losing freshwater thickness, which is important.”
Thanks to an old hydrologic principle, any loss in water at the top of the aquifer means that seawater can push upward a correspondingly larger amount, rising about 40 feet at the bottom of the aquifer for every foot lost at the top as a ballpark rule of thumb.
“If you’re over-pumping during a drought and you don’t really pay attention to it, it's going to get thinner,” McCobb says. “And once that system gets thinner from below, it's really hard to get it to go back. You have these changes that are seasonal at the top — the things that change the shape of the aquifer, like overpumping. A really long term decrease in the thickness, that's what you have to be concerned about in my view. You’ve got to protect the geometry of that freshwater lens.”
It’s a long-term, worst-case scenario threat, on the scale of decades, but it’s worth keeping an eye on, McCobb says.
Turcotte agrees. “We're not out of water, we're just trying to be wise with what we have, and be conservative,” he says. “You never know what the weather is going to do. You don't want to get stuck in the wrong feedback loop.”
Balancing the Water Budget
In absolute numbers, Nantucket is pumping more groundwater than it ever has. In 1975, the Wannacomet Water Company pumped around 30 million gallons of water per month at its summer peak; today it pumps around that much in a summer week. If you turned the entire Nantucket Airport into a swimming pool 10 feet deep, that would be about how much water Nantucket uses in a day.
The aquifer is putting up with it right now, but the island population continues to grow, and by the summer of 2030, when day visitors are projected to reach 100,000 in peak periods, a sustained drought could become harder to manage.
One person charged with anticipating this kind of thing is Vandana Rao, Director of Water Policy at the Massachusetts Executive Office of Energy and Environmental Affairs. “Drought seems to be occurring fairly often in the northeast region of the country,” she says. “Since 2016, we've been in a drought almost every year.”
Climate change might make New England warmer and wetter, but not necessarily in ways that help the aquifer. “We're getting a lot more of these hyper-short-intensity precipitation events,” Rao says. These aren’t effective at recharging groundwater, since most of what falls in a deluge ends up as runoff, in Nantucket’s case going directly into the ocean. “And on the other side, we're also seeing extremes in drought.”
Rao is also co-chair of the state’s Drought Management Task Force, which advises the state on when and where to declare one. “We are continuously monitoring the data,” Rao says. “Once we start seeing a prolonged period of no to low precipitation, then we really start to say, ‘okay, how's the streamflow doing, how's the groundwater doing?’ When we start to see those indices tripping, that's when we call the Drought Task Force together. There’s a lot that happens behind the scenes.”
Once the drought is official, there’s a lot to do: statewide, there are implications from agriculture to firefighting, and water conservation becomes critical. “We want to make sure we have supplies that last through the drought and don't impact our essential uses,” Rao says.
This means not using water outside, and keeping indoor usage efficient. “It just makes sense for us to be careful to use water as efficiently as possible, and to think a little bit more about our everyday use of water, particularly in the summer months,” Rao says.
For example, given the ancient, glacial purity of Nantucket’s aquifer, using it for your garden is basically like watering your plants with San Pellegrino. Rao says a rainwater barrel is a better choice. “Try to avoid using your well water for outdoor water use, really make sure that your good, quality water is used for your indoor essential uses,” she says. “Wash your clothes in a washing machine and your dishes in a dishwasher as opposed to by hand, and make sure there are only full loads.”
These measures aren’t always popular, and in places like Nantucket, where many people are on private wells, water restrictions are only mandatory for people on the public supply. “But the source of water from those wells, for those end users, is the same,” says Rao. “It's coming from the same aquifer, the same groundwater system.”
After working on this issue up close, sometimes with neighbors who are at odds, RJ Turcotte puts it a little more bluntly. “Have you seen There Will Be Blood?” he asks. “I joke with everybody that it’s literally that.” Daniel Day-Lewis’s famous speech, about what happens when you have two straws in the same milkshake, applies to water as well as the oil that was the subject of the movie.
Huge efficiencies in water usage have been achieved before. In the late 1950s, Nantucket cut its water usage from pumped wells in half, simply by installing its first-ever water meters. And in recent years, all sorts of appliances and machinery have become more efficient. “We are already using less. We are one of the most water efficient states per capita,” Rao says.
“All of this is cyclical,” she adds. “It’s about making slight changes in our way of life during those periods when there is a deficit.”
There are bigger picture things Nantucket can do, too, like convincing major water-users like golf courses and agricultural operations to become more efficient, and ensuring that as much land as possible is free of development and available to absorb precipitation.
“We want to make sure that when we get rain water, we allow that water to soak into the ground, to replenish our systems, and not just wash [away] off hard surfaces like roads and other impervious cover,” Rao says.
In fact, by protecting permeable landscapes that absorb rainwater, the island can actually increase how much water it can legally withdraw. In April, the Nantucket Land Bank voted to devote a parcel of Milestone road for drinking water mitigation, a status that will add another several hundred thousand gallons per day to the town’s water withdrawal permit from the state.
“You can conserve a lot of water very easily,” Turcotte says. “This isn't even as hard as recycling.”
