An image of Monhegan Island from the 2013 Lincoln County Sea Level Rise Coastal Hazard Study shows saltwater filling a marshland that serves as the public water supply.
(Source: Lincoln County Regional Planning Commission)
An image of Monhegan Island from the 2013 Lincoln County Sea Level Rise Coastal Hazard Study shows saltwater filling a marshland that serves as the public water supply. (Source: Lincoln County Regional Planning Commission)
Washed-out roads are almost certainly going to be an inconveniece that comes with sea-level rise. But what if the road that washed out was the only thing between the ocean and your drinking water supply? That’s a scenario that officials on Monhegan, an island 10 miles from the mainland, have started taking seriously.

Most year-round residents on Monhegan have private wells into bedrock, but the town water supply that supports the island’s ballooning summer population comes from a series of wells in a low-lying marsh on the southwest side of the island a stone’s throw from the sea.

The U.S. Environmental Protection Agency designated it as a “sole source aquifer” in 1988. Islesboro, North Haven and Vinalhaven have the same designation and are the only others in the state. But Ryan Gordon, a hydrologist with Maine Geological Survey, said the type of aquifer on Monhegan and its proximity to the sea make it unique on the coast of Maine, and uniquely susceptible.

“It’s almost like a pond that’s filled in, and there’s a little outlet that goes to the ocean,” he said. The outlet is a culvert under Main Street. “As sea level rises, it’s in danger of slopping water up into that wetland source and contaminating their drinking water.”

Maine Geological Survey created an interactive map to simulate sea-level rise, starting with the highest astronomical tide and flooding it in increments — 1.2 feet, 1.6 feet, and so on (see “Resources,” left). At 3.9 feet, Monhegan’s marsh would be low-lying enough to flood.

Peter Slovinsky, a coastal geologist at Maine Geological Survey who worked on the simulations, said, at that level, there wouldn’t be a connection to the sea yet. But the simulation doesn’t account for waves.

“Six feet of sea-level rise, or a storm surge on top of a high tide,” he said, “and you could potentially have a breach of the road which could bring saltwater into their drinking water supply.”

Slovinsky was part of a tour organized by the Island Institute last fall at which a geologist from the University of Rhode Island shared notes on coastal conditions and climate change with representatives of Monhegan Water Company, Lincoln County Regional Planning Commission, Maine Department of Environmental Protection, Island Institute, Maine Sea Grant, Mid-Coast Regional Planning Commission and the National Oceanic and Atmospheric Administration.

Andrew Dalrymple, manager of the Monhegan Island Water Supply, was also on the tour. Speaking on April 10, he said the problem isn’t imminent, but the water company is already planning to stay ahead of it — if not for the possibility of getting swamped by seawater, then for drought.

The town aquifer exists entirely due to rainwater that finds its way downhill through adjacent meadows to the marsh. The most recent study of the aquifer, in the 1990s, found a capacity of 1.3 million gallons. The same study found that demand in the summer was double that.

“We rely heavily on the recharge rate,” Dalrymple said, “and in recent years that rainwater has been less dependable. Right now the meadow is flooded with the snow melt, but in the middle of the summer we have dry spells. The water company has to address that.”

Monhegan is trying to stay ahead of the problem. The town got a $30,000 Maine Shore and Harbor Grant last year and another $10,000 from Island Institute to study the island’s suseptibilites to climate change. As part of that planning, Dalrymple said the water company is exploring the possibility of drilling deep wells into confined aquifers in the bedrock, where the drinking water supply would be impervious to seawater.

“We’re preparing now for issues that can occur in the next 25 years or so,” he said. “It’s not realistic to say a wave could come across the road right now, but waves are getting higher, and in the next 10 or 15 years that could happen.”

Seawater doesn’t have to leap over obstacles to cause problems. In peninsula communities, where houses with private wells lie close to the ocean, saltwater can enter through fissures in the bedrock, Gordon, the hydrologist, said. These are harder to predict, he said, but tend to turn up in clusters where fractures in the rock are oriented in a certain relation to the ocean. He called it “more of a neighborhood problem.”

“As the elevation of the ocean goes up, the pressure on those fractures goes up,” he said. The greater the pressure, the more likely that saltwater is pushed into residential wells. “It’s a hard thing to put a number on,” Gordon said, “but I predict that will happen more with sea-level rise.”