The Divers, Robots, and Toilets Saving NYC's Water Supply

The first man to notice New York City’s $1.6 billion problem was a utility worker. Standing near the Hudson River in 1988, he saw it was at low tide, which revealed a separate body of water bubbling up near the shore and spilling into the main current.

That didn’t seem right at all. The worker notified the city’s Department of Environmental Protection (DEP). Back then, New York was using copper sulfate to control algae in the Delaware Aqueduct supply, one of the city’s three main arteries for water. At 85 miles long, it’s also the longest continuous tunnel in the world.  

Scientists tested the gurgling, burbling pond. It was positive for copper sulfate.

Somewhere 700 feet below the surface was a pressurized tunnel approaching 50 years of age that was going to need to be repaired to stop the 15 to 35 million gallon hemorrhage occurring daily. “The dilemma was not just the fact there are cracks in a tunnel hundreds of feet below ground,” Adam Bosch, DEP’s Director of Public Affairs, tells mental_floss. “It was, where is New York City going to get its water if we shut the Aqueduct down for a year or more?”

The answer is a succession of engineering feats that rival any in the city’s history: enlisting skilled divers to maneuver a 23,000 pound submerged bulkhead in place, assembling a massive drill underground to tunnel two miles horizontally, and campaigning citizens to begin conserving water for the day the Aqueduct—which delivers over half of the city’s drinking water—is drained down to its last drop.

Shaft 6, the access point where the Aqueduct will eventually be drained. Image courtesy of Global Diving.

The blame falls mainly with limestone. The coffee cake of rock, it crumbles easily and provides poor support when civilization decides to burrow underground. The men who labored to install the Aqueduct in the 1940s lined the weaker areas with steel, trusting the bedrock in other areas wouldn’t need any additional support.

They were right—up to a point. “We’re seeing the cracks right where the steel liner ends,” Bosch says. “The belief is, if the workers had just gone a few hundred yards further with the liner, we wouldn’t have any leaks right now.”

After the symptoms—the leaks—were confirmed in the late 1980s, the city spent most of the 1990s working on a diagnosis. It's been a slow excavation of information that frustrated nearby residents who were suffering from the consequences of water seepage: The town of Wawarsing saw flooded basements and mold issues that were so severe they prompted city buyouts.

“You have to consider everything,” Bosch says. “There are no small problems.”

After establishing the tunnel wasn’t in danger of collapse—under pressure, it can’t crumble inwardly—the DEP was able to confirm the location of the two leak sites by using a remote-controlled submersible vehicle that took pictures of the cracks in the early 2000s. Photographs taken five years later, Bosch says, have shown the leaks haven’t gotten any worse.

More recently, a vehicle able to inject dye into suspected areas confirmed the site affecting Wawarsing had coin-sized holes that could be repaired by simple grouting once the Aqueduct is drained. The other site, near the Hudson, is long past the point of bandaging: It will need a 2.5-mile long bypass installed to circumvent the damage entirely.

In order to connect the bypass and repair the leaks, engineers will have to drain the tunnel. To do that, they’ll have to upgrade the pump system in Shaft 6, one of the key access points to the Aqueduct located in Wappinger. That, too, would have to be drained in order to install the pumps.

The need to inspect, reinforce, and prepare Shaft 6 for that forthcoming duty fell to a team of six divers who spent weeks at a stretch living and working in a pressurized environment. Their job would be to insert a massive bulkhead that will help handle the millions of pounds of water pressure looming near workers—a fit so precise it allows less than a quarter of an inch of room on any one side.

To dive nearly 700 feet below the surface to perform the work needed in Shaft 6, however, wasn’t going to be easy. It would require 12-hour shifts, one after another. Having men work just one day and then decompressing was not only impractical, it would render an already glacial process almost interminable.

The solution: live under pressure.

Global Diving

Global Diving, the salvage operation out of Seattle contracted by DEP to handle duties for Shaft 6 in 2007, had six divers spending weeks at a stretch cut off from the outside world. This is known as saturation diving, which allows for divers to avoid decompression until the end of their tenure—typically a month. The “saturation” is the maximum amount if nitrogen that’s been built up in the body: it’s not going to be any more whether the diver spends a day or a week under compression.

In order to remain at pressure, the divers lived in a customized chamber built over the mouth of the shaft. The 24-foot enclosure resembled a kind of mobile home by way of NASA, with bedding, a shower, and a “med lock” that allowed support staff to deliver fresh laundry, food, and other supplies without compromising the crushing, oppressive air the divers had to endure.

“Say you go down 600 feet,” says Donald Hosford, one of the divers on the project. “It’s about .445 pounds per square inch for every foot. That’s about 300 PSI. That’s like me sitting on your chest and you trying to breathe.” Divers had to avoid major exertion—“no jumping jacks,” Hosford says—and some suffered a degree of muscle atrophy. “You’re sitting on a rack and not using your leg muscles.” Hosford, at 6-foot-6, didn’t spend a lot of time standing up.

Because there’s too much nitrogen in oxygen at that depth, the divers would breathe a 97 percent solution of helium. Their voices were always balloon-high, which meant some of the crew had to use a descrambler to understand them. (While initially bizarre, divers eventually develop “helium ear,” and the high-pitched tones begin to sound normal to everyone but the support staff.)

Before any restoration work could begin, Global first took a sample of the bronze door that separates Shaft 6 from the Aqueduct to assess its condition. It was in immaculate shape, but DEP wanted to take precautions. Global fabricated a 23,000 pound bulkhead, 5 feet wide and 7 feet tall, made from concrete that would fit so snugly—with just a quarter-inch of give on any one side—that the company rehearsed its fitting before attempting it underwater. When DEP was satisfied it could be done, the bulkhead was lowered down the shaft on a crane and glided across a train track assembly to connect to the existing door.

Because everything needed for the job had to fit in Shaft 6’s 13-foot diameter opening, tools to facilitate the job were built from scratch. And since most were bigger than the 8-foot diameter diving bell could contain, they had to be lowered and retrieved each time.  

Fitting the bulkhead took roughly two weeks. By the time divers performed a 12-hour shift and returned to the chamber, they had just enough time to sleep and get an hour or two of reading in before the next shift began. (Because of fire concerns, electronic devices are largely forbidden.)

After five years of scouting work, planning, fabrication, and fitting, Global finished the project in June 2012. To decompress, the divers spent roughly a day in the chamber for every 100 feet they had been under. After a week of that, Hosford says, “it was just about getting re-acclimated to society.”

The drift split off from Shaft 6 where divers were lowered to work on reinforcing the bronze door bulkhead. Once drained, it will have to withstand millions of pounds of force from the Aqueduct. Image courtesy of Global Diving.

Though New York City’s population has grown by over a million since the 1980s, water consumption has gone down. “The peak water use was 1.6 billion gallons in 1979,” Bosch says. “Today it’s roughly a billion. That’s down by a third.”

Part of the reason is an effort by officials and citizens to become eco-conscious, installing low-flow toilets, shower heads and front-loading washers in residential and commercial buildings. The conservation couldn’t have come at a better time, since reduced usage has allowed the city to rely on the existing Catskill and Croton source as the replacement water supplies while the Delaware tunnel is dry for the six to 15 months it will take to allow for the bypass connection. “It’s enough to sustain the new normal of one billion,” Bosch says.

Currently, workers are boring into ground in the towns of Newburgh and Wappinger to create new access tunnels between 700 and 900 feet below the Hudson. Once they’ve hit bottom—or their version of it—a formidable boring machine will be lowered in pieces and assembled under Newburgh. From there, it will begin the 2.5 mile journey to Wappinger. Bosch expects the drill will move 50 feet a day, upchucking earth to make room for the bypass tunnel.

The tunnel will be gravity-fed, meaning Wappinger’s side of the bypass will rest below Newburgh’s—but only by about 5 feet. “It’s incredibly precise,” Bosch says. (And one of the reasons two drills can’t just plow toward one another in half the time.)

The Delaware Aqueduct is expected to be back online in 2024, ending decades of painstaking assessment and problem-solving. “This is the largest repair of the city’s water supply in its 180-year history,” Bosch says. “We wanted to stop the losses as soon as possible, but we had to make sure the repair is the right repair.”