WAPPINGER, N.Y. » Workers are planning to go under the river and through the rocks to replace part of a massive tunnel leaking New York City’s drinking water.

They’re blasting through hundreds of feet of shale and building a shaft on each side of the Hudson River, about 60 miles north of New York City. Once they reach about 600 feet below the river next year, they will begin building a 2.5-mile bypass tunnel between the shafts to replace a profusely leaking section of the 85-mile Delaware Aqueduct.

The work on the World War II-era tunnel illustrates the ingenious complexity of a water supply system for 9 million people and the intricate efforts to keep water flowing smoothly as the sprawling network of tunnels ages. The Delaware Aqueduct carries about half the city’s water and will have to be temporarily drained for repairs in 2022 during the end stage of the project. A portion of the $1.5 billion price tag is devoted to making sure other water sources continue to flow smoothly during the shut-off.

"The goal of the city in constructing the waterworks was to construct something comparable to the great works of Rome, to last through the ages," said Paul Rush, deputy commissioner of New York City’s Department of Environmental Protection. "It’s with that spirit and with that purpose that this work here continues the tradition."

The Delaware Aqueduct carries 500 million gallons of water a day from four reservoirs in the Catskill Mountains in southeastern New York state to a distributing reservoir at the city’s doorstep. The tunnel, wide enough to drive a car through, is one of two major arteries that carry unfiltered water from bucolic areas of the Catskill and Delaware watersheds.

The aqueduct, which the city says is the longest continuous tunnel in the world, relies solely on pressure and gravity. It’s an engineering marvel, but it’s aging.

The aqueduct is so leaky in one stretch through rural Wawarsing that New York City is buying dozens of homes above it that were plagued with flooded basements and squishy lawns. The leaks are far worse where the tunnel travels under the Hudson by Newburgh, but homes are not affected. Between 15 million and 35 million gallons of water seep through cracks each day, enough to quench a small city.

The bypass will permanently replace the leakiest portion of the tunnel. The other cracked stretch will be repaired. Before workers begin digging the bypass tunnel next year, they first need to complete the entry shafts on opposite sides of the Hudson River at Newburgh and Wappinger.

Since late 2013, workers have been drilling into the shale, scooping up the blasted rocks with a crane and then repeating the process. The result is a deep, 30-foot diameter shaft in Wappinger. Workers in hard hats guide giant buckets of rock from the dark, drizzly bottom to the disc of daylight high above.

"It’s a very repetitive process," said George Schmitt, a project manager.

The massive project, scheduled to finish in 2023, is just one big-ticket item needed to keep water flowing smoothly through the massive system. A filtration plant in the Bronx will cost $3.5 billion, and $4.7 billion has been spent to build a third water tunnel beneath the city.

The work that will repair, rather than replace, the other leaky stretch is especially tricky because the aqueduct carrying half the city’s water will have to be shut down for six months starting in 2022.

Turning off the aqueduct for the first time since the late 1950s means four of the city’s major reservoirs will be out of service temporarily. Keeping the water flowing while patching up a main artery can be as tricky as repairing an airplane in flight. But Rush said the city will rely on alternate water sources and conservation.

The roughly century-old Catskill Aqueduct will be scrubbed and updated to increase its flow in time for the shutdown of its companion aqueduct, and the city will rely on water from Croton and wells in Queens. On the conservation side, the New York City plans to update bathroom fixtures in its public schools and has started a voucher program for up to 800,000 water-saving toilets for qualified houses and apartments. If all goes as planned, New Yorkers turning their faucets will never notice.

"It will happen seamlessly in the city," Rush said. "They will continue to get their water."