This enormous monitoring apparatus is one critical part of New York City’s drinking water supply, ensuring the safety of more than a billion gallons of water flowing daily through a sprawling network of three pristine lakes, 19 reservoirs, and mile after mile of aqueducts and tunnels. About 90 percent of that water never sees the inside of a filtration plant, flowing from huge reservoirs as far as 125 miles away in the rural Catskill Mountains.

New York has spent more than $1.7 billion to protect this unfiltered water supply since the early 1990s, in return for being granted a succession of federal and state waivers exempting it from costly filtration requirements. It is one of only five cities nationally — along with Boston, San Francisco, Seattle, and Portland, Ore. — that have an unfiltered water supply. This marvel of water engineering has attracted visits from scientists and government officials from Australia, China, India, Singapore and Colombia.

The financial stakes are high. Vincent Sapienza, the commissioner of the city’s Department of Environmental Protection, said that if the city were refused a waiver, it would have to spend more than $10 billion to build a massive filtration plant, and at least another $100 million annually on its operation — which would be “the largest capital project that the city has ever taken on.” Water bills would have to rise significantly to cover the cost, he said.

The city already filters 10 percent of its drinking water from a dozen small reservoirs surrounded by development in Westchester and Putnam counties. In 2015, it opened a $3.2 billion filtration plant under a golf driving range at Van Cortlandt Park in the Bronx.