The City of New York just bought 48 acres of prime real estate in Westchester County to keep it from being developed.

Eric Goldstein, an advocate with the National Resources Defense Council, had waited for more than 25 years for the city's Department of Environmental Protection to buy the key stretch of undeveloped land in the Kensico Reservoir watershed in Valhalla.

The streams and hills naturally filter stormwater before it reaches the reservoir.

DEP officials said Monday if the soil here were to be disturbed by construction of buildings and homes, the natural filtration system could be ruined and several harmful kinds of run-off would have a straight shot at the reservoir.

"To prevent pollution from entering the reservoirs in the first place, is to acquire vulnerable lands around the reservoirs," Goldstein said, "that if improperly or haphazardly developed, would drain pollution into the reservoirs."

At a press conference Monday atop the Kensico Dam DEP leaders said the agency is paying $12.56 million for the land, reminding water customers that if the reservoir were to become contaminated the cleanup project would cost much more.

Nine million people in New York City and Westchester County rely on the the reservoir for clean water.

DEP officials said the customers would end up paying for any decontamination and advanced filtration systems through their water bills.

DEP Commissioner Rohit Aggarwala said the agency had been monitoring the status of the land for some time.

The religious order, Legion of Christ, had previously owned several acres including the 48 the DEP bought.

"When they decided to sell it, of course, it became far more important for us to get control of the portion that flows into the reservoir," Aggarwala said.

DEP leaders said they also keep national security in mind when considering how to guard the reservoir, a crucial part of one of the nation's most important water systems.

They are looking to buy more land in the watershed.

"There might be things that we no longer should keep doing," Aggarwala said. "I don't see a world in which buying as much land as we can with the willing partnership of the local communities, in the Kensico watershed is ever a bad idea."

The land the DEP just bought was the largest watershed protection land purchase, officials said, in New York City history.

The land will stay undeveloped and might be open to recreation in the future, Aggarwala said.

With the land purchase the DEP remains Westchester County's single largest taxpayer.