The city added to its massive land holdings upstate yesterday with the purchase of 1,775 acres of woodlands around the sprawling reservoir system — the latest buy in a land-acquisition program to keep the drinking-water supply pristine.

The Department of Environmental Protection bought the land, which spans six counties, for more than $7 million.

The DEP said the acquisitions are saving the city from having to build a water-filtration plant that which would cost roughly $10 billion.

Since the land-acquisition program began, the city has spent $541 million to expand its watershed holdings and protect a reservoir system that produces 1 billion gallons of water a day for 9 million people.