From Arbor Day Site (https://www.arborday.org/trees/treefacts/): Trees help clean our drinking water.

Forested watersheds provide quality drinking water to more than 180 million Americans.
share U.S. Forest Service, 2013
In 1997, New York City spent $1.5 billion to preserve the forested watershed that supplies New York City’s drinking water by purchasing thousands of upstate acres of forested watershed. A filtration plant large enough to clean New York City’s water supply would have cost more than $6 billion dollars.
share Ecosystem Marketplace, 2006
Today, New Yorkers enjoy some of the cleanest drinking water in the world, and New York City has won regional water taste competitions.
share New York City Department of Environmental Protection, 2013