New York City has long been proud of its excellent municipal drinking water. Approximately 90% of that water comes from the Catskill/Delaware Watershed, which covers about 1,900 square miles (nearly 5,000 square kilometers) of rugged, densely forested land north of the city and west of the Hudson River. Stored in six hard-rock reservoirs and transported through enormous underground tunnels, the city water is outstanding for so large an urban area. Yielding 1.2 billion gal (450,000 cubic meters) per day, and serving more than 9 million people, this is the largest surface water storage and supply complex in the world. As the metropolitan agglomeration has expended, however, people have moved into the area around the Catskill Forest Preserve, and water quality is not as high as it was a century ago.

When the 1986 U.S. Safe Drinking Water Act mandated filtration of all public surface water systems, the...