On the streets of New York or Denver or San Mateo this summer, it seems the telltale cap of a water bottle is sticking out of every other satchel. Americans are increasingly thirsty for what is billed as the healthiest, and often most expensive, water on the grocery shelf. But this country has some of the best public water supplies in the world. Instead of consuming four billion gallons of water a year in individual-sized bottles, we need to start thinking about what all those bottles are doing to the planet’s health.

Here are the hard, dry facts: Yes, drinking water is a good thing, far better than buying soft drinks, or liquid candy, as nutritionists like to call it. And almost all municipal water in America is so good that nobody needs to import a single bottle from Italy or France or the Fiji Islands. Meanwhile, if you choose to get your recommended eight glasses a day from bottled water, you could spend up to $1,400 annually. The same amount of tap water would cost about 49 cents.

Next, there’s the environment. Water bottles, like other containers, are made from natural gas and petroleum. The Earth Policy Institute in Washington has estimated that it takes about 1.5 million barrels of oil to make the water bottles Americans use each year. That could fuel 100,000 cars a year instead. And, only about 23 percent of those bottles are recycled, in part because water bottles are often not included in local redemption plans that accept beer and soda cans. Add in the substantial amount of fuel used in transporting water, which is extremely heavy, and the impact on the environment is anything but refreshing.

Tap water may now be the equal of bottled water, but that could change. The more the wealthy opt out of drinking tap water, the less political support there will be for investing in maintaining America’s public water supply. That would be a serious loss. Access to cheap, clean water is basic to the nation’s health.

Some local governments have begun to fight back. Earlier this summer, San Francisco Mayor Gavin Newsom prohibited his city’s departments and agencies from buying bottled water, noting that San Francisco water is “some of the most pristine on the planet.” Salt Lake City has issued a similar decree, and New York City recently began an advertising campaign that touted its water as “clean,” “zero sugar” and even “stain free.”

The real change, though, will come when millions of ordinary consumers realize that they can save money, and save the planet, by turning in their water bottles and turning on the tap.