In this heat, one prop binds the human diversity together, however: the plastic bottle of water.

Never mind that dotted conspicuously at decent intervals all around the shady park are public water fountains. On this sultry afternoon, they are lonely and neglected, except for one where a nanny and infant are playing at putting fingers over the spout to see how far they can make it spray. A newly purchased bottle of spring water rests in a cup holder of the pram, in case either one of them should actually need to slake their thirst.

Even a decade ago, there would have been queues for the fountains. No longer. They have been rendered almost redundant by bottled water, rather as public phones have been by mobiles. We used to live perfectly well without either of these modern accessories. Today, they are vital to our happiness.

No one would suggest returning to the pre-wireless age, of course. But as the heat builds across America this summer, a backlash against packaged water is gathering. It is led by environmentalists, consumer activists and increasingly by local political leaders. The media is gulping down the brouhaha, and the beverage business, with much to lose, is starting to squeal.

The H2O militants, who include the mayors of cities such as New York, Salt Lake City and San Francisco, are asking why spend your hard-earned cash on buying fancy brands of bottled water such as Fiji, Poland Spring, Aquafina and Dasani, when the stuff that comes out of the tap at home . . .

or from the fountains in public spaces . . . is perfectly good? The quality controls imposed on public water supplies here by federal regulators are, after all, far more stringent than those required of bottled water.

But it is not just an issue of wastefulness. Rather, the principal objection to our love affair with bottled water has to do with ecology. It takes energy and oil to make those bottles, all of which contributes to global warming. And then there is the question of where you throw them when you are done.

According to most estimates, less than a fifth of empty bottles in the United States are actually returned by consumers for recycling. The rest end up in landfills where they will take hundreds of years to decompose.

New York launched a public service advertising campaign in June, reminding residents that the metropolis has some of the most pristine public water supplies in the country, all of it from the Catskill Mountain watershed, north of the Hudson Valley. Just a few weeks ago, the federal government delayed plans to build an expensive new filtration system for the city because the quality of its water right now is still unimpeachable.

Earlier this month, Mayor Michael Bloomberg, responding to a new controversy about where some of the bottled water actually comes from . . . neither springs nor glaciers . . .

repeated his pitch. "New Yorkers can turn on the tap whenever they want and fill up a glass of some of the most delicious water on the planet, " he said. "And that's something I hope New Yorkers will continue to do, because drinking tap water, instead of bottled water, is not just easier on your wallet, it's also easier on the environment."

The economics of the bottled water industry in America are astounding, and there is little to suggest that the "just say no" pleadings of Mr Bloomberg or anyone else will have any impact. In 2006, wholesale revenue for the purveyors of bottled water, including beverage giants Pepsi and Coca-Cola, topped $11bn ( 8bn). That's more than Americans pay to go to the movies every year. And the industry is set to grow at the rate of about 10% annually.

Americans consumed 8.25 billion gallons of bottled water last year. Equally unsettling is the background story of how all this water is distributed. Upwards of one billion bottles of the stuff are shipped around the US every week. That's a lot of exhaust fumes. To shift that amount of water (it is a heavy commodity) you need a fleet of no fewer than 37,800 18-wheeler lorries. It doesn't help that the most popular bottles are the small, slip-in-your-backpack size, making more plastic to chuck.

The poster-child for the packaged hydration objectors is Fiji Water, a popular US brand that stands out from others in part because it is among the most expensive and, therefore, the most chic. It is usually $1.50 for half a litre, more expensive than petrol. True, there are even more pocketburning varieties, most famously a brand called Bling H2O that sells at one suburban New York bar for $55 per bottle.

Paris Hilton (of course) has been spotted sipping Bling. No one would deny that spending that much on water is plain obscene.

Buying Fiji water is not entirely rational either, because it really does come from a remote corner of the island of the same name. First the bottles have to be shipped there. They are filled, trucked to port and then brought by ship across the Pacific before distribution across the US. The best way to expand your carbon footprint? Drink Fiji.

What, on this bright day, does Sean Gallagher, 26, an actor from Los Angeles in New York for an audition, have hiding in his shoulder bag? (Sean has been picked because he is sitting on a bench directly across from one of the Union Square water fountains. ) Oops, there it is: Fiji Water.

"I needed a container, " he blurts almost immediately.

And it is true, the square Fiji container with its patterns of South Pacific blossoms has more capacity than most of the competition.

He swears when he is finished, he will refill it with regular water. Sean admits that buying Fiji is particularly ironic in his case because he has three municipal water engineers for uncles who lecture him for ever about the purity of tap water. "I am not into this yuppy bullshit of buying Fiji because of the brand."

Sitting beside him is mathematics student Russell Avdek, 21. He has eschewed the fountain in favour of Poland Spring. He too is contrite, swearing tap water is his usual tipple. But he bought the bottle from a vendor across the park "because it's just convenient." Fair enough. What he finds "outlandish", he says, is people buying whole cases of bottled water in supermarkets to take home.

He too says he will refill from a tap.

Then there is Marilyn Smith from Nebraska, in New York on a Christian mission and found lingering by the dog run. Poking from her bag is a bottle of Nestle Pure Life.

She would drink from the fountains, she says, but fears that in "a big city like this it might not be sanitary." What she did not know, however, is that Pure Life is among a number of leading bottled water brands that are not quite what they seem.

Last week, the Pepsi company succumbed to pressure from a group called Corporate Accountability International, to change the labels on its Aquafina water, the largest-selling brand in the US. Up to now, three initials, PWS, are all that tells you that what is inside actually comes from the same sources as the water in your kitchen.

In future, the words will be spelled out in full: public water source.

Other popular brands that are little more than tap water, if refined a little with extra filtering, are Dasani from Coca Cola and, yes, Pure Life.

"That is a rip-off, " said a disgusted Ms Smith.

It is the dodgy marketing of some varieties of bottled water that has riled San Francisco's mayor, Gavin Newsom, who has just banned city employees using public funds to buy any kind of bottled water, for functions like meetings or conferences.

"We are now exposing an underbelly truth . . . a big percentage of bottled water in this country is tap water, " he declared the other day. Some of America's poshest restaurants, including Chez Panisse and the Larder, both in the San Francisco area, are also taking a stand, refusing to offer bottled water to patrons.

Rocky Anderson, mayor of Salt Lake City, a town as proud of its water quality as New York, has even gone so far as to forbid firemen from taking bottled drinking water with them on emergency calls.

There is not a major news programme in the United States that has not run a bottled-water story in the past few weeks, focusing particularly on the provenance of brands like Aquafina. "Aquafina.

"It sounds so nice, so refined, so special, " said Katie Couric on the CBS evening news the other night.

"You might think this biggest-selling brand of bottled water must come from someplace special. Well, today we found out it doesn't." Ouch.

Offended by this assault, the industry contends that consumers should be allowed the choice. "We think it's unfortunate it's turned into this either-or battle, " comments Joseph Doss, president of the International Bottled Water Association. "We do feel like we're being unfairly targeted."

This reporter, meanwhile, will return now to the Union Square and the dog run with his pooch. But, because it is still hot, first he has a purchase to make. A bottle of PetRefresh.

It is a product designed exclusively for our four-legged companions which, according to the company's website, "begins with natural water from high mountain streams of unusual purity, clarity, and beneficial mineral contents."