Filmmaker Jessica Yu harnesses the celebrity power of actor Jack Black and environmental activist Erin Brockovich - immortalized by Julia Roberts in the 2000 movie about Brockovich's work - to give the looming U.S. water crisis a thorough wringing out in "Last Call at the Oasis".

"A third of U.S. counties face water shortage by the year 2050," Yu told Reuters. "It's not really a solvable problem but we can manage it so much better."

"Last Call at the Oasis" follows environmental activists as they try to hold accountable those who contaminate the Earth's most precious natural resource - clean water.

In Las Vegas, they find a desert city is straining limited resources as it grows exponentially. Rural mid-western states are home to industrial cattle farms where tons of manure is improperly disposed, contaminating streams and drinking water. In farming communities, local towns see a spike in cancer cases after chemicals are used in pesticides.

According to Yu's research, in just 60 years the aquifer in California's Central Valley could be depleted, leaving barren an area that provides one fifth of the nation's produce.

Brockovich, who won a 1996 multi-million dollar settlement against energy giant Pacific Gas and Electric for polluting the water supply of a California town, said that water pollution is causing health issues throughout the United States.

"There are 4000 individual communities on my map now, and I can barely keep up with the incoming data," she told Reuters.

"Tropic Thunder" comic actor Black appears in a spoof commercial for bottled water, dubbed Porcelain Springs, that has been reclaimed from sewage - a concept that has been a hard sell in the United States despite being practiced elsewhere.

Singapore, for instance, satisfies 30 percent of its requirements through reclaimed water, the documentary notes.

"We're taught that in a survival situation if you don't have any water, you can drink your own urine," laughed Brockovich. "I just think none of us want to be in a position where we find ourselves drinking our urine if we can just make other options and choices now."

The sources of pollution include household products, pesticide manufacturers and the natural gas industry, to name a few. While the movie refrains from pointing the finger at any one company or group, industry representatives nevertheless declined to be interviewed for the film.

"The film is not about a bad guy," said Yu. "These industries are representative of a system that lets these things happen. We give the benefit of the doubt to industry. The burden of testing being on the producers of the chemicals - that seems like something that is fundamentally flawed."

Solutions discussed in the film also include better oversight by the Environmental Protection Agency and tighter regulations particularly on the natural gas industry and chemicals used for hydraulic fracturing, or fracking, in drilling for gas.

"Nobody wants industry and those companies to go away because these people need jobs, but they don't want you to poison them," said Brockovich.

"There's a moment here where industry does not have to be the villain. You could create jobs to better dispose of waste - how we're going to reclaim and recycle that water, so that it's usable," she added.

In a 2008 report, the UN Intergovernmental Panel on Climate Change estimated that by 2080 nearly half the world's population will be without clean water.

"We see third world countries that have these problems," noted Brockovich. "If you think it can't be us, then think again."

(Reporting By Jordan Riefe, Editing by Jill Serjeant and Bob Tourtellotte)

• A Permanent Economic Emergency