The Great Lakes hold six quadrillion gallons of water. That’s 20 percent of the world’s fresh surface water. As scarcity grows, there’s concern more and more people are eying that water — it’s been likened to death by a thousand straws.

“We are leaving the century of oil and we are entering the century of water,” said author Peter Annin. “And so in the next 100 years and beyond, I really do think it’s going to be all about water. We really don’t know how much pressure will come in the future on the Great Lakes.”

A historic compact designed to protect the Great Lakes against that pressure from diversions was signed into law in 2008.

Now it’s facing its first big test in a thirsty suburb of Milwaukee.

To get a sense for just how vast the lakes are, and what is at stake, I went out to the shores of Lake Michigan with Joel Brammeier, head of the Alliance for the Great Lakes. We saw a tiny corner of Indiana, and then just the blue horizon.

“It’s pretty dramatic,” Brammeier said.

“When you stand on the shore of the Great Lakes, you absolutely believe that there’s no way this water could ever all be used up and yet we’ve seen things play out in the Southeast and Southwestern United States where resources that were thought to be inexhaustible eventually found their bottom. We don’t want to get anywhere close to that here in the Great Lakes basin.”

Consider the mighty Colorado River, where so much water’s diverted for irrigation, the river often slows to a trickle by its end.

That’s why the Great Lakes compact became law. It’s part of an agreement between the eight Great Lakes states and two Canadian provinces to decide who gets Great Lakes water. It’s like this invisible international shield that keeps Great Lakes water inside the basin.

What helped spur the agreement was a fear that Great Lakes water would end up around the world, or out West, filling swimming pools in the desert.

“To people in the Great Lakes region, suggesting that that water could be transferred to other parts of the continent is like someone suggesting that the Rocky Mountains could be transferred to other parts of the United States,” said Peter Annin, the author of “The Great Lakes Water Wars.”

That fear has a basis in fact. In 1998, Canadian officials OK’d a proposal to let the Nova Group ship tankers of water to Asia.

“It was seen as the nightmare potential legal precedent because if you can send Great Lakes water to Asia, where can’t you send it?” Annin said.

Lawmakers realized they had nothing on the books to stop it. After years of negotiations, they announced their agreement. Annin says it did more than just prevent diversions. It contained provisions to conserve water and protect the ecosystem.

“It wasn’t just copying other compacts in the past,” he said. “What this compact was trying to do was recognize the environment had a seat at the table, and that humans weren’t just carving up the water for human’s sake.”

“The compact sets a very high bar for decisions to be made about who gets Great Lakes water and who doesn’t,” said Joel Brammeier from the Alliance for the Great Lakes. “If you don’t have strong legal protections in place for Great Lakes water, anybody can come and make a claim and say I want some of that.”

But for now, it’s not Asia or the parched West going after Great Lakes water — it’s our own suburbs. The compact is getting its first major test in Waukesha, a city just outside the basin. Waukesha is draining down its aquifer. It’s had to build an elaborate system to make its water drinkable.

Dan Duchniak, the general manager of the Waukesha Water Utility, said they have to add a slurry of chemicals here in the well to filter out radium. That’s a radioactive element that can increase the risk of bone cancer.

“As you draw down deeper and deeper in these aquifers, you get to a point where it’s brackish water or higher levels of salt,” Duchniak said.

“Do you have any wells that aren’t a problem?” I asked.

“No,” Duchniak said. “Simple answer? No.”

Under the compact, towns outside the basin aren’t allowed to get Great Lakes water. That’s because the basin is like a giant bathtub, and outside of it, water flows away, so water is lost to the Great Lakes. The compact was designed to keep water in.

There are a few exceptions. Towns that are right on the basin line or like Waukesha – in a county that is – can apply, but even then it’s a tough process. To get a sense how tough that process is, I went to the Wisconsin Department of Natural Resources. Eric Ebersberger showed me Waukesha’s application.

“Oh, we’ve probably got 8 inches of documentation here,” he said.

“And this isn’t everything?

“No, it’s not everything,” he said. “[The] significance is, it’s the first application for a diversion to a community within a straddling county. So a lot of people see it setting a precedent for future such applications if there are any.”

Waukesha will need approval from all eight Great Lakes governors. From now on, every town that wants to divert Great Lakes water is going to have to go through something like this.

But does that mean the compact is ironclad? An upcoming Natural Resources Defense Council report finds some states already missed deadlines for water efficiency and conservation goals. Several environmentalists worried that could put the compact at risk, as water scarcity increases.

Henry Henderson‘s director of the NRDC’s Midwest Program:

“Clearly Congresses can undo what they have done,” he said. “What becomes vulnerable is having a lackadaisical, tattered and underinvested set of institutions. Things that work right don’t invite reform.”

A law professor at Wayne State University, Noah Hall, thought it was “very” unlikely though, that the federal government would tamper with the compact:

“There are dozens of interstate water compacts,” Hall said. “Some have been in existence for almost 100 years. And Congress has never disturbed the settled agreements in any of those compacts.”

And if the states do fall behind, he said, there’s a tried-and-true remedy: lawsuits.

“You rarely get environmental protection without citizens going to court and enforcing it.”

Citizens can be powerful outside of court, too. When the Nova Group wanted to ship water to Asia, it was public outcry that turned the tide. Either way, along with politicians and advocates, vigilance must also come from citizens who have made their lives on the Great Lakes, like a group of retirees who keep a close eye on Lake Superior.

“I know everyone is trying to take our water away from us,” said one of the men. They gather every morning for coffee near the lake’s western border.

“You’ve got to watch these governors from Michigan and Wisconsin and Illinois and Minnesota,” one of the other men said. “Once they get in a group in a room together, a lot of things can happen. They’ll go right through their Congressmen and within two years, you can have water trailing out of here faster than you can make it.”

The compact was designed to prevent that — even in the face of climate change and exploding population counts — if everyone does their part or is made to do so in court.

For more information about the compact or Waukesha’s application:

Council of Great Lakes Governors

Peter Annin’s The Great Lakes Water Wars

Waukesha’s application

Waukesha water history

The compact’s legislative history

For an international take on water issues, see Worldviews stories on Pakistan’s flood victims and China’s water diversion.