The recent decision by eight U.S. governors to grant a small Wisconsin town access to the Great Lakes water basin has sparked concern about the precedent this may set for other thirsty towns and cities.

It also prompted a dire warning by at least one vocal Ontario mayor, who said extending access to the water basin threatens the future of this fresh water supply.

"If you open it up to one, how do you then deny it to, let's say, the State of California, which is in a drought condition," Leamington, Ont., Mayor John Paterson said to CBC News. "If this continues, the Great Lakes won't be very great anymore. They'll be gone."

The agreement with Waukesha, Wis., a city of 72,000 people, isn't exactly going to dry up Lake Michigan. The amount of drinking water being allocated, about 31 million litres a day, may sound like a lot. But roughly 40 billion litres of water a day come out of that lake, based on 2014 data compiled by the Great Lakes Commission.

(Gregory Bull​/Associated Press)

This means Waukesha's take will represent roughly 0.07 per cent of the total. Meanwhile, the city has promised to return the same amount back to the lake as treated wastewater.

The world's largest freshwater lake system

The Great Lakes and St. Lawrence River, taken together, are considered the world's largest freshwater lake system, and hold around 20 per cent of the world's supply of surface freshwater. The water supplies eight states — Illinois, Indiana, Michigan, Minnesota, New York, Ohio, Pennsylvania and Wisconsin — and two provinces, Ontario and Quebec.

The total amount withdrawn from the whole basin is roughly 160 billion litres per day, based on 2014 figures. Public water supplies made up only 13 per cent of that amount, with the largest quantity — almost 70 per cent — going toward cooling for thermoelectric power production. Ontario is the biggest water guzzler, accounting for 41 per cent of the total withdrawal.

Much of the withdrawn water eventually makes its way back into the basin. For example, despite the 160 billion litres used a day, the reported water loss to the basin was roughly 1.4 billion litres annually in 2014, and marked a 56 per cent decrease from the 2013 net water loss of 3.2 billion litres.

Access to this abundant source of fresh water is limited to those communities within the Great Lakes watershed; most diversions across the watershed boundary are prohibited. However, there are cities that straddle the watershed line — including Waukesha, which is located only 27 kilometres from Lake Michigan.

Water use is regulated by the Great Lakes-St. Lawrence River Basin Sustainable Water Resources Agreement, a deal hammered out by the eight states and two provinces in 2005.

'Stringent regulatory regimes'

Back in 1985, those same jurisdictions forged the Great Lakes Charter, a good-faith deal rather than a legally binding agreement.

Then, in 1998, Ontario granted a permit that would have allowed water tankers to take drinking water to China, sparking a massive outcry.

While those permits were eventually dropped, the incident demonstrated that the charter had no teeth, and prompted negotiations for a binding agreement on large scale diversions and bulk water removal, explains Theresa McClenaghan, executive director and counsel of the Canadian Environmental Law Association.

That led to the 2005 agreement (which wasn't ratified until last year), which included limited exceptions for water diversion.

'We do now have a [relatively] stringent regulatory regimes dealing with water taking," McClenaghan says.

The Great Lakes are surrounded by two provinces and eight states that have rights to the watershed, but a new decision may change access rights 2:18

Waukesha is the first community to request water under that limited exception provision. While Ontario and Quebec are part of that resources agreement, only the states who are signatories to the agreement were given legal authority to consider the request for U.S. water diversions. (Ontario and Quebec were allowed to provide input.)

Waukesha is under court order to address the radium contamination of its groundwater wells. But Keith Brooks, campaign director for Environmental Defense Canada, says his organization doesn't believe the city's application met the requirements to take water from Lake Michigan.

(Michael Dalder/Reuters)

An applicant must demonstrate that seeking water from the basin is a last resort — that there are no other options available. Environmental Defense Canada believes Waukesha failed to demonstrate that need.

"Everybody would probably love to take that water, because water is probably in short supply in lots of places," Brooks says. "The body was created to manage the resource, to protect and conserve it."

McClenaghan, of the Canadian Environmental Law Association, says they too were very disappointed that the first applicant for a water diversion was granted access.

The Fox River flows through downtown Waukesha, which says its groundwater is contaminated with radium. (John Flesher/Associated Press)

She isn't particularly concerned that jurisdictions far from the Great Lakes, like California, will come seeking water, saying that the energy costs involved in transporting water over those distances make it economically unrealistic.

She also takes comfort that a return flow requirement was part of the 2005 agreement: "Not only that we would have an agreement and getting a definition about water bulk removals and diversions, but then starting to set up some of the parameters so if you had [diversions] you'd have return flow."

But, McClenaghan says, "We certainly hope it will prove the true exception to the rule, not turn out to be a pattern in the future."