One of the sharpest exchanges in Thursday's French language debate occurred during a discussion of a topic that is on very few Canadians' political radar — bulk water exports to the U.S.

Liberal Leader Justin Trudeau accused NDP leader Tom Mulcair of favouring water exports when Mulcair was environment minister in Quebec in 2004.

Trudeau had made the same accusation during the debate sponsored by the Globe and Mail earlier in the campaign. This time, Mulcair was ready for him.

"It's completely false," Mulcair fired back. But Trudeau persisted, saying he had seen a video in which Mulcair expressed his support for selling water.

Mulcair insisted he was only trying to stimulate a debate on the issue, and he then delivered this ad hominem zinger.

You don't like debates because your staff has to write all your lines for you," he said to Trudeau. "I write my own."

Ouch.

Mr. Mulcair frequently draws on his ministerial experience to burnish both his environmental and administrative credentials, so it's not surprising that the Liberals would try to poke some holes in that balloon.

But is Trudeau's accusation fair?

The spin

There are a series of remarks that the Liberals have amassed to back up their claims that Mulcair supported the idea of bulk water exports in the past, despite what he now claims. 

The Liberals have even uploaded a video to YouTube with Mulcair talking about water exports on three separate occasions. 

Tom Mulcair on the idea of water exports

Speaking on the topic in the National Assembly on April 29, 2004, for example, Mulcair argued "this is a renewable natural resource unlike a mine. As soon as you take the ore out of the ground it's over, you can't anymore.

"But here it's water. If we manage it properly, if we take care of it as we should, why can't we even talk about it? It is this quasi-religious approach that I can't explain…"

He later told journalists that he could not understand why it would be a "mortal sin" for Quebec to export water in bulk, arguing that Canada's ban against bulk water exports "isn't religion."

The counter-spin

Trudeau is not the first political opponent to give Mulcair a hard time about past statements involving bulk water exports.

In a debate during the NDP leadership race in 2012, leadership hopeful Paul Dewar challenged Mulcair to "clarify" his position on the issue. Mulcair's defence that day was the same as the one he used with Mr. Trudeau last night.

"Come on, Paul," Mulcair responded. "I've always fought for the protection of our freshwater resources. The means of achieving that result can vary and that was the object of a discussion, a debate."

Nor is Mulcair alone in raising the idea for debate. Former Liberal prime minister Jean Chretien broached the topic himself in 2011, after leaving politics, when he told reporters: "We have to be able to debate any issue.

"We're selling oil. It's finite. We're selling natural gas. It's finite. Water, it's raining and snowing in Canada every year."

The rinse

The second half of Mulcair's response to Dewar is certainly true. He was clearly anxious to have a debate in 2004 about the wisdom of sending bulk water to the U.S.

And that does leave him vulnerable to the charge that he was willing to entertain the possibility of doing it. Though it would not have been easy.

The federal government first declared itself opposed to bulk water exports in 1987 when the issue was much in the news. And there were three high-profile cases in the late 1990s involving potential exports from B.C., Ontario and Newfoundland that were essentially nixed by public backlash.

The Canada-U.S. Free Trade Agreement in 1987 and the subsequent North American Free Trade Agreement that included Mexico in 1994 both excluded bulk water exports from their provisions.

And in 1999, the federal and provincial governments, including Quebec, entered into a voluntary agreement to prohibit bulk water exports, though Quebec and some Western provinces balked at formalizing this.

In the face of all that, and the strenuous opposition of environmentalists, it does seem rather curious that Mulcair would be musing about whether Quebec should break the voluntary agreement and sell its water.

For that reason, his claim that "I've always fought for the protection of our freshwater resources" seems a bit overblown.

Does any of this matter in 2015?

A heated exchange between Trudeau and Mulcair over water exports1:05

Given the severe drought conditions currently plaguing California, and the drying up of the Ogallala aquifer, which supplies about 30 per cent of the groundwater used for irrigation in the U.S., the pressures on Canada to export water to the U.S. are only going to intensify.

But the obstacles to bulk water exports are even greater today than they were when Tom Mulcair was trying to open up a debate in 2004.

In 2013, a private members bill (C-383), was introduced by Conservative MP Larry Miller.

The bill amended the International Boundary Waters Treaty Act to "prohibit the bulk removal of transboundary waters," which includes the Great Lakes. The Harper government threw its support behind the bill, and it passed with the support of the Liberals and the NDP. 

That still leaves the massive water resources of the North potentially vulnerable, but the fact is, no one has yet to figure out a way to make bulk water exports a viable economic proposition.

Earlier this year, an Alaskan company announced plans to start shipping tens of millions of litres of water from Alaska to Southern California this summer, but that deadline came and went with no water being moved.

"The economics are absurd," former Liberal environment minister David Anderson told the Vancouver Sun. "There are very few, if any, viable commercial schemes out there."

In the end, it really doesn't matter whether Tom Mulcair flirted with the idea of bulk water exports in 2004. Whoever becomes prime minister after October 19, our water isn't going anywhere soon.