It could one day be Alberta's very own Lake District, a recreational haven complete with campgrounds, boating, fishing – even swimming.

Or it could turn into a landscape of ponds sullied by toxins and oil, a malingering presence left by an industrial experiment gone wrong.

It may take a century to find out what is left around Fort McMurray. Because the lakes, 30 of them, will be built by Canada's oil sands industry. When the companies mining heavy crude from northeastern Alberta finish their work, they intend to pump water into old mine pits, some with toxic effluent at their bottoms, before leaving the area to biological processes to restore it to health.

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If those plans go according to form, oil sands miners will leave behind broad stretches of new shore perched beside waters clean enough for fish and people. Success on this front would vindicate industry pledges to limit the mark it leaves on an area now home to sprawling open-pit mines.

But the coming lake district also highlights the scale of the ecological gamble under way in the province. The 30 bodies of water will be what are known as end pit lakes, left behind because it's less costly to fill a mine with water than dirt. Their size and scale are laid out in a new document produced by Alberta's industry-funded Cumulative Environmental Management Association.

The document makes clear the vast permanent environmental imprint being contemplated by one of Canada's most important industries.

It also outlines the uncertainties attached to plans for so many lakes, even though companies have yet to build their first one in the oil sands. Industry has done trials and is confident they will work. At least some of the science, however, can't be done until the first lake is actually built, and it will take decades to measure and assess its effectiveness.

"This is a total crapshoot, in the sense that no one has ever done this before. But really, what are your options?" said one person familiar with the report, which will be released this week.

Perhaps the only certainty is that the lake district will cover substantial ground. The exact size is subject to changing mining plans – including some from projects not yet built – but it seems likely they will take up, in aggregate, more than 100 square kilometres as part of a series of artificial watersheds spreading over 2,500 square kilometres. By comparison, Toronto occupies 630 square kilometres.

The lakes are a project that will engage several generations. Each stands to take a century of work to plan, mine out and establish into a functioning ecological feature. From the moment workers end mining and begin filling the lakes, it could take fully 40 years before governments begin certifying them as environmentally sustainable, the 436-page CEMA report estimates.

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The report is also a striking look at how much remains unknown about lakes that will, in some cases, bury mine effluent called tailings. Those tailings are largely sand and clay, although they are laced with hydrocarbons, salts and naphthenic acids sufficiently toxic that they cannot be released into the environment. In an estimated 50 per cent of the end pit lakes currently contemplated, those tailings will be placed at the bottom of the pit, before being covered with a "cap" of fresh water mixed with dirty mine-processed water. The expectation is that the thicker tailings will remain on the bottom and solidify over the years, while biological processes will work to remediate the water above it.

But without a functioning example to base science on, "scientific uncertainty is high" around the lakes, and there is a likely "potential for substantial negative and environmental and/or social impacts," according to the CEMA report.

The very first end pit lake will begin operations late this year or in early 2013, when Syncrude Canada begins pumping fresh water over 40 vertical metres of mine effluent that it has deposited in what it calls "base mine lake." Syncrude has spent two decades studying end pit lakes, with smaller experimental bodies on its site, although this is its first commercial-scale operation.

Warren Zubot, a senior Syncrude engineering associate, said that in one experimental pond, the dirty water cleaned up so fast it could support rainbow trout in a year.

Syncrude expects its much larger base mine lake to be clean enough to hand back to the province in 25 years – better than the 40 to 50 the CEMA document predicts. But Mr. Zubot acknowledged it's hard to know.

"The major purpose of this [base mine lake] demonstration is to help understand and define these timelines," he said. "We're very confident this technology is going to work."

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Syncrude is also researching methods to help better clean up water flowing out of the lake, like using petroleum coke as a kind of industrial-sized Brita filter.

Still, many in both the research and local communities are skeptical. The oil sands "lake district" – a reasonable description, since those 30 lakes will form part of a broad water system connected together, or in "communication" – will appear on a landscape now dominated by muskeg and forest.

"It's going to be a completely alien landscape," said Peter Fortna, who has worked on environmental issues for the Métis of Fort McKay, a community at the heart of the oil sands development. Local residents aren't convinced end pit lakes are a good choice for anyone but the companies involved.

"They try to find the cheapest option as opposed to the best option," Mr. Fortna said.

And the science is still out on whether they will work – or leave a toxic legacy, notes David Schindler, a respected scientist at the University of Alberta. No further end pit lakes should be approved, he said, until "we must have some assurance that they will eventually support a healthy ecosystem." There is to date, he added, no "evidence to support their viability, or the 'modelled' results suggesting that outflow from the lakes will be non-toxic."

Other types of mines have used end pit lakes with mixed results. Some have become vibrant recreational areas, even prized fishing spots. Others, like Berkeley Lake in Montana, have turned into toxic soup, killing birds and requiring expensive sustained cleanup work.

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Among the unknowns in the oil sands: Will tailings buried beneath water stay there? Probably. But maybe not always. Wind can stir them up in shallow areas. And past experience has seen oil left in the tailings come floating to the surface, such as "the sudden occurrence of bitumen slicks on Suncor's 20-year-old sustainability ponds after years of bitumen-free surface conditions," the CEMA document notes.

And if water quality doesn't reach sufficient quality to sustain fish? The document suggests companies compensate "by planning to perpetually stock the lake or by planning to introduce heartier, less-sensitive species." That may be needed, anyway, since biological processes in the lakes could cause "fish winterkill."

To overcome some potential pitfalls for the oil sands, CEMA makes 14 recommendations, calling on industry to invest in the science needed to understand each lake, share technological developments and commit money to decades-long monitoring. Money is a critical issue: "Expect higher closure costs than initial estimates: in some cases, perpetual water treatment systems may be required to meet closure criterion," the document warns.