I would say that at the current rates of river flow and of seepage, it's probably a very small part of the overall picture, compared to the airborne and surface runoff problems we've documented. With respect to the tailings ponds, however, the scenario I dread would be a tailings pond rupture where several million litres might hit the river at once, particularly under ice.
There was one spill in 1982 of only.... I believe 50 million litres were released into the river, and because it was impossible to clean up under ice--and it still is impossible--that spill made it all the way to Lake Athabaska. If something the size of Mildred Lake were to breach the wall of that dike under winter conditions, I'm sure we'd see the effects of that spill all the way to Great Slave Lake and the McKenzie.
That's not unheard of. A year ago, I had some graduate students look for evidence of tailings ponds breaks and breaches of the walls in the last 20 years. They came up with 184 incidences around the world.