As often happens when they do this sort of thing, they took some samples of the ground, and it turns out it's a horrible mess. Gasoline, diesel, motor oil, PCBs, asbestos, mercury, all it lacks is a little radioactivity and maybe some pathogens and it'd cover all the bases. This is actually not too surprising, since the site used to be a lake (now completely swallowed by urban plumbing), and in fact that whole area is polluted. The Lachine canal, which passes nearby, was opened for pleasure-boating only once it had been established that all the above nastiness was in the muck on the bottom and unlikely to be disturbed. There's a strip of parkland on one side, and on the other is what used to be some pretty sketchy housing, now being replaced by upmarket condos. What the new tenants of the condo think of the toxic waste reclamation site facing them across the canal I don't know. It just looks like a fenced-off grassy berm, with a little museum of sorts explaining how the cleanup works.
What I find most surprising about all this is the origin of the pollution. I associate pollution with heavy industry - silver mines, smelters, pulp mills. But there's none of that here in Montreal, and there never really was. In fact those tend to have their own dedicated waste treatment plants that do a pretty good job of cleaning up after them (at least here in Canada). What caused the pollution in this area seems to be largely the rail yards - a century of variously leaky and dilapidated rail cars filled with any old thing, sitting on sidings, dripping away. There's no treatment system set up for that, and so all the accumulated foulness seeps into the soil.
For the most part this sort of soil contamination in an urban setting is fairly benign - if there's no construction going on, the pollutants tend to just stay put in the soil. The one exception to watch out for is gardens. If you grow food in soil full of mercury, well, the food is liable to have alarming levels of mercury in it. Unfortunately a number of community gardens - otherwise a wonderful idea for a city of apartment-dwellers - have been found to have contaminated soil.
I grow my plants in pots.