Today in class, I asked the question I asked on Ianqui's blog: When it snows in New York City, where do they put all the snow? It just doesn't look like there's room between the buildings to put it all.
Students from the city were eager to explain the process. It gets plowed into piles, then hauled away with construction-type vehicles, big dumptrucks and such. Eventually, it gets loaded onto barges, hauled out into the ocean, and dumped. If you walk down to the piers on a winter day, you can watch this. They can't dump it into the Hudson because that would create glacier-like obstacles.
The snow that falls in a heavily urban area picks up pollutants on its way down. So even if it looks pure and white, it isn't.
That's what my students tell me.