Genetic modification to the rescue! Would be even more valuable if they can do it for toxic metals like arsenic and mercury.

I'm not sure where I read it, but I do remember that plants such as broccoli and similar species can suck out heavy metals from the ground. Not sure how much or how fast, though.

Mustard plants are the ones that readily come to mind; apparently it's called phytoremediation.

Research has already been done on phytoremediation of heavy metals using mustard plants and bioaccumulation with various species of fungi.

Bacteria are already being used to extract economically significant amounts of metals from ore bodies and mine waste.

ATW, up to 25% of the world's supply of new copper is now produced this way.

http://en.wikipedia.org/wiki/Biomining

That Wikipedia article is the least-sourced one I've seen in years. It has one cite, from "June 1994". Not to contradict you, and I'm sure that biomining is important, but I thought it was interesting to see a Wikipedia article like that. Biomining must not be a controversial topic!

That's really unfortunate. I was having visions of literal gold farming becoming a viable real-life vocation. :) 500 trees per ring is probably too inefficient to make it worthwhile.