I know that this is a recent reply to an old thread, but it still seems relevant. One easy solution to the challenge of cleaning roof runoff (toxic or not) is to have the downspout empty into a rain barrel full of good quality biochar to filter the Water at the source before it is applied, then it can be attached to a hose at the bottom for cleaned Water to use in your garden. One such rain barrel I saw had collected a fist size chunk of zinc.
