Ditto to the above. Mine runoff contamination is not identifiable in the field and can include a witches' brew of metals, minerals and acid. Some of California's most contaminated waste sites are old mines, and there are tens of thousands more out there.

We have one mine site with runoff so acidic it has a negative pH, something I never learned of in chemistry class. But my biggest fears are mercury and cyanide. Mercury remains in our streams and rivers from the 1850s.