French scientists are investigating the ways plants can be used to purify water [from the Guardian Weekly 14/1/11], using plant fibres in the roots to digest or trap contaminants. See Flickr pages 1 & 2.
Compare the way nettle beds and reed beds have been used in this country. ICI’s factory at Billingham used to discharge chemicals to the sea until they installed a reed bed instead, in 1990-91. Now it is a famous wildlife reserve! This subject is known as phytoremediation.