Permaculture in New Zealand

Hi,

I've been reading Linda Woodrow's book, The Permaculture Home Garden, in which she advocates collecting water weeds that have absorbed runoff fertiliser from farms so you ge the benefit of it for free.  I can see her rational but am concerned that this still be bringing chemical fetilisers into the garden.  Do the waterweed 'purify' the fertiliser or would this just be the organic equivalent of passive smoking? 

I'd love to hear some opinions on this as we have some great sources of pre-fertilised waterweeds nearby which I'd love to add to my compost.

Thanks