Agricultural runoff

From Appropedia

Agricultural runoff of water from fields often contains fertilizer or livestock manure which becomes a problem when it reaches the ocean and creates dead zones ( I would like to start a discussion on how to solve this problem.

Usually water leaving fields collects and runs down ditches next to roads or through other small waterways before flowing into creeks and rivers. This seems to be an opportunity to install filters to remove the problem compounds. I have no idea what such a filter would look like: it would need to deal with a fair quantity of water moving at a good clip, especially in areas where there is a spring runoff from winter snows or where there are heavy rainfalls. Since we are dealing with dissolved compounds rather than particulates the usual sort of filter likely wouldn't work and chemical processes would need to be used. Pollutants arising from chemical reactions would need to be avoided and we would need a good way of disposing of any resulting material. We may as well think about how to remove herbicide and pesticide residues as well.

This would not do anything about water moving underground but it would reduce problem compounds reaching the ocean considerably.

I would like to invite anyone who has any ideas about this, especially if you have any expertise in chemistry, but even if you don't, to leave your thoughts on the discussion page.