According to this article by the director of the Chesapeake Bay Foundation, strategically placed ditches can play a major role in capturing and removing nutrient runoff by providing a place were these nutrients can be held until plants use them up. One of my biggest disagreements with Bjorn Lomborg in his book "The Skeptical Environmentalist" is the very high cost he puts on the remediation of nutrient pollution is bodies of water. I have not done the math myself, but given the economic benefits of having a clean Chesapeake Bay I cannot see how something as simple as ditch cannot be cost-effective. I would like to see Lomborg complete a second edition of this book that takes this new technique into account.