Hi Cassie,
Here's a link to a report on the pollution that affects the Chesapeake Bay:
http://www.environmentmaryland.org/reports/mde/urban-fertilizers-chesapeake-bay . The upshot is that 30% of the phosphorus load and 10% of the nitrogen load comes from turf, and the turf portion of the land use is increasing. I don't know what the figure would be nationally.
Susan
