"Nearly 40 years after the first Earth Day, this is irony: The United States has reduced the manmade pollutants that left its waterways dead, discolored and occasionally flammable.
But now, it has managed to smother the same waters with the most natural stuff in the world.
Animal manure, a byproduct as old as agriculture, has become an unlikely modern pollution problem, scientists and environmentalists say. The country simply has more dung than it can handle: Crowded together at a new breed of megafarms, livestock produce three times as much waste as people, more than can be recycled as fertilizer for nearby fields.
That excess manure gives off air pollutants, and it is the country's fastest-growing large source of methane, a greenhouse gas."