Iowa has a poop problem. The Hawkeye State’s pigs, cows and chickens produce about as much waste as 134 million people — nearly the population of Russia. Most of that manure is spread onto fields as fertilizer, where significant amounts of it wash into Iowa waterways. The city of Des Moines uses one of the most expensive nitrate removal systems in the world to make its water supply from the Raccoon River safe to drink. “We have to ask if we can reconcile our water quality goals with the idea that we can continue to expand the livestock industry,” said Chris Jones, a professor and water quality...