A new study from the University of Minnesota suggests that household fertilizer and pet waste are the major sources of nutrients polluting Twin Cities lakes, streams and rivers.
The study was the first comprehensive look at the sources of nitrogen and phosphorus in the Twin Cities watershed and how quickly the nutrients move across the land, ending up in the Mississippi River.
Excessive nutrients can cause water quality problems, including toxic algae blooms, said Sarah Hobbie, the lead researcher on the study and a professor in the university's Department of Ecology, Evolution and Behavior.
"Despite long-running efforts to clean up those nutrients, we continue to have poor water quality in many of our cities, including the Twin Cities," Hobbie said.
The study focused on seven sub-watersheds of the Mississippi River in the St. Paul area. Hobbie's team used past work it had done in the Twin Cities, including household surveys that asked about fertilizer use, pet ownership and waste pickup habits.
They also used information about the average number and weight of pets owned by Twin Cities residents, the amount of nutrients in pet food. They also looked at fertilizer use on golf courses and college campuses.
The researchers concluded that urban watersheds are "leaky," meaning they are losing most of the phosphorus and nitrogen that flow into them every year. That's likely because of the dense network of streets and storm sewers in urban areas, Hobbie said.
"Once that phosphorus is in the street, there's nothing to hold onto it," she said. "So it's just readily washed into the storm drain system and into our surface waters, unless there's some kind of storm pond that's going to trap that phosphorus before it moves downstream."
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Phosphorus and nitrogen are probably getting into streets through leaves falling from trees or from snow melting off people's yards, Hobbie said. Trees growing next to the street act like pumps, taking up nutrients from the soil and moving them into their leaves, she said.
The trees reabsorb about half those nutrients before dropping their leaves into the street, where the nutrients are washed into the storm drain if they're not picked up by a street sweeper. Cities can help prevent nutrient pollution by sweeping the streets more often, Hobbie said.
The dominant source of nitrogen in the urban watershed is household fertilizer use, which is 10 times the amount of commercial fertilizers used by golf courses and college campuses. Reducing excessive fertilizer use is another thing people can do to help reduce nutrient pollution, Hobbie said.
The study zeroed in on pet waste as the biggest source of phosphorus in the urban watershed. People are generally good about picking up waste while walking their dog, but they often aren't as diligent about cleaning up the backyard, Hobbie said. Rain gardens also help by capturing water and filtering it before it goes into streams and lakes.
While this study focused on urban watersheds, Hobbie noted that they are a relatively minor source of nitrogen and phosphorus in the overall Mississippi River watershed compared to agricultural land.