The major pollutants that flow from Twin Cities neighborhoods into the Mississippi River can be traced back to three of the characteristics that define cities: lawns, pavements and dogs.

In one of the most the most detailed studies yet of phosphorus and nitrogen in an urban area, researchers at the University of Minnesota tracked where the pollutants come from and where they go in the seven small watersheds that make up the Capitol Region Watershed district in St. Paul.

They found that 76 percent of the phosphorus, the nutrient that turns lakes green and scummy, comes from pet waste — a number they calculated by estimating the number of dogs that live in the area, how much they eat, and how much they leave behind. Leaves, clippings and yard waste provide much of the rest.

Nitrogen, which contaminates drinking water and helps create the so-called "dead zone" in the Gulf of Mexico, comes from the atmosphere, car and truck emissions — and mostly from the relatively few residents who go overboard with fertilizing their lawns. And thanks to pavement, roofs and other hard urban surfaces that rush water as quickly as possible out of town, a good portion of both ends up in groundwater and the Mississippi River.

In short, "Every home in Minnesota is waterfront property, whether the owners know it or not," said Trevor Russell, program director for the Friends of the Mississippi River. "What we do at home influences water quality all around us."

Which is another way of saying that virtually all of the contaminant load comes from the way residents live and manage their property, which makes up 70 percent of the land in the watershed. Industry, commercial enterprises, golf courses and local government properties provided very little of the loads.

Stopping the pollution

Identifying the sources of the pollution — the input — is the first step toward identifying how to stop it from getting into the water — the output, said Sarah Hobbie, the University of Minnesota ecology professor who led the study. It is the first time a study has figured out the entire "budget" for urban pollutants, a calculation that can be used by urban watersheds across the country, she said.

Urban areas contribute a small percentage of the total load that goes into the Mississippi along its entire length, Hobbie said. Runoff from farmland, which makes up most of the Mississippi River basin, contributes about 60 percent of the nitrogen that creates the dead zone in the Gulf, according to other analyses. Wastewater treatment plants and other industrial sources produce a large percentage as well.

But household sources do have a major impact on water quality close to home — the lakes and streams that urban residents use the most, Hobbie said.

"The green lakes we see in the city are green because of phosphorus from the city," Hobbie said.

And, not surprisingly, most of the output comes via storm sewers. In fact, most of the phosphorus produced in the watershed, everything from pet waste — both kinds — lawn clippings, yard waste, and leaves that fall to the pavement from trees, ended up in the water rushing below the streets. That's the opposite of what happens in more natural environments: Phosphorus tends to stay in the ground, attached to soil or as part of organic matter that slowly degrades where it falls.

"We engineered these [storm sewer] systems to get rid of water because we don't want flooding, but we created a system that transports phosphorus really easily," she said.

Nitrogen, however, tends to stick around more, she said, and contaminates groundwater that eventually makes its way to the Mississippi. The primary source is household lawn fertilizer — an amount that exceeded the total used on golf courses, cemeteries, parks and campuses combined.

Hobbie said that her earlier research on Twin Cities neighborhoods found that only a fifth of homeowners were responsible for 70 percent of fertilizer inputs. Over the course of the summer, they distribute far more than their lawns can absorb, and the rest of it washes down the storm sewer or seeps into the ground.

Dog droppings: A puzzle

Solving the problems is, of course, a daunting task. Take pet waste. It's impossible to know how much gets picked up and how much gets left behind, or how promptly homeowners clean up their own backyards.

It's not exactly a well-researched issue. One survey around Chesapeake Bay found that 40 percent of the dog waste stayed where it landed in the first place.

But no matter how good people are at picking up after their dogs, there's still the problem of urine, which generates just under half of the pollution they leave behind. So preventing waste from running off into urban streams might be the next challenge.

"We don't have a lot more room to reduce inputs into these watersheds," Hobbie said. "We could maybe make a little headway with pet waste. But now we have to figure out how we manage these impervious surfaces we put in everywhere."

Now, cities usually sweep the streets of leaves once in the fall. Increasing that to four times would decrease the phosphorus output by 23 percent, Hobbie said. And that would be cheaper than building stormwater retention ponds and other phosphorus removal systems.

Still, that's a hard task as well. Leaves drop at different times of the fall, rains come unexpectedly, and neighbors can get annoyed at the parking restrictions.

"Street sweeping is a challenge," said Mark Doneux, executive director of the Capitol City Watershed District.

Josephine Marcotty• 612-673-7394