Of feces and fertilizer: residential sources of urban water pollutants

A man fishes from a pond impaired from high levels of nitrogen and phosphorus in St. Paul, Minnesota. Photo by Dan Ackerman.

Hobbie, S. E., Finlay, J. C., Janke, B. D., Nidzgorski, D. A., Millet, D. B., & Baker, L. A. (2017). Contrasting nitrogen and phosphorus budgets in urban watersheds and implications for managing urban water pollution. Proceedings of the National Academy of Sciences, 201618536. DOI: 0.1073/pnas.1618536114


Do you get upset when your neighbor doesn’t clean up after their dog’s business on the sidewalk? Now there’s extra justification for your righteous anger. Poorly managed pet waste is a major source of phosphorus pollution to urban waterways, according to research conducted in St. Paul by a team from the University of Minnesota.

Lakes and rivers provide benefits to city residents, including drinking water, fisheries, and spots to cool off on a sweltering summer day. However, degraded water quality has long threatened the health of urban waters. The prime suspects? Phosphorus and nitrogen, two nutrients found in excess in cities that fuel the growth of harmful algae and bacteria. Although St. Paul banned the use of phosphorus in fertilizer for household lawns in 2004, both nutrients continue to plague the city’s waterways, which feed into the Mississippi River.

Building a budget

Led by Dr. Sarah Hobbie, the research team set out to determine the main sources of nitrogen and phosphorus to St. Paul’s lakes and streams. They constructed nutrient “budgets”, carefully accounting for rates of nutrient inputs to the urban environment, as well as rates of nutrient export into waterways. For example, they used an atmospheric transport model to estimate how much nitrogen and phosphorus enter the urban environment from rain and wind. Surveys of land managers, homeowners, and pet owners revealed the quantity of nutrients entering the landscape from activities like fertilization, composting, and, of course, failure to pick up dog poop.

Not all of these nutrient inputs end up in lakes in streams. The landscape naturally retains some nutrients before they enter the waterways. For instance, plants take up nitrogen and phosphorus, incorporating them into their stems and leaves. Wetlands can also bury phosphorus in sediments while expelling nitrogen into the atmosphere as gas. The researchers calculated the landscape’s retention of nutrients by comparing the sum of nutrient inputs to the quantity of nutrients exported to waterways, which they measured by sampling nutrients in storm drains that bring water from streets into lakes and streams.

A tale of two elements

Pet waste (a.k.a. dog poop) proved to be the primary contributor of phosphorus to the urban environment. About three quarters of phosphorus entering the city’s ecosystem came from pet waste, with the remainder coming from processes like windblown dust from construction sites. To make matters worse, the researchers found that the urban environment retained very little of this phosphorus, just 22%, before it reached the storm drains and entered waterways. Because water flows primarily over concrete and through pipes in the urban environment, there isn’t much opportunity for phosphorus to be taken up by plants or buried in wetlands along the way.

On the other hand, the landscape retained about 80% of nitrogen inputs from reaching waterways. The researchers attribute much of this retention to nitrogen leaching into groundwater or being respired by microbes into the atmosphere. Similar to the phosphorus in pet waste, nitrogen inputs came mostly from household sources like lawn fertilization, rather than from non-residential sources like golf courses or corporate campuses. About one third of nitrogen inputs came from atmospheric deposition.

Decisions, decisions…

This study demonstrates that decisions made at the household level can have a huge effect on a city’s water quality. Using lawn fertilizer more sparingly can cut the biggest source of urban nitrogen pollution. Picking up pet waste can have a similar effect for phosphorus, which is especially important since the urban landscape does a poor job of buffering waterways from high phosphorus inputs.

So the next time your neighbor neglects to clean up after their dog, don’t just complain to them about what’s stuck to the bottom of your shoe. Make sure to mention water quality as well.

This diagram shows fluxes of nitrogen and phosphorus in urban ecosystems, as measured by Dr. Hobbie’s team. The thickness of each arrow is proportional to the size of the flux labeled. More information is available in figure 4 of Hobbie et al. 2017.
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Daniel Ackerman

I'm completing a PhD in Ecology, Evolution, and Behavior at the University of Minnesota. My research subjects vary quite a bit, from the arctic tundra of Alaska's North Slope to urban lakes near my home in Minneapolis. I study how carbon and nutrient cycles in these natural and built environments are responding to human activity in a rapidly changing world. When I escape fieldwork and labwork, you can catch me canoeing in the Boundary Waters, birdwatching, or reading.

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