City Trees

Removal of Fallen Leaves Can Improve Urban Water Quality The timely removal of leaf litter can reduce harmful phosphorus concentrations in stormwater by over 80 percent in Madison, Wisconsin, according to our recent U.S. Geological Survey study. Autumn leaf litter contributes a significant amount of phosphorus to urban stormwater, which then runs off into waterways and lakes. Excessive amounts of nutrients like phosphorus and nitrogen can cause eutrophication, or the depletion of oxygen in water, resulting in death of aquatic animals like fish. The USGS-led study found that without removal, leaf litter and other organic detritus in the fall of 2015 contrib- uted 56 percent of the annual total phosphorus load in urban stormwater compared to only 16 percent when streets were cleared of leaves prior to a rain event. Our study found that leaf removal is one of the few treatment options available to environmental managers for reducing the amount of dissolved nutrients in storm- water. These findings are applicable to any city that is required to reduce phosphorus loads from urban areas. We compared concentrations of phosphorus and nitrogen in stormwater from two residential catchments in western Madison that had similar tree cover. The City applied a leaf litter removal program from late September through mid-No- vember at one site but not the other. We found significantly lower amounts of phosphorus and nitrogen at the site where leaves were removed. The study also found that stormwater nutrient levels were highest during the fall months when the amount of organic debris on streets was at its peak. This finding suggests that leaf removal programs are most effec- tive during fall in Madison, and that sources other than leaves, such as street dirt and grass clippings, were likely the primary contributors of phosphorus and other nutrients during spring and summer. The efficiency, frequency, and timing of leaf removal and street cleaning are the primary factors to consider when developing a leaf management program. During 2013 through 2015, the City of Madison used municipal leaf collection and street cleaning to remove leaf litter from residential areas, and it asked resi- dents to pile their leaves adjacent to the street to limit excess debris. Leaf collection and street cleaning occurred about every seven days from late September through mid-November. Additionally, leaf blowers were used to clean debris off streets prior to storm event. Results showed more than an 80 percent reduc- tion in phosphorus load compared to no cleaning. Since then, the USGS continued to evaluate other municipal leaf collection and street cleaning programs that vary in both method, frequency, and amount of overhead tree canopy. Results of that portion of the study should be available by the end of 2020. The USGS collaborated with the City of Madison, City of Oshkosh, City of Fond du Lac, League of Wisconsin Municipalities, the Clean Lakes Alliance, Yahara WINS, the Fund for Lake Michigan, and the Wisconsin Department of Natural Resources on the study. —Bill Selbig, Research Hydrologist, USGS – Upper Midwest Water Science Center, Middleton, Wisconsin The USGS Madison, Wisconsin study found that stormwater nutrient levels were highest during the fall months when the amount of organic debris on streets was at its peak. Photo by Bill Selbig 20 CityTREES

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