The timely removal of leaf litter could reduce harmful phosphorus concentrations in storm water by over 80% in Madison, Wisconsin, according to a recent U.S. Geological Survey (USGS) study.

Autumn leaf litter contributes a significant amount of phosphorus to urban storm water, which then runs off into waterways and lakes. Excessive amounts of nutrients such as phosphorus and nitrogen can cause eutrophication, or the depletion of oxygen in water, resulting in the death of aquatic animals.

Leaf removal is one of the few options available to environmental managers for reducing the amount of dissolved nutrients in storm water. Image credit: USGS. “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,” says USGS scientist Bill Selbig. “These findings are applicable to any city that is required to reduce phosphorus loads from urban areas.”

During April through November of 2013 through 2015, scientists compared concentrations of phosphorus and nitrogen in storm water 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-November at one site but not the other.

The researchers found significantly lower amounts of phosphorus and nitrogen at the site where leaves were removed. The study further determined that, without removal, leaf litter and other organic debris in the fall contributed 56% of the annual total phosphorus load in urban storm water compared to only 16% when streets were cleared of leaves prior to a rain event.

The study also concluded that storm water 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 effective 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,” Selbig says.