Trees are good, right?
Statewide phosphorus reduction credits for leaf collection
Urban trees provide many benefits to our communities. They help us save energy, reduce noise, and improve air quality. Trees are aesthetically pleasing, can increase property value, and provide natural homes for insects, birds, and other wildlife. Trees are also an important part of the earth’s water cycle. Transpiration from plants and trees is released into the atmosphere, and later becomes precipitation. The rain and snow return valuable moisture to our crops and forests, and the cycle continues.
However, trees can cause problems for our lakes if the leaves that fall from them each autumn are not regularly removed from streets and parking lots. When leaves collect on streets, they create a phosphorus-rich “tea” whenever it rains. The rain water passes through the leaf litter, and allows phosphorus to drain from the leaves. The leaf tea washes into storm drains and flows directly into our lakes, causing water quality to deteriorate.
Phosphorus is a naturally occurring element, found in leaves, dirt, manure, and other organic matter. It acts as a nutrient to the land, but is the main cause of water quality problems in our region. When high amounts of phosphorus enter our lakes, it can fuel toxic cyanobacteria (blue-green algae) blooms, which can harm people and animals. By improving leaf management efforts, we can reduce the amount of phosphorus that reaches our waters.
In 2017, the U.S. Geological Survey (USGS) expanded ongoing research to determine how much phosphorus could be diverted from local waterways by removing leaves from the street. The project worked in cooperation with the Wisconsin Department of Natural Resources (Wisconsin DNR), and received funding support from the Fund for Lake Michigan, Yahara Watershed Improvement Network (Yahara WINS), the City of Madison, Madison Area Municipal Stormwater Partnership (MAMSWaP), and $10,000 from Clean Lakes Alliance. Clean Lakes Alliance also served on an advisory group to help oversee the effort. In turn, the Wisconsin DNR adopted a statewide phosphorus-crediting system, which will allow municipalities to count leaf management initiatives toward compliance with stormwater permits.
Why is this important?
Numerous studies have researched sources of phosphorus in urban locations. Many agree that in areas of dense overhead tree canopy, organic debris can act as a major source of phosphorus. End-of-pipe measurements demonstrate high concentrations of phosphorus loading in urban stormwater coinciding with leaf accumulations on streets.
The multi-year study conducted by the USGS in Madison, Wisconsin, quantified phosphorus reductions through leaf collections and street sweeping. The study used a number of test and control sites to compare phosphorus concentration levels in stormwater. Results showed that more than 50% of the annual amount of phosphorus in urban stormwater can come from leaves in the street.
The future of leaf management and our lakes
Removing leaves from the streets, especially before a precipitation event, is one of the best ways we can positively impact our lakes. However, keeping our streets leaf-free is a shared effort. Clean Lakes Alliance is working hard with our community this fall to keep our streets leaf-free and our waters clean. Many small actions help create sustainable change for our watershed.
To learn more about leaf management rules and guidelines in your community, visit: cleanlakesalliance.org/leaves.