Originally posted on 09.12.16 by William Selbig

William Selbig
U.S. Geological Survey – Wisconsin Water Science Center
8505 Research Way
Middleton, WI  53562
608-821-3823
wrselbig@usgs.gov

Organic detritus and particulate matter can be major sources of phosphorus and other nutrients in the urban environment, especially in areas with dense overhead tree canopy. In order to meet impending regulation to reduce phosphorus loads, Wisconsin cities will require information on structural and non-structural Stormwater Control Measures (SCMs) that target organic detritus and particulate matter. Implementing a city-wide leaf collection and street cleaning program is one way to reduce organic detritus and particulate matter in urban areas before it becomes entrained in runoff. Non-structural SCMs may have advantages over structural SCMs because most Wisconsin cities already conduct some level of leaf collection and existing street cleaning programs, thus may benefit from increased phosphorus reduction with minor changes to existing practices; however, few studies have quantified the water-quality benefits of such a program.

The U.S Geological Survey measured the water-quality benefits of a municipal leaf collection program coupled with street cleaning in Madison, WI during the months of April through November of 2014 and 2015. The calibration phase of the study (2014) characterized nutrient concentrations and loads in runoff from two paired basins (each less than 15 acres) without leaf collection or street cleaning. During the treatment phase (2015), street cleaning was done in the test basin by city personnel on a weekly basis from April through September with leaf collection added in October and November. Additionally, prior to each precipitation event, USGS personnel removed as much organic debris from the street surface as reasonably possible. The control remained without street cleaning or leaf collection for the entire monitoring period.

During the summer, street cleaning alone did not significantly decrease nutrient load in runoff. During the spring and fall, however,  leaf collection and street cleaning was able to remove the increased amount of organic debris and leaf litter from the curb and street surface which resulted in statistically significant (p<0.05) reductions in phosphorus and nitrogen loads. In the spring, weekly street cleaning was able to reduce total and dissolved phosphorus and nitrogen loads by 45 to 50 percent. As the mass of leaf litter on streets increased in the fall, so too did the removal efficiency with percent reductions ranging from 70 to 80 percent.

In the control basin, 56 percent of the annual phosphorus load occurred in fall (winter excluded) compared to only 16 percent in the test basin. The majority of phosphorus was in the dissolved fraction making source control through leaf removal one of the few treatment options available to environmental managers when reducing the amount of dissolved nutrients in stormwater runoff. Subsequently, the efficiency, frequency, and timing of leaf removal and street cleaning are the primary factors to consider when developing a leaf management program.

Results of the study have been summarized in an article contained in the journal Science of the Total Environment, available here.