Reducing leaf litter contributions of phosphorus and nitrogen to urban stormwater through municipal leaf collection and street cleaning practices

As the boundaries of urban land use continue to expand, environmental managers are looking for innovative ways to reduce export of nutrients from urban sources. Municipal services such as leaf collection and street cleaning have the potential to reduce nutrient pollution at its source while continuing to offer services valued by residents. This study characterized reductions of total and dissolved forms of phosphorus and nitrogen in stormwater runoff from paired catchments, testing the method and frequency of municipal leaf collection and street cleaning programs.

Overall, the performance of municipal programs was related to the frequency and not the form of treatment. Catchments receiving a weekly street cleaning by a regenerative-air street cleaner had the highest reduction in phosphorus load, ranging from 65 to 71 percent (probability value [p] is less than 0.05) for total phosphorus and 57 to 70 percent (p is less than 0.05) for dissolved phosphorus, regardless of leaf collection method or frequency. Reduction in nitrogen load was generally mixed, with many of the catchments showing no statistically significant changes after treatment. In general, nutrient concentrations, and subsequent percent reduction of nutrient loads, were positively correlated with street tree canopy. Collection of only leaf piles, leaving streets unswept, showed no significant reduction in loads of total or dissolved phosphorus and an 83 percent increase in load of total nitrogen. The majority of nutrient concentrations were in the dissolved fraction making source control through leaf collection and street cleaning more effective at reducing the amount of dissolved nutrients in stormwater runoff than structural practices such as wet detention ponds. Based on the results of this study, municipal leaf management programs would be most effective with weekly street cleaning in areas of high street tree canopy, whereas the method and frequency of leaf pile collection is of less importance to the mitigation of nutrients in stormwater runoff.