Evaluation of leaf removal as a means to reduce nutrient concentrations and loads in urban stormwater
Graphical abstract
Introduction
Excessive amounts of nutrients in stormwater runoff, such as phosphorus and nitrogen, have long been identified as accelerating the effects of eutrophication in urban streams and lakes (U.S. Environmental Protection Agency, 1972, Browman et al., 1979, Schindler, 2006, Smith et al., 2006, Carpenter, 2008, Lusk and Toor, 2014). Unlike an undisturbed terrestrial ecosystem, urban watersheds can dramatically increase the export of phosphorus to receiving waters (Duan et al., 2012). With the conversion of rural to urban landscape comes a proliferation of impervious surfaces creating a directly connected pathway by which pollutants can migrate from source to stream in what has been coined the “urban stream syndrome” (Walsh et al., 2005, Meyer et al., 2005, Wallace et al., 2008). The replacement of natural drainage networks with urban conveyances has dramatically increased streamflows and altered subsidies and fluxes of organic matter creating a complex suite of stressors to downstream receiving waters (Kaushal and Belt, 2012). Duan et al. (2014) recognized the importance of leaf litter in regulating ecological function in headwater forest streams. However, the release of nutrients from leaf litter in hydrologically flashy urban systems is a complex process requiring additional research (Belt, 2012). Increased export of nutrients from the urban landscape to urban lakes and streams can have ecosystem and human health implications by increasing the occurrence of algal blooms which can block sunlight for other aquatic plants, clog the gills of fish and produce toxins that are harmful if ingested. As such, management of nitrogen and phosphorus from urban sources should be considered when developing watershed plans to protect and preserve the ecological function of streams and lakes.
Sources of phosphorus and nitrogen in the urban landscape include anthropogenic (fertilizers, automotive detergents, pet waste) and biogenic (leaves, pollen, grass clippings) materials (Berretta and Sansalone, 2011, Hochmuth et al., 2012). Of these, organic detritus and particulate matter are often considered the primary contributors of nutrients to urban stormwater, especially in areas with high overhead tree canopy (Waller, 1977; Waschbusch et al., 1999). Previous studies have noted the positive correlation between tree canopy and phosphorus and nitrogen loads on streets which vary seasonally and by prevalent tree species (Kalinosky et al., 2014, Baker et al., 2014). Early research by Cowen and Lee (1973) has shown the concentration of leachable phosphorus from leaves that are subjected to stormwater can vary considerably between tree species. This was also supported by Dorney (1986) who concluded leaf litter was a major source of phosphorus in Milwaukee, WI. Based on laboratory experiments, as much as 9% of total leaf phosphorus leached from leaves within 2 h (Dorney, 1986). Despite past research confirming the high nutrient content of leaves, the potential loading from urban tree canopy is still not fully understood. Using urban tree models, Scheuler et al. (2016) estimated the average load of phosphorus and nitrogen associated with leaf litter in the city of Baltimore to be 2.95 and 28.8 lbs./ac/year, respectively. What is still uncertain, however, is how much of the leaf litter made it to the street gutter where it becomes available for washoff during precipitation events. In a review of studies linking nutrients with plant debris, Hochmuth et al. (2012) concluded that plant debris can be a significant source of nutrients in stormwater. Hochmuth et al. (2012) also noted that the removal of plant debris should be done as soon as possible because stormwater can easily and rapidly extract nutrients from the debris.
While many studies have documented the phosphorus content of leaves and their potential effect on water quality in urban watersheds, few have quantified the potential benefit of their removal on stormwater quality. Templer et al. (2015) estimated reductions in carbon and nitrogen from urban areas in the city of Boston via leaf litter removal during the fall leaf collection period. Their results showed removal of leaves may cause nutrient limitation in vegetation due to diminished nutrient cycling thereby creating spatial heterogeneities of urban ecosystems that are either nitrogen limited or saturated depending on leaf collection practices; however, the effect of leaf removal on stormwater quality was left unaddressed. Similarly, Kalinosky et al. (2014) and Law et al. (2008) used material collected by street cleaners to estimate nutrient removal from urban streets. Although they concluded street cleaners were capable of removing an appreciable amount of seasonal organic detritus from streets, their data served only as a proxy for improvements to water quality through nutrient reduction in stormwater runoff. Stack et al. (2013) made a similar conclusion when estimating the nutrient removal benefits of street cleaners, catch basins, and trash nets in Talbot County, MD but also noted additional research was needed to statistically quantify the impact of leaf litter on urban stream nutrient loadings.
Understanding the role of municipal practices such as street cleaning or leaf collection on preventing nutrient release from organic material on impervious surfaces is important in the context of stormwater management (Hobbie et al., 2014). Implementation of structural stormwater control measures (SCMs) may help remove leaves and coarse particulates entrained in stormwater but may do little to remove dissolved nutrients leached from leaves. Furthermore, cities around the Nation are often faced with limited open space available for the construction of new structural SCMs. While the possibility of retrofitting existing areas remains an option for environmental managers, costs may be prohibitive. Improving our knowledge of the potential reduction of nutrients to stormwater through removal of leaves and other organic detritus on streets could help tailor more targeted municipal leaf collection programs.
The purpose of this study was to characterize the potential for a municipal leaf collection and street cleaning program to reduce nutrient concentrations and loads from stormwater runoff. The U.S. Geological Survey, in cooperation with the City of Madison and the Wisconsin Department of Natural Resources, measured concentrations of phosphorus and nitrogen in stormwater from two residential catchments in Madison, WI, USA. One catchment was established as a control in which there was no effort to remove leaf litter and other organic detritus from streets. The second catchment served as the test catchment in which removal of leaf litter was done through a combination of municipal leaf collection, street cleaning, and leaf blowers. Relations were established between nutrient loads from both the control and test catchments during a calibration and treatment period to quantify the effect of leaf removal from streets during precipitation events. This study supports an ongoing effort to identify existing and new methods to reduce nonpoint source pollution from urban areas.
Section snippets
Site description
This study characterized concentrations of select nutrients in urban stormwater runoff from two residential catchments in Madison, Wisconsin, USA (Fig. 1). The climate in Madison is typical of interior North America, with a large annual temperature range and frequent short-period temperature changes. Months of the year were lumped by season in which leaves were either emerging (spring), mature (summer), or in recession (fall). Spring is defined as April–May, summer as June–September, and fall
Results
A total of 71 paired samples were collected over the study period, with 40 and 31 paired samples representing the calibration and treatment phases, respectively. A complete list of paired concentrations, loads, and measured weather parameters can be found in Appendix 1 in the supplemental online material (also available in Selbig, 2016.
Leaves as a source of phosphorus in urban stormwater
Results from this study confirm what others (e.g. Kluesner and Lee, 1974, Waller, 1977, Hochmuth et al., 2012) previously concluded, that in an urban residential area with high overhead street tree canopy (> 15% in this case), leaf litter could be the primary source of phosphorus in stormwater during the fall months (for this study defined as October and November). This study provides additional evidence that significant reductions in loads of the total and dissolved forms of phosphorus and
Acknowledgements
The author would like to thank Neely Law from the Center for Watershed Protection and Angela Brennan from the U.S. Geological Survey for their helpful comments. The author would also like to acknowledge Nicolas Buer for his dedication to ensuring field-collected data remained of the highest quality. The Wisconsin Department of Natural Resources, City of Madison, Fund for Lake Michigan, Yahara WINS, and Dane County Land and Water Resources provided financial support necessary to complete this
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2023, Science of the Total EnvironmentCitation Excerpt :For one, no numeric federal or state standards apply. While reviews have indicated that fOP ≈ 0.45(fTP) on average (Song et al., 2015), stormwater studies indicate a variable relationship (e.g., 0.25–0.91) depending upon canopy coverage, season, and solids composition (Selbig, 2016; Bratt et al., 2017; Janke et al., 2017). At the median fOP herein (0.39), the aforementioned Minnesota [TP] standards (μg/L) translate roughly to [OP] of 25 (lakes) and 39 (rivers); across IESFs, median effluent [OP] (26 μg/L) was below (river) and near (lake) these translated benchmarks.