Pollutant removal of traditional structural stormwater best management practices, such as bioretention and permeable pavement, can be determined by determining pollutant concentrations entering and leaving the practice and knowing the volume of runoff being treated. For example, constructed Design Level 2 stormwater ponds are assumed to remove 84 percent of total suspended solids, 50 or 68 percent of total phosphorus depending on absence or presence of an iron amendment, and 30 percent of total nitrogen (see Information on pollutant removal by BMPs).
Unlike stormwater practices such as bioretention, permeable pavement, and constructed ponds, direct measurement of water quality performance for street sweeping cannot be performed. Pollutant removal is a function of the amount of material removed and the chemical composition of the material removed. This simplified approach also assumes all material and pollutants on impervious surfaces would reach a receiving water, although this same assumption applies to structural practices, except for infiltration practices. Another complicating factor is that concentrations of some pollutants, such as nutrients, varies considerably during the year, with the result that sweeping effectiveness also varies with timing. This differs from a traditional practice, such as bioretention, which treats runoff throughout the year, thus allowing calculation of annual average removal.
The following discussion summarizes general conclusions on the effectiveness of street sweeping in removing pollutants. There is also a section on cost effectiveness of street sweeping compared to other stormwater practices.
Hobbie et al. (2020) determined phosphorus concentrations in street sweepings from six municipalities. Results indicate significantly greater concentrations of phosphorus during fall when leaf drop occurs. Combined with the much greater mass of material generated at this time of year, targeted sweeping during fall leaf drop provides by far the most effective way of reducing phosphorus concentrations in stormwater runoff (Bratt et al., 2017; Hixon and Dymond, 2018). Selbig (2016) observed loads of total and dissolved phosphorus were reduced by 84 and 83% (p < 0.05) with an active leaf removal program. Without leaf removal, 56% of the annual total phosphorus yield (winter excluded) was due to leaf litter in the fall compared to 16% with leaf removal.
Phosphorus removal will be greatest where organic inputs are greatest. Janke et al. (2017) observed a strong correlation between total phosphorus and street canopy cover. The observed relationship was linear, with concentrations ranging from about 0.2 mg/L to 0.45 mg/L at 40% street canopy coverage. Waschbusch et al. (1999) observed a similar relationship in two Madison, Wisconsin neighborhoods, with total phosphorus ranging from about 0.1 mg/L with 5 percent street canopy coverage to 0.40 mg/L with 40 percent street canopy coverage. Selbig (2016) measured monthly concentrations of total phosphorus in a residential area of Madison, Wisconsin having 17 percent street tree canopy coverage. Mean concentrations in spring were 0.67-0.74 mg/L, decreasing to 0.41-0.45 in summer, and rapidly increasing following Fall leaf drop, when concentrations increased to more than 2.5 mg/L. Janke et al. (2017) observed similar patterns in Minnesota.
For information on targeted street sweeping for phosphorus, see Recommended street sweeping practices for water quality purposes.
Selbig (2016, Madison WI) observed loads of total and dissolved nitrogen by 74 and 71% (p < 0.05) with an active leaf removal program. Despite significant reductions in load, total nitrogen showed only minor changes in fall yields without and with leaf removal at 19 and 16%, respectively. Smith et al. (2020, Columbus, OH) observed spring total nitrogen concentrations had a significantly (p < 0.05) higher median concentration (2.19 mg/L) than fall (1.55 mg/L) and summer (1.50 mg/L). Hobbie et al. (2020) found that TN concentration increased with increasing canopy cover and was higher in the mid to late spring (April, May), early summer (June), and autumn (September, October, November). Jani et al. (2020, Florida) observed increased concentrations of nitrate, total organic nitrogen, and ammonium-N as dry periods between runoff events increased in length, while dissolved organic nitrogen was only correlated with storm intensity.
While nitrogen in runoff is correlated with canopy cover and organic inputs, the relationships appear more complicated than with phosphorus. First, there are several forms of nitrogen, and transformations may occur between the different forms. Second, seasonal relationships do not appear to be as strong as with phosphorus. Specifically, targeted fall sweeping will reduce nitrogen inputs but likely not to the extent as with phosphorus.
In Minnesota, nitrogen concentrations in urban runoff do not generally represent a significant water quality concern, unlike areas along the eastern coast of the United States. Since nitrogen concerns are not driving stormwater programs, targeted sweeping for phosphorus should also provide benefits for nitrogen reduction.
This page was last edited on 30 January 2023, at 19:03.