A brief review of extant literature on sources of nutrients in urban stormwater and options for communication, education and outreach campaigns to address critical sources of nutrient loads on our urban water systems.
Nutrient Sources in Urban Areas
Research shows that leaf litter, atmospheric deposition, fertilizers and (to a lesser extent) pet waste are all potential sources of nutrients in stormwater. When the above sources are deposited onto impervious surfaces, they accumulate as street solids and become “directly-connected” to the stormwater system. Rough estimates of average annual nutrient loads show that the nutrients measured in street solids are on the same order of magnitude as those measured in urban stormwater runoff. Although leaf litter is a “natural” source of nutrients, education and outreach programs could focus on keeping fallen leaves out of the stormwater system through proper collection and recycling.
Activities such as blowing leaves and other vegetative debris into roadways should be discouraged. A side benefit of this type of program would be reduced maintenance to the stormwater system due to leaves clogging pipes and inlets.
The control of nutrients contributed by atmospheric deposition through a local education and outreach program would likely be limited as many of the sources are non-local and/or naturally-occurring. However, several studies did identify vehicle emissions as a significant localized source of nitrogen. This suggests that any education and outreach campaign that focuses on reducing vehicle usage may also be contribute to a reduction of nitrogen in stormwater.
Fertilizer use is the most often targeted source of nutrients in stormwater management campaigns. While it may seem most intuitive that a reduction or elimination of the use of fertilizers may be the most effective education and outreach approach, in fact research shows that poorly maintained lawns (those with low/no fertilizer applied) actually export more nutrients to runoff compared to fertilized lawns. Research also shows that the highest export of nutrients from lawns occurs in during the late fall through early spring months when the ground is frozen and/or vegetation growth is limited. Therefore, an education and outreach program targeting fertilizer use should focus on applying the proper amounts of fertilizer (according to manufacturer and/or Extension recommendation) during the proper seasons.
Research on the contribution of pet waste to stormwater nutrients is rather limited, but few studies have identified it as a potential source. Many cities already have education and outreach programs targeting the proper collection and disposal of pet waste for bacteria reduction, however the same programs may also contribute to nutrient reduction.
Lastly, a review of research on street solids was included because they represent an aggregate measure of nutrients accumulating in the urban environment. Street sweeping programs operated by local utilities are an obvious activity that can significantly reduce nutrients in stormwater, however such operations may be limited to certain streets and do not specifically target private citizens. Education and outreach programs that aim to “keep the streets clean” could educate citizens that what appears to be relatively innocuous “dirt” is actually akin to trash and litter collecting on the streets. Engaging citizens to prevent materials such as leaves, grass clippings, fertilizer from reaching streets and potentially even remove dirt and other accumulated materials as they would trash and litter would be an interesting (and potentially highly effective) program.