Innovation Inspiration: Rain Gardens

Increasing stormwater runoff, a consequence of urbanization, is often considered one of the biggest problems in developed areas. As urban spaces continue to develop, permeable green spaces are replaced with impervious parking lots or building developments resulting in less infiltration and more stormwater runoff when it rains. This then causes hydrological problems like flooding in urban areas.

Urban stormwater is highly contaminated, often more so than groundwater, roof runoff, rainwater, or effluent from wastewater treatment plants [1]. This contamination is typically an accumulation of chemicals picked up along roads, hydrocarbons from vehicles, pesticides applied along highways, and pollutants present in building and roofing materials [2-4].


Graph from the US EPA Technical Guidance on
Implementing Stormwater Runoff Requirements:
https://www.epa.gov/sites/production/files/2015-08/documents/epa_swm_guidance.pdf

But as humans become more urbanized, we also become more creative. The field of low impact development (LID), for example, uses innovative design to return the hydrology of developed areas to pre-development levels [5]. Some methods in this category include constructed wetlands, permeable pavements, green roofs, and rain gardens or bioretention cells.

As the topic of my master’s thesis, rain gardens and bioretention cells are close to my heart. To be clear, these two things are not EXACTLY the same – they have subtle differences in their designs that alter how they work – but they are similar enough in concept that I will combine them for the purposes of this post. In fact, I’m just going to go ahead and use the term “rain garden” from here on out because it does a better job of describing how they look.

Rain gardens are exactly what they sound like: gardens. Only they’re more like specially-engineered superhero gardens. Rain gardens are designed to infiltrate more water than a normal garden so that they can reduce stormwater runoff volumes and limit urban flooding [6]. They can do this because rain gardens use soils that contain more sand and less organic matter than soils used in normal gardens. A high organic matter content is what causes soils to expand when they get wet, so less organic matter and more sand help rain gardens to maintain their permeability in a rain event [7]. Typically, rain gardens also incorporate vegetation capable of high rates of evapotranspiration, meaning that the roots can take up large amounts of water and the plants release the water as vapour [8].

Rain garden in Toronto, ON, used as a field site for grad students. This is the same site as the one in the header photo, but before our research group adapted the inlet to conduct a hydrological study.

Not only do rain gardens reduce stormwater runoff and flooding, but they re-introduce green spaces into our urbanized lives that are beautiful to look at and provide habitats for wildlife [5]. BUT THAT’S NOT ALL! Rain gardens are also capable of reducing some of the contamination in the water before it enters other water supplies like surface water or groundwater [6]. Contaminants can attach to soil particles, be taken up by plants, or degrade thanks to microorganisms in the soil [8]. To date, rain gardens have proved effective at removing many typical stormwater contaminants such as heavy metals, hydrocarbons, nitrogen, and phosphorus [9,10].

Rain gardens and other LID systems are becoming increasingly popular treatment methods in urban areas because they are aesthetically pleasing, flexible in design, and they leave small environmental footprints [11]. These methods are great for incorporating sustainability into our everyday lives and for making our urban areas – literally – greener.

Cool, right?!

Unless specified, photos were taken by students at the University of Toronto.

[1] Sablayrolles, C.; Breton, A.; Vialle, C.; Vignoles, C.; Montréjaud-Vignoles, M. Priority Organic Pollutants in the Urban Water Cycle (Toulouse, France). Water Sci. Technol. 2011, 64 (3), 541–556.

[2] Pitt, R.; Field, R.; Lalor, M.; Brown, M. Urban Stormwater Toxic Pollutants: Assessment, Sources, and Treatability. Water Environ. Res. 1995, 67 (3), 260–275.

[3] Huang, X.; Pedersen, T.; Fischer, M.; White, R.; Young, T. M. Herbicide Runoff along Highways. 1. Field Observations. Environ. Sci. Technol. 2004, 38 (12), 3263–3271.

[4] Lundy, L.; Ellis, J. B.; Revitt, D. M. Risk Prioritisation of Stormwater Pollutant Sources. Water Res. 2012, 46 (20), 6589–6600.

[5] Clark, S. E.; Pitt, R. Targeting Treatment Technologies to Address Specific Stormwater Pollutants and Numeric Discharge Limits. Water Res. 2012, 46 (20), 6715–6730.

[6] Davis, A. P. Field Performance of Bioretention: Water Quality. Environ. Eng. Sci. 2007, 24 (8), 1048–1064.

[7] Hsieh, C.-H.; Davis, A. P. Evaluation and Optimization of Bioretention Media for Treatment of Urban Storm Water Runoff. J. Environ. Eng. 2005, 131 (11), 1521–1531.

[8] Davis, A. P.; McCuen, R. H. Stormwater Management for Smart Growth; Springer Science+Business Media, Inc.: New York, NY, 2005.

[9] Hong, E.; Seagren, E. A.; Davis, A. P. Sustainable Oil and Grease Removal from Synthetic Stormwater Runoff Using Bench-Scale Bioretention Studies. Water Environ. Res. 2006, 78 (2), 141–155.

[10] Davis, A. P.; Shokouhian, M.; Sharma, H.; Minami, C. Water Quality Improvement through Bioretention Media: Nitrogen and Phosphorus Removal. Water Environ. Res. 2006, 78 (3), 284–293.

[11] Hatt, B. E.; Fletcher, T. D.; Deletic, A. Pollutant Removal Performance of Field-Scale Stormwater Biofiltration Systems. Water Sci. Technol. 2009, 59 (8), 1567–1576.

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