Bioswales: Performance

While few field studies of the pollutant removal capacity of bioswales are available from cold climate regions like Ontario, it can be assumed that they would perform similar to bioretention cells.
Bioretention provides effective removal for many pollutants as a result of sedimentation, filtering, plant uptake, soil adsorption, and microbial processes. It is important to note that there is a relationship between the water balance and water quality functions.
If a bioswale infiltrates and evaporates 100% of the flow from a site, then there is essentially no pollution leaving the site in surface runoff. Furthermore, treatment of infiltrated runoff will continue to occur as it moves through the native soils. 
Volumetric runoff reduction from bioswales
LID Practice 	Location 	Runoff Reduction* 	Reference
Bioswale without underdrain 	Washington 	98% 	Horner et al. (2003)Horner RR, Lim H, Burges SJ. HYDROLOGIC MONITORING OF THE SEATTLE ULTRA-URBAN STORMWATER MANAGEMENT PROJECTS: SUMMARY OF THE 2000-2003 WATER YEARS. Seattle; 2004. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.365.8665&rep=rep1&type=pdf. Accessed August 11, 2017.
Bioswale without underdrain 	Scotland 	94% 	Jefferies (2005)Jefferies, C. 2004. Sustainable drainage systems in Scotland: the monitoring programme. Scottish Universities SUDS Monitoring Project. Dundee, Scotland. https://www.climatescan.nl/uploads/projects/8126/files/1277/SNIFFERSR_02_51MainReport.pdf
Bioswale with Underdrain 	Maryland 	46 to 54% 	Stagge (2006)Stagge, J. 2006. Field evaluation of hydrologic and water quality benefits of grass swales for managing highway runoff. Master of Science Thesis, Department of Civil and Environmental Engineering, University of Maryland. https://drum.lib.umd.edu/items/42be6ce6-e4ef-4162-a991-c273607d422d
Bioretention without underdrain	China 	85 to 100%* 	Gao, et al. (2018)Gao, J., Pan, J., Hu, N. and Xie, C., 2018. Hydrologic performance of bioretention in an expressway service area. Water Science and Technology, 77(7), pp.1829-1837.
Bioretention without underdrain	Connecticut 	99% 	Dietz and Clausen (2005) Dietz, M.E. and J.C. Clausen. 2005. A field evaluation of rain garden flow and pollutant treatment. Water Air and Soil Pollution. Vol. 167. No. 2. pp. 201-208. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.365.9417&rep=rep1&type=pdf
Bioretention without underdrain	Pennsylvania 	80% 	Ermilio (2005)Ermilio, J.F., 2005. Characterization study of a bio-infiltration stormwater BMP (Doctoral dissertation, Villanova University). https://www1.villanova.edu/content/dam/villanova/engineering/vcase/vusp/Ermilio-Thesis06.pdf
Bioretention without underdrain	Pennsylvania 	70% 	Emerson and Traver (2004)Emerson, C., Traver, R. 2004. The Villanova Bio-infiltration Traffic Island: Project Overview. Proceedings of 2004 World Water and Environmental Resources Congress (EWRI/ASCE). Salt Lake City, Utah, June 22 – July 1, 2004. https://ascelibrary.org/doi/book/10.1061/9780784407370
Bioretention with underdrain 	 	 	
Bioretention with underdrain 	Ontario 	64% 	CVC (2020) Credit Valley Conservation. 2020. IMAX Low Impact Development Feature Performance Assessment. https://sustainabletechnologies.ca/app/uploads/2022/03/rpt_IMAXreport_f_20220222.pdf
Bioretention with underdrain 	Maryland and North Carolina	20 to 50% 	Li et al. (2009) Li, H., Sharkey, L.J., Hunt, W.F., and Davis, A.P. 2009. Mitigation of Impervious Surface Hydrology Using Bioretention in North Carolina and Maryland. Journal of Hydrologic Engineering. Vol. 14. No. 4. pp. 407-415.
Bioretention with underdrain 	North Carolina 	40 to 60% 	Smith and Hunt (2007)Smith, R and W. Hunt. 2007. Pollutant removals in bioretention cells with grass cover. Proceedings 2nd National Low Impact Development Conference. Wilmington, NC. March 13-15, 2007.
North Carolina 	33 to 50% 	Hunt and Lord (2006) Hunt, W.F. and Lord, W.G. 2006. Bioretention Performance, Design, Construction, and Maintenance. North Carolina Cooperative Extension Service Bulletin. Urban Waterways Series. AG-588-5. North Carolina State University. Raleigh, NC.	
Runoff Reduction Estimate* 	Runoff Reduction Estimate* 	85% without underdrain;
45% with underdrain 	85% without underdrain;
45% with underdrain