Exfiltration: Performance

Ability of exfiltration systems to meet SWM objectives
Water balance benefit	Water quality improvement	Erosion control benefit
Yes 	Yes 	Partial: depending on soil infiltration rate
===Water balance===
The degree to which water balance objectives are met will depend on the underlying native soil type on which the system is located. Several Ontario studies have assess the performance of exfiltration systems in cold climates. 
 Volumetric runoff reduction achieved by exfiltration systems
Practice 	Location 	Underlying soil type 	Runoff reduction
Grass swale/perforated pipe	Nepean, ONJ.F. Sabourin and Associates Incorporated. 2008a. 20 Year Performance Evaluation of Grassed Swale and Perforated Pipe Drainage Systems. Project No. 524(02). Prepared for the Infrastructure Management Division of the City of Ottawa. Ottawa, Ontario.	Silty till 	73 %
Grass swale/perforated pipe	Nepean, ON 	Sandy silty till 	86 %
Perforated pipe 	Etobicoke, ONStormwater Assessment Monitoring and Performance (SWAMP) Program. 2005. Synthesis of Monitoring Studies Conducted Under the Stormwater Assessment
Monitoring and Performance Program.. Toronto and Region Conservation Authority,
Toronto, Ontario. 	Clay/clayey silty till over silty sand	95 %
Perforated pipe 	North york, ON 	Silty sand 	89 %
Runoff reduction estimate = 85 % on HSG A and B soils.
Runoff reduction estimate = 45 % on HSG C and D soils.
===Pollutant removal capacity===
Performance results from a limited number of field studies indicate that subsurface stormwater infiltration practices are effective BMPs for pollutant removal Toronto and Region Conservation (TRCA). 2009. Review of the Science and Practice of Stormwater Infiltration in Cold Climates. Prepared under the Sustainable Technologies Evaluation Program (STEP). Toronto, Ontario.. These types of practices provide effective removal for many pollutants as a result of sedimentation, filtering, and soil adsorption. It is also important to note that there is a relationship between the water balance and water quality functions. If an infiltration practice infiltrates and evaporates 100% of the runoff from a site, then there is essentially no pollution leaving the site in surface runoff. Furthermore, treatment of infiltrated runoff continues to occur as it leaves the facility and moves through the native soil. The performance of perforated pipe systems would be expected to reduce pollutants in runoff in a manner similar to infiltration trenches.
Several studies of exfiltration systems in Ontario have examined their water quality benefits. Seasonal contaminant load reductions in the order of 80% were observed for most constituents, with the exception of chloride, in the study of the system installed in a low density residential neighbourhood in Etobicoke Stormwater Assessment Monitoring and Performance (SWAMP) Program. 2002. Performance Assessment of a Swale/Perforated Pipe Stormwater Infiltration System – Toronto, Ontario. Toronto and Region Conservation Authority, Toronto, Ontario.. Perforated pipe systems that incorporate grassed swales as pretreatment have been observed to reduce loads of suspended sediment, phosphorus, nitrogen, copper, lead and zinc in runoff flowing from the system between 75 to 90% in comparison to a similar catchment with conventional catchbasins and storm sewers J.F. Sabourin and Associates Incorporated. 1999. Research Project for the Updated
Investigation of the Performance Evaluation of Grass Swales and Perforated Pipe
Drainage Systems. Executive Summary. Prepared for the Infrastructure Management
Division of the City of Ottawa. Ottawa, Ontario.. The Nepean systems were shown to release significantly less pollutants than the conventional sewer system, even after 20 years of operation.
Pollutant removal efficiencies of exfiltration systems
Practice 	Location 	Lead % 	Copper %	Zinc % 	Total suspended solids (TSS) %	Total Phosphorus %	Total Nitrogen (TKN) %
Soakaway 	Valence, FranceBarraud, S., Gautier, A., Bardin, J.P., Riou, V. 1999. The Impact of Intentional Stormwater Infiltration on Soil and Groundwater. Water Science and Technology. Vol.
39. No. 2. pp. 185-192.	98 	- 	54 - 88	- 	- 	-
Infiltration trench 	VariousASCE
(2000)Pollutant removal efficiencies are reported as ranges because they are based on a synthesis of several performance monitoring studies that were available as of 2000.	70 - 90	70 - 90 	70 - 90	70 - 90 	50 - 70 	40 - 70
Grass swale/perforated pipe	North York, ON	75 	96 	93 	24 	84 	84
Grass swale/perforated pipe	Nepean, ON 	- 99 	66 	0 	81 	81 	72
Grass swale/perforated pipe	Nepean, ON 	> 99 	> 99 	90 	96 	93 	93
----
category: performance