Exfiltration: Performance

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Starting after TRIECA (end March) members of STEPSustainable Technologies Evaluation Program will be undertaking a literature review on the performance of our most popular BMPs. The results will be combined with the information we have to date from the development of the Treatment Train Tool and agreed performance metrics established. Until then, please feel free to continue to ask questions via email or the feedback box below.

Ability of exfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). systems to meet SWMStormwater Management objectives

Water balanceThe accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle. benefit Water quality improvement Erosion controlIncludes the protection of soil from dislocation by water, wind or other agents. benefit
Yes Yes Partial: depending on soil infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour.

Water balanceThe accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

The degree to which water balanceThe accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle. objectives are met will depend on the underlying native soilThe natural ground material characteristic of or existing by virtue of geographic origin. type on which the system is located. Several Ontario studies have assess the performance of exfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). systems in cold climates.

Volumetric runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. reduction achieved by exfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). systems
Practice Location Underlying soil type RunoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. reduction
Grass swaleVegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales./perforated pipe Nepean, ON[1] Silty till 73 %
Grass swaleVegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales./perforated pipe Nepean, ON[1] Sandy silty till 86 %
Perforated pipe Etobicoke, ON[2] Clay1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System)./clayey silty till over silty sandMineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm. 95 %
Perforated pipe North york, ON[2] Silty sandMineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm. 89 %

RunoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. reduction estimate = 85 % on HSG A and B soils.
RunoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. 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 [3]. These types of practices provide effective removal for many pollutants as a result of sedimentationDeposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.Settling-out or deposition of particulate matter suspended in runoff., filtering, and soil adsorptionThe attachment of gas, vapour or dissolved matter onto the surface of solid materials.. It is also important to note that there is a relationship between the water balanceThe accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle. and water quality functions. If an infiltration practice infiltrates and evaporates 100% of the runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. from a site, then there is essentially no pollution leaving the site in surface runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface.. Furthermore, treatment of infiltrated runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. continues to occur as it leaves the facility and moves through the native soilThe natural ground material characteristic of or existing by virtue of geographic origin.. The performance of perforated pipe systems would be expected to reduce pollutants in runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. in a manner similar to infiltration trenches.

Several studies of exfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). 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 [4][2]. Perforated pipe systems that incorporate grassed swales as pretreatment have been observed to reduce loads of suspended sedimentSoil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls., phosphorus, nitrogen, copper, lead and zinc in runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. flowing from the system between 75 to 90% in comparison to a similar catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. with conventional catchbasins and storm sewers [5]. The Nepean systems were shown to release significantly less pollutants than the conventional sewer system, even after 20 years of operation[1].

Pollutant removal efficiencies of exfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). systems
Practice Location Lead % Copper % Zinc % Total suspended solids (TSSTotal suspended solids) % Total Phosphorus % Total Nitrogen (TKN) %
SoakawayA pit into which liquids may flow and then percolate slowly into the subsoil.An excavated area lined with geotextile filter cloth and filled with clean granular stone or other void forming material, that receives runoff and allow it to infiltrate into the native soil; can also be referred to as infiltration galleries, French drains, dry wells or soakaway pits. Valence, France[6] 98 - 54 - 88 - - -
Infiltration trench Various[7] 70 - 90 70 - 90 70 - 90 70 - 90 50 - 70 40 - 70
Grass swaleVegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales./perforated pipe North York, ON[4] 75 96 93 24 84 84
Grass swaleVegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales./perforated pipe Nepean, ON[1] - 99 66 0 81 81 72
Grass swaleVegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales./perforated pipe Nepean, ON[1] > 99 > 99 90 96 93 93

  1. 1.0 1.1 1.2 1.3 1.4 J.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.
  2. 2.0 2.1 2.2 Stormwater 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.
  3. 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.
  4. 4.0 4.1 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.
  5. 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.
  6. Barraud, 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.
  7. ASCE (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.