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File:Bioretention ~~No Infiltration~~.png |thumb|600px|Bioretention cell ~~schematic capturing runoff from an adjacent~~ parking lot. ~~Design~~ for this LID ~~BMP can~~ vary, but ~~generally all facilities~~ include a ~~gravel~~ storage ~~layer~~, an optional choker layer, media layer, mulch and vegetation. This specific variation does not infiltrate water into the surrounding native soil, potentially due to the site possessing soils prone to subsidence, or the site may be considered to be a pollution hot spot. There are two other types of bioretention variations, [[Bioretention: Full infiltration|full infiltration]] and [[~~Bioretention: Partial infiltration~~|~~partial~~ infiltration]], which can be explored with their own image maps. For more details on this variation of a bioretention feature and the other two previously mentioned, click here for the [https://www.toronto.ca/legdocs/mmis/2017/pw/bgrd/backgroundfile-107514.pdf City of Toronto's Green Streets Technical Guidelines (GSTG).] ''~~A note~~: The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.''

File:Bioretention Full Partial infiltration placementswap.png|thumb|600px|'''[[Bioretention: Partial infiltration|Partial infiltration bioretention]]''' cell draining a parking lot. Designs for this type of LID practice vary, but typically include a water storage reservoir, optional choker layer, filter media layer, mulch and vegetation. Other design variations include [[Bioretention: Full infiltration|'''full infiltration''']] and [[Stormwater planters|'''no infiltration''']] configurations which can be explored with their own image maps. '''''Note''': The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.''

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rect 1023 3114 1113 3189 [[Reservoir aggregate|Clear Stone / Aggregate]]

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</imagemap>

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*Spaces for herbaceous flowering plants can be included. This may be attractive at a community entrance location or in a residential rain garden.

Tables for identifying ideal species for bioretention are found in the [[Plant lists]]. See [[plant selection]] and [[planting design]] for supporting advice.

===Variations of Bioretention Cells===

Below, find three alternate Bioretention cell configurations, that differ based off whether developers want the system to infiltrate incoming water fully, partially, or not at all due to sites which possess contaminated soils or shallow bedrock, and/or zero-lot-line developments (i.e. condo developments and dense urban infill).

All the images below are image map drawings from the following pages to compare side-by-side the differences between varying configurations.

File:Bioretention Full infiltration placementswap.png|thumb|left|400px|[[Bioretention: Full infiltration|'''Full infiltration bioretention''']] cell draining a parking lot. This design variation includes a surface overflow pipe/structure to allow excess water to leave the practice. A monitoring well is included so drainage performance can be evaluated over its operating lifespan.'''''Note''': The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.''

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poly 870 241 866 322 953 333 1017 371 1045 400 1078 322 1129 312 1182 316 1235 328 1388 245 1263 245 1388 247 1155 243 [[Mulch|Mulch]]

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rect 866 1034 990 1406 [[Mulch|Mulch]]

rect 1300 1061 1392 1171 [[Mulch|Mulch]]

rect 864 359 1015 585 [[Stone|Erosion Control - Stone]]

rect 1257 1206 1394 1420 [[Stone|Erosion Control - Stone]]

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poly 982 3227 941 3144 917 3160 953 3217 970 3219 1015 3227 1059 3152 1031 3148 1023 3191 1064 3176 [[Reservoir aggregate|Clear Stone / Aggregate]]

</imagemap>

File:Bioretention Full Partial infiltration placementswap.png|thumb|right|400px|[[Bioretention: Partial infiltration|'''Partial infiltration bioretention cell''']] draining a parking lot. This design variation includes an underdrain and surface overflow pipes that allow excess water to leave the practice. A monitoring well is included so drainage performance can be evaluated over its operating lifespan. '''''Note''': The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.''

rect 1134 3187 1159 3246 [[Digital technologies|Water Level Sensor]]

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</imagemap>

File:Bioretention No infiltration placementswap.png|thumb|center|400px|[[Stormwater planters|'''Stormwater planter / No infiltration bioretention''' ]] cell draining a parking lot. This design variation includes an impermeable liner, an underdrain and surface overflow pipes to allow excess water to leave the practice. '''''Note''': The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.''

~~Tables for identifying ideal species for bioretention are found in the~~ [[Plant lists]]~~. See~~ [[~~plant selection~~]] ~~and~~ [[~~planting design~~]] ~~for supporting advice.~~

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poly 1214 1389 1263 1419 1339 1440 1390 1446 1384 1491 1161 1497 [[Mulch|Mulch]]

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==Performance==

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===Water Balance===

Bioretention practices have been shown to reduce runoff volume through both means of evapotranspiration and infiltration. The primary body of research is separated into bioretention practices either with underdrains and those without (therefore, relying solely on full infiltration into underlying soils). Volumetric performance improves when:

* Native soils have high infiltration capacity.

* Native soils have high infiltration capacity and facility is designed for full infiltration, without an underdrain;

* ~~General size~~ of the ~~practice~~.

* Size of the impervious drainage area relative to the facility permeable footprint area (i.e., I:P area ratio) is kept within recommended range of 5:1 (HSG C and D soils) to 20:1 (HSG A and B soils).

* ~~Underdrain~~ is elevated above the ~~native soil and/or a~~ flow restrictor is installed on the underdrain.

* Perforated pipe or outlet connection is elevated above the bottom of the practice in the underdrain cross-section;

* A flow restrictor (e.g., orifice, valve) is installed on the underdrain or storm sewer outlet pipe.

{|class="wikitable"

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|-

!'''LID Practice'''

!'''Location'''

!'''Runoff Reduction*'''

!'''Reference'''

|-

|rowspan="4" style="text-align: center;" | Bioretention without underdrain

|style="text-align: center;" |China

|style="text-align: center;" |'''<span title="Note: Runoff reduction estimates are based on SWMM and RECARGA models applied to generate the runoff reduction percentages of a bioretention installation near one of China's and expressway service area.">85 to 100%*'''

|style="text-align: center;" |Gao, ''et al.'' (2018)<ref>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.</ref>

|-

|style="text-align: center;" |Connecticut

|style="text-align: center;" |99%

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|style="text-align: center;" |70%

|style="text-align: center;" |Emerson and Traver (2004)<ref>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</ref>

|-

|rowspan="12" style="text-align: center;" | Bioretention with underdrain

|-

|style="text-align: center;" |Ontario

|style="text-align: center;" |64%

|style="text-align: center;" |CVC (2020)<ref> Credit Valley Conservation. 2020. IMAX Low Impact Development Feature Performance Assessment. https://sustainabletechnologies.ca/app/uploads/2022/03/rpt_IMAXreport_f_20220222.pdf</ref>

|-

|style="text-align: center;" |Ontario

|style="text-align: center;" |66%

|style="text-align: center;" |STEP (2019)<ref> Sustainable Technologies Evaluation Program. 2019. Improving nutrient retention in bioretention. https://sustainabletechnologies.ca/app/uploads/2019/06/improving-nutrient-retention-in-bioretention-tech-brief.pdf</ref>

|-

|style="text-align: center;" |Texas

|style="text-align: center;" |'''82%*'''

|style="text-align: center;" |Mahmoud, ''et al.'' (2019)<ref>Mahmoud, A., Alam, T., Rahman, M.Y.A., Sanchez, A., Guerrero, J. and Jones, K.D. 2019. Evaluation of field-scale stormwater bioretention structure flow and pollutant load reductions in a semi-arid coastal climate. Ecological Engineering, 142, p.100007. https://www.sciencedirect.com/science/article/pii/S2590290319300070</ref>

|-

|style="text-align: center;" |China

|style="text-align: center;" |'''~~~~<span title="Note: Runoff reduction estimates are based on SWMM and RECARGA models applied to generate the runoff reduction percentages of a bioretention installation near one of China's and expressway service area.">85 to ~~100~~%*</span~~></u~~>'''

|style="text-align: center;" |'''<span title="Note: Runoff reduction estimates are based on SWMM and RECARGA models applied to generate the runoff reduction percentages of a bioretention installation near one of China's and expressway service area.">35 to 75%*'''

|style="text-align: center;" |Gao, ''et al.'' (2018)<ref>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.</ref>

|-

|~~rowspan~~="8" style="text-align: center;" | ~~Bioretention with underdrain~~

|style="text-align: center;" |Ohio

|style="text-align: center;" |36 to 59%

|style="text-align: center;" |Winston ''et al.'' (2016)<ref>Winston, R.J., Dorsey, J.D. and Hunt, W.F. 2016. Quantifying volume reduction and peak flow mitigation for three bioretention cells in clay soils in northeast Ohio. Science of the Total Environment, 553, pp.83-95.</ref>

|-

|style="text-align: center;" |Ontario

|style="text-align: center;" |90%

|style="text-align: center;" |STEP (2015)<ref> Sustainable Technologies Evaluation Program. 2015. Performance Comparison of Surface and Underground Stormwater Infiltration Practices. https://sustainabletechnologies.ca/app/uploads/2016/08/BioVSTrench_TechBrief__July2015.pdf</ref>

|-

|style="text-align: center;" |~~Texas~~

|style="text-align: center;" |Ontario

|style="text-align: center;" |~~'''82~~%*'''

|style="text-align: center;" |91 to 96%

|style="text-align: center;" |~~Mahmoud, ''et al.''~~ (~~2019~~)<ref>~~Mahmoud, A., Alam, T., Rahman, M.Y.A., Sanchez, A., Guerrero, J.~~ and ~~Jones, K~~.~~D. 2019~~. Evaluation of ~~field-scale stormwater bioretention structure flow and pollutant load reductions in~~ a ~~semi~~-~~arid coastal climate. Ecological Engineering, 142~~, p.~~100007~~. https://~~www~~.~~sciencedirect.com~~/~~science~~/~~article~~/~~pii~~/~~S2590290319300070~~</ref>

|style="text-align: center;" |TRCA (2014)<ref> Toronto and Region Conservation Authority. 2014. Performance Evaluation of a Bioretention System - Earth Rangers, Vaughan. Sustainable Technologies Evaluation Program. https://sustainabletechnologies.ca/app/uploads/2014/09/STEP-Bioretention-Report_2014.pdf</ref>

|-

|style="text-align: center;" |Virginia

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|style="text-align: center;" |DeBusk and Wynn (2011)<ref>DeBusk, K.M. and Wynn, T.M., 2011. Storm-water bioretention for runoff quality and quantity mitigation. Journal of Environmental Engineering, 137(9), pp.800-808. https://www.webpages.uidaho.edu/ce431/Articles/DeBusk-ASCE-2011.pdf</ref>

|-

|style="text-align: center;" |~~China~~

|style="text-align: center;" |Maryland and North Carolina

|style="text-align: center;" |'''<span title="Note: Runoff reduction estimates are based on SWMM and RECARGA models applied to generate the runoff reduction percentages of a bioretention installation near one of China's and expressway service area.">35 to 75%*'''

|style="text-align: center;" |20 to 50%

|style="text-align: center;" |~~Gao,~~ ''et al.'' (~~2018~~)<ref>~~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~~.</ref>

|style="text-align: center;" |Li ''et al.'' (2009) <ref>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.</ref>

|-

|style="text-align: center;" |North Carolina

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|style="text-align: center;" |North Carolina

|style="text-align: center;" |33 to 50%

|style="text-align: center;" |Hunt and Lord (2006). <ref>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~~.</ref>~~

|style="text-align: center;" |Hunt and Lord (2006) <ref>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.</ref>

|-

~~|style="text-align: center;" |Maryland and North Carolina~~

~~|style="text-align: center;" |20 to 50%~~

|style="text-align: center;" |Li ''et al.'' (2009). <ref>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.</ref>

|-

~~|style="text-align: center;" |Ohio~~

~~|style="text-align: center;" |36 to 59%~~

|style="text-align: center;" |Winston ''et al.'' (2016). <ref>Winston, R.J., Dorsey, J.D. and Hunt, W.F. 2016. Quantifying volume reduction and peak flow mitigation for three bioretention cells in clay soils in northeast Ohio. Science of the Total Environment, 553, pp.83-95.</ref>

|-

|rowspan="5" style="text-align: center;" | Bioretention with underdrain & liner

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|style="text-align: center;" |15 to 34%

|style="text-align: center;" |[https://sustainabletechnologies.ca/app/uploads/2019/10/STEP_Bioretention-Synthesis_Tech-Brief-New-Template-2019-Oct-10.-2019.pdf STEP (2019)] <ref>STEP. 2019. Comparative Performance Assessment of Bioretention in Ontari0. Technical Brief. https://sustainabletechnologies.ca/app/uploads/2019/10/STEP_Bioretention-Synthesis_Tech-Brief-New-Template-2019-Oct-10.-2019.pdf.</ref>

|-

~~|style="text-align: center;" |Maryland~~

~~|style="text-align: center;" |49 to 58%~~

|style="text-align: center;" |Davis (2008). <ref>Davis, A.P. 2008. Field performance of bioretention: Hydrology impacts. Journal of hydrologic engineering, 13(2), pp.90-95. https://ascelibrary.org/doi/abs/10.1061/(ASCE)1084-0699(2008)13:2(90)</ref>

|-

|style="text-align: center;" |Queensland, Australia

|style="text-align: center;" |33 to 84%

|style="text-align: center;" |Lucke and Nichols (2015). <ref>Lucke, T., & Nichols, P. W. B. 2015. The pollution removal and stormwater reduction performance of street-side bioretention basins after ten years in operation. Science of The Total Environment, 536, 784-792. doi:http://dx.doi.org/10.1016/j.scitotenv.2015.07.142</ref>

|style="text-align: center;" |Lucke and Nichols (2015) <ref>Lucke, T., & Nichols, P. W. B. 2015. The pollution removal and stormwater reduction performance of street-side bioretention basins after ten years in operation. Science of The Total Environment, 536, 784-792. doi:http://dx.doi.org/10.1016/j.scitotenv.2015.07.142</ref>

|-

|style="text-align: center;" |Victoria, Australia

|style="text-align: center;" |15 to 83%

|style="text-align: center;" |Hatt ''et al.'' (2009). <ref>Hatt, B. E., Fletcher, T. D., & Deletic, A. 2009. Hydrologic and pollutant removal performance of stormwater biofiltration systems at the field scale. Journal of Hydrology, 365(3), 310-321. doi:http://dx.doi.org/10.1016/j.jhydrol.2008.12.001</ref>

|style="text-align: center;" |Hatt ''et al.'' (2009)<ref>Hatt, B. E., Fletcher, T. D., & Deletic, A. 2009. Hydrologic and pollutant removal performance of stormwater biofiltration systems at the field scale. Journal of Hydrology, 365(3), 310-321. doi:http://dx.doi.org/10.1016/j.jhydrol.2008.12.001</ref>

|-

|style="text-align: center;" |Maryland

|style="text-align: center;" |49 to 58%

|style="text-align: center;" |Davis (2008)<ref>Davis, A.P. 2008. Field performance of bioretention: Hydrology impacts. Journal of hydrologic engineering, 13(2), pp.90-95. https://ascelibrary.org/doi/abs/10.1061/(ASCE)1084-0699(2008)13:2(90)</ref>

|-

| colspan="2" style="text-align: center;" |'''<span title="Note: This estimate is provided only for the purpose of initial screening of LID practices suitable for achieving stormwater management objectives and targets. Performance of individual facilities will vary depending on site specific contexts and facility design parameters and should be estimated as part of the design process and submitted with other documentation for review by the approval authority." >Runoff Reduction Estimate*'''

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===Water Quality===

Performance results from both laboratory and field studies indicate that bioretention systems have the potential to be one of the most effective BMPs for pollutant removal ([https://sustainabletechnologies.ca/app/uploads/2014/10/SW_Infiltration-Review_10.15.2014.pdf TRCA, 2009]). Bioretention provides effective removal for many pollutants as a result of sedimentation, filtering, soil adsorption, microbial processes and plant uptake. It is also important to note that there is a relationship between the water balance and water quality functions. If a bioretention cell 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 moves through the native soil.

Performance results from both laboratory and field studies indicate that bioretention systems have the potential to be one of the most effective BMPs for pollutant removal ([https://sustainabletechnologies.ca/app/uploads/2014/10/SW_Infiltration-Review_10.15.2014.pdf TRCA, 2009]). Bioretention provides effective removal for many pollutants as a result of sedimentation, filtering, soil adsorption, microbial processes and plant uptake. It is also important to note that there is a relationship between the water balance and water quality functions. If a bioretention cell infiltrates and evaporates 85 to 100% of the runoff from the drainage area during the design storm event, then there is little to no pollution leaving the site in surface runoff. Furthermore, treatment of infiltrated runoff continues to occur as it moves through the native soil.

A comparative performance assessment of bioretention in Ontario was conducted comparing 9 different bioretention facilities in the GTA. The results showed total suspended solids (TSS) ~~concentration~~ reductions between 73 to 99%. [https://sustainabletechnologies.ca/app/uploads/2019/10/STEP_Bioretention-Synthesis_Tech-Brief-New-Template-2019-Oct-10.-2019.pdf (STEP, 2019)<ref>STEP. 2019. Comparative Performance Assessment of Bioretention in Ontario - Technical Brief.</ref>]. Other STEP studies in the Greater Toronto Area have displayed similar results, with 90% reduction in TSS when compared to nearby asphalt runoff samples having median TSS concentrations ~~below~~ the provincial 30 mg/L standard (median = ~19 mg/L) [https://sustainabletechnologies.ca/app/uploads/2015/01/ER-Bio-Tech-Brief-Final.pdf STEP, 2014]<ref>~~STEP~~. 2014. Performance Evaluation of a Bioretention System - Earth Rangers. Prepared by Toronto and Region Conservation. September 2014. https://sustainabletechnologies.ca/app/uploads/2014/09/STEP-Bioretention-Report_2014.pdf</ref>.

A comparative performance assessment of bioretention in Ontario was conducted comparing 9 different bioretention facilities in the GTA. The results showed total suspended solids (TSS) load reductions between 88 to 99%, and total phosphorus load reductions between 68 and 92% for unlined facilities. Results for a lined bioretention swale for TSS and Total Phosphorus load reduction were 73 to 79% and -18 to -21% respectively.[https://sustainabletechnologies.ca/app/uploads/2019/10/STEP_Bioretention-Synthesis_Tech-Brief-New-Template-2019-Oct-10.-2019.pdf (STEP, 2019)]<ref>STEP. 2019. Comparative Performance Assessment of Bioretention in Ontario - Technical Brief.</ref>. Negative TP load reduction values were observed because effluent concentrations were higher than influent concentrations, and volume reduction through evapotranspiration was not sufficient to offset the increase in phosphorus concentration in biofilter effluent. [https://sustainabletechnologies.ca/app/uploads/2020/11/CC-Bioswale-Tech-brief-2018-FINAL.pdf (STEP, 2018)]<ref> Sustainable Technologies Evaluation Program. 2018. Effectiveness of Retrofitted Roadside Biofilter Swales - County Court Boulevard, Brampton Technical Brief. https://sustainabletechnologies.ca/app/uploads/2020/11/CC-Bioswale-Tech-brief-2018-FINAL.pdf </ref>. Other STEP studies in the Greater Toronto Area have displayed similar results, with 90% reduction in TSS load when compared to nearby asphalt runoff samples having median TSS concentrations near the provincial 30 mg/L standard (median = ~19 mg/L) [https://sustainabletechnologies.ca/app/uploads/2015/01/ER-Bio-Tech-Brief-Final.pdf STEP, 2014] <ref>Sustainable Technologies Evaluation Program. 2014. Performance Evaluation of a Bioretention System - Earth Rangers. Prepared by Toronto and Region Conservation. September 2014. https://sustainabletechnologies.ca/app/uploads/2014/09/STEP-Bioretention-Report_2014.pdf</ref>.

Another group of studies of bioretention facilities examines nutrient removal of these LID installation, with mixed results. Some facilities have been observed to increase total phosphorus in infiltrated water (Dietz and Clausen, 2005<ref>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.</ref>; Hunt ~~''et al''.~~, 2006<ref>Hunt, W.F. and W.G. Lord. 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</ref> ; TRCA, 2008<ref>. Performance Evaluation of Permeable Pavement and a Bioretention Swale, Seneca College, King City, Ontario. Prepared under the Sustainable Technologies Evaluation Program (STEP). Toronto, Ontario. </ref>). These findings have been attributed to leaching from filter media soil mixtures which contained high phosphorus content. To avoid phosphorus export, the phosphorus content ~~(i.e., Phosphorus Index)~~ of the filter media soil mixture should be examined prior to installation and kept between 10 to 30 ppm (Hunt and Lord, 2006<ref>~~Hunt~~, W.F. ~~and~~ W.G. ~~Lord~~. ~~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</ref>). While moderate reductions in total nitrogen and ammonia nitrogen have been observed in laboratory studies (Davis ''et al''., 2001<ref>Davis, A., M. Shokouhian, H. Sharma and C. Minami. 2001. Laboratory Study of Biological Retention for Urban Stormwater Management. Water Environment Research. 73(5): 5-14.</ref>) and field studies (Dietz and Clausen, 2005<ref>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.</ref>), nitrate nitrogen has consistently been observed to be low. ~~Little data exists~~ on the ability of bioretention to reduce bacteria concentrations~~, but preliminary~~ laboratory and field study results report good removal rates for fecal coliform bacteria (Rusciano and Obropta, 2005; Hunt ''et al''., 2006<ref>Hunt, W.F., A.R. Jarrett, J.T. Smith, and L.J. Sharkey. 2006. Evaluating Bioretention Hydrology and Nutrient Removal at Three Field Sites in North Carolina. ASCE Journal of Irrigation and Drainage Engineering. 132(6): 600-608.</ref>; TRCA, 2008<ref>. Performance Evaluation of Permeable Pavement and a Bioretention Swale, Seneca College, King City, Ontario. Prepared under the Sustainable Technologies Evaluation Program (STEP). Toronto, Ontario.</ref>).

Another group of studies of bioretention facilities examines nutrient removal of these LID installation, with mixed results. Some facilities have been observed to increase total phosphorus in infiltrated water (Dietz and Clausen, 2005<ref>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.</ref>; Hunt and Lord, 2006<ref>Hunt, W.F. and W.G. Lord. 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</ref> ; TRCA, 2008<ref>. Toronto and Region Conservation Authority. 2008. Performance Evaluation of Permeable Pavement and a Bioretention Swale, Seneca College, King City, Ontario. Prepared under the Sustainable Technologies Evaluation Program (STEP). Toronto, Ontario. https://sustainabletechnologies.ca/app/uploads/2013/03/PP_FactsheetSept2011-compressed.pdf</ref>). These findings have been attributed to leaching from filter media soil mixtures which contained high phosphorus content. To avoid phosphorus export, the plant-available (extractable) phosphorus content of the filter media soil mixture should be examined prior to installation and kept between 12 to 40 ppm (see [[Bioretention: Filter media | Filter media]]; Hunt and Lord, 2006). A design option to increase phosphorus removal performance of bioretention is to incorporate [[Additives | additives]] into the filter media bed, either blended into the media or as a layer in the aerobic portion of the filter bed, such as iron filings (i.e., zero valent iron)<ref>Erickson, A.J., Gulliver, J.S., Weiss, P.T. 2012. Capturing phosphates with iron enhanced sand filtration. Water Research. 46(9). 3032-3042. https://www.sciencedirect.com/science/article/abs/pii/S0043135412001728 </ref>, fly ash<ref>Zhang, W., Brown, G.O., Storm, D.E., Zhang, H. 2008. Fly-ash amended sand as filter media in bioretention cells to improve phosphorus removal. Water Environment Research. 80(6). 507-516. https://onlinelibrary.wiley.com/doi/abs/10.2175/106143008X266823 </ref> <ref>Kandel, S., Vogel, J., Penn, C., Brown, G. 2017. Phosphorus Retention by Fly Ash Amended Filter Media in Aged Bioretention Cells. Water. 9, 746. https://www.mdpi.com/2073-4441/9/10/746</ref>, iron (ferric) or aluminum hydroxide-based water treatment residuals (by-product from drinking water treatment)<ref>O'Neill, S.W., Davis, A.P. 2012a. Water treatment residual as a bioretention amendment for phosphorus. I. Evaluation studies. Journal of Environmental Engineering. 138(3). pp 318-327. https://ascelibrary.org/doi/10.1061/%28ASCE%29EE.1943-7870.0000409</ref> <ref>O'Neill, S.W., Davis, A.P. 2012b. Water treatment residual as a bioretention amendment for phosphorus. II. long-term column studies. Journal of Environmental Engineering. 138(3). pp 328-336. https://ascelibrary.org/doi/10.1061/%28ASCE%29EE.1943-7870.0000436</ref>, biochar <ref>Nabiul Afrooz, A.R.M., Boehm, A.B. 2017. Effects of submerged zone, media aging, and antecedent dry period on the performance of biochar-amended biofilters in removing fecal indicators and nutrients from natural stormwater. Ecological Engineering. 102. 320-330. https://www.sciencedirect.com/science/article/abs/pii/S0925857417301209 </ref> <ref>Mohanty, S.K., Valenca, R., Berger, A.W., Yu, I.K.M., Xiong, X., Saunders, T.M., Tsang, D.C.W. 2018. Plenty of room for carbon on the ground: Potential applications of biochar for stormwater treatment. Science of the Total Environment. 625. 1644-1658. https://www.sciencedirect.com/science/article/abs/pii/S0048969718300378 </ref>, proprietary filter media additives or blends, or by using iron-rich sand in the filter media blend. Read about a field evaluation comparing the phosphorus retention performance of parking lot bioretention cells featuring iron-rich sand and proprietary reactive media additive (Sorptive PTM) in the STEP [https://sustainabletechnologies.ca/app/uploads/2019/06/improving-nutrient-retention-in-bioretention-tech-brief.pdf technical brief]<ref>Sustainable Technologies Evaluation Program. 2018. Improving nutrient retention in bioretention. Technical Brief. https://sustainabletechnologies.ca/app/uploads/2019/06/improving-nutrient-retention-in-bioretention-tech-brief.pdf</ref>. While moderate reductions in total nitrogen and ammonia nitrogen have been observed in laboratory studies (Davis ''et al''., 2001<ref>Davis, A., M. Shokouhian, H. Sharma and C. Minami. 2001. Laboratory . Study of Biological Retention for Urban Stormwater Management. Water Environment Research. 73(5): 5-14.</ref>) and field studies (Dietz and Clausen, 2005<ref>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.</ref>), nitrate nitrogen removal has consistently been observed to be low. Design innovations to enhance nitrate-nitrogen removal performance of bioretention is an area of active research. Promising results have been observed from laboratory column and field-scale evaluations of underdrained practices featuring [[Bioretention: Internal water storage |internal water storage reservoirs]] containing mixtures of clear stone aggregate and shredded newspaper or wood chips, which creates low oxygen or anoxic conditions and promotes conversion of nitrate-nitrogen to nitrogen gas via denitrification <ref>Kim, H., Seagren, E.A., Davis, A.P. 2003. Engineered bioretention for removal of nitrate from stormwater runoff. Water Environment Research. 75(4). 335-367. https://onlinelibrary.wiley.com/doi/abs/10.2175/106143003X141169 </ref> <ref> Brown, R.A., Hunt, W.F. 2011. Underdrain configuration to enhance bioretention exfiltration to reduce pollutant loads. Journal of Environmental Engineering. 137(11). 1082-1091. https://ascelibrary.org/doi/abs/10.1061/(ASCE)EE.1943-7870.0000437 </ref> <ref> Wang, C., Wang, F., Qin, H., Zeng, X., Li, X. Yu, S. 2018. Effect of Saturated Zone on Nitrogen Removal Processes in Stormwater Bioretention Systems. Water. 10, 162. https://www.mdpi.com/2073-4441/10/2/162 </ref>.

Roseen et al. (2013) conducted both field and laboratory testing on the performance of bioretention cells featuring filter media amended with drinking water treatment residuals (WTR) with low solids content (5-10% solids) as an [[Additives| additive]]. Water treatment residuals were included at 10-15% of the total filter media mix by volume. Amended bioretention cells had median orthophosphate removal efficiencies of 90-99%. A second study found a bioretention design featuring WTR amended filter media and an [[Bioretention: Internal water storage|internal water storage zone]] optimized to remove phosphorus and nitrogen had an orthophosphate removal efficiency of 20% and effluent concentrations below 0.02 mg/L.<ref>Roseen, R.M., Stone, R.M. 2013. Evaluation and Optimization of Bioretention Design for Nitrogen and Phosphorus Removal. U.S. Environmental Protection Agency. https://www3.epa.gov/region1/npdes/stormwater/research/epa-final-report-filter-study.pdf</ref> More recently, LeFevre et al. (2015) present a state-of-the-art review of dissolved stormwater pollutant sources (focusing on nutrients, toxic metals and organic compounds), typical concentrations, and removal mechanisms and fate in bioretention, along with design options to enhance their retention <ref>LeFevre, G.H., Paus, K.H., Natarajan, P., Gulliver, J.S., Novak, P.J., Hozalski, R.M. 2015. Review of Dissolved Pollutants in Urban Storm Water and Their Removal and Fate in Bioretention Cells. Journal of Environmental Engineering. 141(1). https://ascelibrary.org/doi/abs/10.1061/(ASCE)EE.1943-7870.0000876 </ref>.

[[File:Bioretnetion TSS.jpg|200px|thumb]]

The two box plot figures to the right show combined stormwater effluent quality results from STEP monitoring projects conducted over a 16-year time period (between 2005 and 2021) at sites within Greater Toronto Area (GTA) municipalities. Total Suspended Solid (TSS) effluent concentration results for bioretention practices represent the combined results from 9 sites in the GTA and a total of 301 monitored storm events. Median TSS concentration was found to be 9.5 mg/L and exceeded the Canadian Water Quality Guideline of 30 mg/L (CCME, 2002<ref>Canadian Council of Ministers of the Environment (CCME). 2002. Canadian water quality guidelines for the protection of aquatic life: Total particulate matter. In: Canadian Environmental Quality Guidelines, Canadian Council of Ministers of the Environment, Winnipeg</ref>) during only 15% of the 301 monitored storm events. Median TP concentration was found to be 0.09 mg/L and exceeded the Ontario Provincial Water Quality Objective (PWQO) of 0.03 mg/L (OMOEE, 1994<ref>Ontario Ministry of Environment and Energy (OMOEE), 1994. Policies, Guidelines and Provincial Water Quality Objectives of the Ministry of Environment and Energy. Queen’s Printer for Ontario. Toronto, ON.</ref>) during 86% of monitored storm events. In comparison, median TP effluent concentration for bioretention in the International Stormwater BMP Database was found to be 0.240 mg/L, based on 850 monitored storm events (Clary et al. 2020)<ref>Clary, J., Jones, J., Leisenring, M., Hobson, P., Strecker, E. 2020. International Stormwater BMP Database: 2020 Summary Statistics. The Water Research Foundation. [https://www.waterrf.org/system/files/resource/2020-11/DRPT-4968_0.pdf</ref>, which is well above the Ontario PWQO of 0.03 mg/L. These results indicate that the design of bioretention draining to phosphorus-limited receiving waterbodies should include variations to improve [[Phosphorus]] retention. An example of such a design variation is including sorption [[Additives| additives]] in [[Bioretention: Filter media]]. Please refer to the [[Phosphorus]] and [[Additives]] pages for further guidance.

[[File:Bioretnetion TP.jpg|200px|thumb]]

The mechanisms involved in, and ability of bioretention to reduce bacteria and other microbial pathogen concentrations is also an area of active research. Preliminary laboratory and field study results report good but variable removal rates for fecal coliform bacteria from biofilters and bioretention cells (Rusciano and Obropta, 2005<ref> Rusciano, G.M., Obropta, C.C. 2007. Bioretention Column Study: Fecal Coliform and Total Suspended Solids Reductions. Transactions of the ASABE. 50(4): 1261-1269. https://elibrary.asabe.org/abstract.asp??JID=3&AID=23636&CID=t2007&v=50&i=4&T=1 </ref>; Hunt ''et al''., 2006<ref>Hunt, W.F., A.R. Jarrett, J.T. Smith, and L.J. Sharkey. 2006. Evaluating Bioretention Hydrology and Nutrient Removal at Three Field Sites in North Carolina. ASCE Journal of Irrigation and Drainage Engineering. 132(6): 600-608.</ref>; TRCA, 2008<ref>. Performance Evaluation of Permeable Pavement and a Bioretention Swale, Seneca College, King City, Ontario. Prepared under the Sustainable Technologies Evaluation Program (STEP). Toronto, Ontario.</ref>). In a recent review, Clary et al. (2020) report bioretention E.coli removal efficiency of 42.5% and fecal coliform removal efficiency of 99.4% based on median inlet and outlet concentrations from 12 and 8 studies, respectively <ref> Clary, J. Jones, Leisenring, M., Hobson, P., Strecker, E. 2020. International Stormwater BMP Database 2020 Statistical Summary. https://www.waterrf.org/system/files/resource/2020-11/DRPT-4968_0.pdf</ref>. In a recent article, Peng et al. (2016) review factors influencing microbial removal and effects of design choices on treatment performance. They found that approaches for improving the removal of microorganisms by biofilters could involve altering the grain size range and surface properties of the filter media. This could involve the use of filter media with smaller average grain sizes, the inclusion of [[Additives |additives]] (e.g., activated carbon, zeolite, or biochar) to improve filtration rates, or chemical modifications of filter media grain surfaces (e.g., with biocides) to promote microbial die-off. Including an [[Bioretention: Internal water storage |internal water storage reservoir]] was also found to improve microbial removal rates <ref> Peng, J., Cao, Y., Rippy, M.A., Nabuil Afrooz, A.R.M., Grant, S.B. 2016. Indicator and Pathogen Removal by Low Impact Development Best Management Practices. Water. 8. 600. https://www.mdpi.com/2073-4441/8/12/600 </ref>.

Recent research into the role of plants in bioretention confirms they play an important roles in hydraulic and nitrogen removal performance. In a recent review of scientific literature, Dagenais ''et al.'' (2018) found that planted facilities are more effective than unplanted ones, as the presence of plants increases filter bed permeability and nitrogen removal. Plant species selection can considerably affect hydraulic and nitrogen removal performance, with root traits (e.g., thickness and depth) identified as playing important roles. They identified further research needed to test the hypothesis that native or diversely-planted facilities perform better than ones planted with exotic or fewer species.<ref>Dagenais, D., Brisson, J. and Fletcher, T.D. 2018. The role of plants in bioretention systems; does the science underpin current guidance?. Ecological Engineering, 120, pp.532-545. http://www.phytotechno.com/wp-content/uploads/2018/10/Dagenais-2018-Bioretention.pdf</ref>

===Stream Channel Erosion===

The feasibility of storing the channel erosion control volume within bioretention areas will be dependent on the size of the drainage area and available space~~. It may prove infeasible due to the large footprint needed to maintain the recommended maximum ponding depth of 200 mm~~. Meeting the channel erosion control requirement through bioretention is most feasible in the regions of the Greater Toronto Area with [[A and B soils~~|Soil groups~~]]. In these situations, the reduction in runoff volume through infiltration and evapotranspiration may be sufficient. It is important to note that the bioretention practice will infiltrate runoff ~~throughout~~ the ~~course~~ of the storm~~; so the actual capacity of the bioretention cell~~ to ~~capture runoff from~~ the ~~drainage area will be larger than its designed storage volume~~.

The feasibility of storing the channel erosion control volume within bioretention areas will be dependent on the size of the drainage area and available space. Meeting the channel erosion control requirement through bioretention is most feasible in the regions of the Greater Toronto Area with [[Soil groups|Hydrologic Soil Group A and B soils]]. In these situations, the reduction in runoff volume through infiltration and evapotranspiration may be sufficient. For facilities with constrained footprints, include a flow restrictor on the underdrain perforated pipe or outlet storm sewer to control the release rate, and size the internal water storage reservoir to retain the necessary volume of water. Where acceptable, consider increasing the maximum ponding depth beyond the recommended 350 mm. It is important to note that the bioretention practice will infiltrate runoff into the filter media bed and underlying native soil over the duration of the design storm event, which can be factored into surface ponding and internal water storage reservoir sizing to optimize the design. See [[Bioretention: Sizing | Sizing]] for further guidance.

===Other Benefits===

The benefits of bioretention reach beyond the specific stormwater management goals to other social and environmental benefits, including:

*''Reduced thermal aquatic impacts'': Bioretention and other filtration and infiltration practices benefit aquatic life by reducing thermal impacts on receiving waters from urban runoff (Jones and Hunt, 2009<ref>Jones, M.P. and Hunt, W.F. 2009. Bioretention Impact on Runoff Temperature in Trout Sensitive Waters. Journal of Environmental Engineering. Vol. 135. No. 8. Pp. 577-585.</ref>). Unlike detention ponds, bioretention does not raise water temperature and can help maintain baseflows through infiltration.

*''Snow Storage'': Bioretention areas can be used for snow storage and snow melt treatment from the contributing drainage area during winter, especially those located adjacent to parking lots and roadways. To function as snow storage, bioretention must include an overflow for snow melt in excess of the designed ponding depth. Additionally, the plant material must be salt-tolerant, perennial and tolerant of periodic inundation.

*''Reduced Urban Heat Island'': Bioretention is able to reduce the local urban heat island by introducing soils and vegetation into urban areas, such as parking lots. Vegetation absorbs less solar radiation than hard urban surfaces. Also, the water vapor emitted by plant material also cools ambient temperatures.

==See also==

*[[Rain gardens]]

*[[Bioswales]]

*[[~~Rain gardens~~]]

*[[Stormwater planters]]

*[[~~Trees~~]]

*[[Stormwater Tree Trenches |Stormwater tree trenches]]

*[[Bioretention: Streetscapes]]

==External links==

*[https://www.toronto.ca/ext/digital_comm/pdfs/transportation-services/green-streets-technical-guidelines-document-v2-17-11-08.pdf City of Toronto Green Streets Technical Guidelines - Version 1.0 (2017) Schollen & Company Inc., Urban Forest Innovators, TMIG, DMG]

*[https://store.csagroup.org/?cclcl=en_US/ CSA W200-18 Design of Bioretention Systems (2018) CSA Group]

*[https://store.csagroup.org/?cclcl=en_US/ CSA W201-18 Construction of Bioretention Systems (2018) CSA Group]

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