Bioretention: Filter media

It is recommended that the mixture comprises:
One of these two blend options
 	Blend A: Drainage rate priority 	Blend B: Water quality treatment priority
Application	Impervious area to pervious area (I:P) ratio of 15:1 or greater 	
Proportions	
3 parts sand 
1 part organic soil components and additives 	
3 parts sand 
2 parts topsoil 
1 part organic soil components and additives
Porosity 	This mixture may be assumed to have a porosity of 0.4 unless demonstrated otherwise	This mixture may be assumed to have a porosity of 0.35 unless demonstrated otherwise
Filter media should be obtained premixed from the vendor and meet all municipal, provincial and federal environmental standards. To minimize transportation-related environmental impacts, filter media should be obtained from local vendors and blended from locally sourced materials. Topsoil used to produce the mix should be passed through a 5 centimetre (2 inch) screen to remove large rocks, roots and other debris, while retaining soil peds. Samples of the filter media should be dried, ground and tested by a certified soil testing laboratory to ensure they meet the following specifications:
 Bioretention filter media
Characteristic 	Criterion 	Recommended test method
Particle-size distribution (PSD) 	< 25% silt- and clay-sized particles (smaller than 0.05 mm) combined; 3 to 12% clay-sized particles (0.002 mm or smaller)	ASTM D6913/D6913M, Standard Test Methods for Particle-Size Distribution (Gradation) of Soils Using Sieve Analysis (pebble to sand fraction); and 
ASTM D7928, Standard Test Method for Particle-Size Distribution (Gradation) of Fine-Grained Soils Using the Sedimentation (Hydrometer) Analysis (silt and clay fraction).
Organic matter (OM) 	3 to 10% by dry weight 	ASTM F1647, Standard Test Methods for Organic Matter Content of Athletic Field Rootzone Mixes.
Phosphorus, plant-available or extractable	12 to 40 ppm 	As measured by the 'Olsen' method for alkaline and calcareous soils (common in Ontario). Alternatives include 'Mehlich I or III', or 'Bray', which is better suited to acidic to slightly alkaline and non-calcareous soils. NB: Results from different test methods are not directly comparable.Sawyer JE, Mallarino AP. Differentiating and Understanding the Mehlich 3, Bray, and Olsen Soil Phosphorus Tests 1. http://www.agronext.iastate.edu/soilfertility/info/mnconf11_22_99.pdf. Accessed August 1, 2017.
Cationic exchange capacity (CEC) 	> 10 meq/100 g 	ASTM D7503, Standard Test Methods for Measuring the Exchange Complex and Cation Exchange Capacity of Inorganic Fine-Grained Soils.
Hydraulic conductivity, saturated (Kf) 	> 75 mm/h; Blend A > 25 mm/h; Blend B < 300 mm/h; Blend A and B 	ASTM D2434, Standard Test Method for Permeability of Granular Soils (Constant Head), when the sample is compacted to 85% of its maximum dry density in accordance with ASTM D698, Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort.
Note that you may choose not to use particle-size distribution as a criterion for acceptance of a filter media blend, but saturated hydraulic conductivity should be one. While information on particle-size distribution and soil texture are useful in selecting plants, the disconnect between hydraulic conductivity and uniformity of gradation makes it far less important than measuring the saturated hydraulic conductivity directlyCRC for Water Sensitive Cities. (2015). Adoption Guidelines for Stormwater Biofiltration Systems: Appendix C - Guidelines for filter media in stormwater biofiltration systems. 
==Sand==
==Topsoil==
==Organic component==
This is the first big opportunity to manage phosphorus export from a bioretention or stormwater planter system. While compost is the most common choice, designers working in nutrient-sensitive receiving waters are encouraged to explore the alternatives listed below. Some of these materials may be combined 50:50 with compost to balance providing the nutrients required by the plants with limiting the potential for leaching of excess nutrients. 
 
===Compost===
Compost is the most widely used organic component. It's use in bioretention facilities is well established and documented. In Ontario, compost should comply with mandatory Ontario Compost Quality Standards for Category 'AA'.Ontario Ministry of the Environment and Climate Change (OMOECC). 2012. Ontario Compost Quality
Standards, July 25, 2012. PIBS 8412. Queen’s Printer of Ontario, Toronto, ON.
https://www.ontario.ca/page/ontario-compost-quality-standards. See Compost page for a summary of Category 'AA" compost standards. Compost should also be certified to meet quality parameters recommended under the Compost Council of Canada Compost Quality Alliance (CQA) program.A & L Canada Laboratories. 2004. Compost Management Program. London, ON.
http://www.alcanada.com/index_htm_files/compost_handbook.pdf. Low available phosphorus composts should always be sought for use in low impact development facilities, including bioretention. Low available phosphorus composts are typically created from feedstocks including yard, leaf, and wood waste, and excluding manures, biosolids, and food scraps.Hurley S, Shrestha P, Cording A. Nutrient Leaching from Compost: Implications for Bioretention and Other Green Stormwater Infrastructure. J Sustain Water Built Environ. 2017;3(3):4017006. doi:10.1061/JSWBAY.0000821.
===Organic component alternatives===
Even low-phosphorus composts are known to export phosphorus over many years. The use of compost is not recommended in nutrient-sensitive watersheds where phosphorus pollution is a concern, or an additive to enhance nutrient retention of the media should also be included in the blend, or a layer be included above the stone reservoir, or a reactive media vault be included in the treatment train downstream of the bioretention (see Additive below for available options). There are a number of alternative sources of soil organic matter which have undergone field studies which have benefits and potential concerns:
 Organic soil components
Material 	Benefits 	Concerns
Coconut coirRheaume, A., Hinman, C., and Ahearn, D. (2015). “A Synthesis of Bioretention Research in Pacific Northwest.” Herrera, <http://www.modularwetlands.com/new/wp-content/uploads/2015/11/2-Bioretention-Synthesis-2015-DAhearn.pdf>	Doesn't leach phosphorus 	Must be imported
Wood chips 	Doesn't leach phosphorusPromotes nitrogen removal from water	
Peat moss 	Doesn't leach phosphorus 	Must be unsustainably extracted from natural wetlands
Shredded paper (e.g., Pittmoss) 	Doesn't leach phosphorusPromotes denitrification 	
===Wood derivatives===
The 2017 guidance from New Hampshire specifically rules against the inclusion of compost in their bioretention media.UNHSC Bioretention Soil Specification. (2017). Retrieved from https://www.unh.edu/unhsc/sites/default/files/media/unhsc_bsm_spec_2-28-17_0.pdf Instead they recommend "Shredded wood, wood chips, ground bark, or wood waste; of uniform texture and free of stones, sticks". The use of wood chip has been common in New Hampshire for some time, in this 2006 thesis 20% wood chips (not characterized) were incorporated into all of the test cases to match current practices at the time. Stone, R. M. (2013). Evaluation and Optimization of Bioretention Design for Nitrogen and Phosphorus Removal. University of New Hampshire. Retrieved from https://www.unh.edu/unhsc/sites/unh.edu.unhsc/files/STONE THESIS FINAL.pdf
Shredded paper has been tested as an additional source of carbon and as an electron-donor to promote denitrification in a number of successful laboratory and field studies. 
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==Additives==
Typically these components would make up 5 to 10% by volume of the filter media mixture. 
==References==