Bioretention: Filter media

Filter media should be obtained premixed from a vendor and meet all municipal, provincial and federal environmental standards. Mixing of sand, topsoil and compost should be done in a manner that preserves topsoil peds. The mixture should be free of stones, stumps, roots, or other debris larger than 50 mm diameter. Samples of the filter media should be dried, ground and tested to ensure they meet the following specifications:

It is recommended that the mixture comprises:

• 70 - 80 % coarse sand
• 10 - 15 % topsoil
• 10 - 15 % organic soil component

The final blended product should have the following properties:

Characteristic	Criterion	Recommended method
Texture	<20 % fines	Hygrometer
Organic matter (OM)	5 - 10%	ASTM D2974-14, Standard test methods for moisture, ash and organic matter of peat and other organic soils.
Phosphorus	10 - 30 ppm	'Bray' is most common in Ontario. Alternatives include 'Mehlich I or III', or 'Olsen'.
Cationic exchange capacity(CEC)	> 10 meq/100g	ASTM D7503-10, Standard test method for measuring the exchange complex and cation exchange capacity of inorganic fine grained soils.
Hydraulic conductivity	> 25 mm/hr <250 mm/hr	Falling head or constant head KSAT

Sand

Particle size distribution graph for ASTM C33 sand, as described in table

• Coarse sand for LID construction shall be washed clean and free of toxic materials.
• The pH of the sand shall be ≤ 7.0.
• The coarse sand shall have a fineness modulus index between 2.8 and 3.1 according to ASTM C33/C33M, or otherwise meet the gradation below.

Particle size distribution

Sieve	Percent passing
9.5 mm	100
4.75 mm (No.4)	95 - 100
2.36 mm (No.8)	80 - 100
1.18 mm (No.16)	50 - 85
0.60 mm (No.30)	25 - 50
0.30 mm (No.50)	5 - 30
0.15 mm (No.100)	0 - 10
0.075 mm (No.200)	≤ 3

Topsoil

• Topsoil may be material that was stripped from the project site and stored in stockpiles for re-use, or material imported to the site from a supplier provided the physical and chemical characteristics are within acceptable ranges.
• Topsoil shall be in compliance with Ontario Regulation 153/04 Record of Site Condition standards for soil quality or as amended through Ontario Management of Excess Soil - A Guide for Best Management Practices.
• Soil laboratory reports shall certify the material to be suitable for re-use on residential, parkland, institutional, industrial, commercial, or community landscapes for the germination of seeds and the support of vegetative growth.

The factors to consider in determining if a topsoil is suitable for use as planting soil for a vegetated stormwater practice, or use in producing a bioretention filter media mixture include the following:

• Must be friable and capable of sustaining vigorous plant growth;
• Must be free from toxic material and roots, stones or debris over 50 mm (2") in diameter;
• Should not have been passed through sieves or screens smaller than 50 mm (2”) to avoid eliminating peds;
• Should have a Loamy Sand, Sandy Loam, Sandy Clay Loam, Loam or Silty Loam soil texture;
• For use as planting soil for a vegetated stormwater practice , the topsoil must contain a minimum of 5% organic matter by dry weight or be amended so, through addition of an organic soil conditioner;
• For use in producing bioretention filter media Blend B (water quality treatment priority), the topsoil must contain at least 9%, and not greater than 36% clay-sized particles and at least 2% organic matter by dry weight.
• Must have a pH of between 6.0 and 8.0;
• Must have a sodium absorption ratio less than 15;
• Must have a cationic exchange capacity greater than 10 milliequivalents per 100 grams (meq/100 g).

Specify that 4 litre samples of topsoil, from each source to be drawn upon, be provided to the consultant for visual inspection, along with topsoil quality test results from an accredited soil testing laboratory, or a quality assurance certificate from the supplier.

We recommend a planting soil or filter media depth of 300 mm to support grasses, 600 mm for shrubs and perennials, and 1000 mm for trees.

Organic components

Compost

Compost is the most widely used organic component. It's use in bioretention facilities is well established and documented. Low-phosphorus composts should always be sought for use in low impact development facilities, including bioretention. These are typically created from feedstocks including yard, leaf, and wood waste, and must exclude manures, biosolids, and food scraps.

Compost Specifications

Even low-phosphorus composts are known to export phosphorus over many years. The use of compost is not recommended in watersheds for which phosphorus pollution is a concern. There are alternatives which have undergone field study, each of which has a number of benefits and potential concerns:

Organic soil components

Material	Benefits	Potential concerns
Coconut coir	Doesn't leach P	Requires importation
Wood chip	Doesn't leach P Promotes nitrogen removal from water	TBD

Coconut coir

http://www.modularwetlands.com/new/wp-content/uploads/2015/11/2-Bioretention-Synthesis-2015-DAhearn.pdf

Wood derivatives

https://www.unh.edu/unhsc/sites/default/files/media/unhsc_bsm_spec_2-28-17_0.pdf https://www.unh.edu/unhsc/sites/unh.edu.unhsc/files/STONE%20THESIS%20FINAL.pdf https://jbioleng.biomedcentral.com/articles/10.1186/s13036-017-0057-4 (focus on denitrification)

Amendments

Typically these type of amendments would comprise 5- 10 % by volume of the filter media mixture. It is recommended that this fraction be taken from the topsoil or the organic portion otherwise; keeping the sand as minimum 65 % by volume.

Amendments

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  Sandy filter media mix

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The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

Mineral 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.

Decayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

Soil particles with a diameter less than 0.050 mm.

Mineral 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.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

1. 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).

1. 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).

Decayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.