Engineered (bioretention) media mixes for stormwater applications

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Caution: Mixes A, B, E, and F should not be used in BMPs having an underdrain unless the mix is amended to retain phosphorus.

The selection of an engineered media mix depends on the conditions and objectives at a site.

Media mixes for filtration practices

Mixes C and D are acceptable for filtration practices (e.g. BMPs with an underdrain). Mixes A, B, E, and F, discussed in the next section, should be avoided when phosphorus is a surface water quality concern unless amended to retain phosphorus. Amendments include substituting a source of organic matter less prone to leaching phosphorus (e.g. coir, biochar), or chemicals that attenuate phosphorus (e.g. iron, aluminum).

Mix C: North Carolina State University water quality blend

Source: North Carolina Department of Environment and Natural Resources, 2009. See Section 12.3.4

This mix is a homogenous soil mix of

• 85 to 88 percent by volume sand (USDA Soil Textural Classification);
• 8 to 12 percent fines by volume (silt and clay, with a maximum clay content of 5% recommended); and
• 3 to 5 percent organic matter by volume (ASTM D 2974 Method C) MnDOT Grade 2 compost (See Specification 3890, page 685) is recommended.

A higher concentration of fines (12 percent) should be reserved for areas where nitrogen is the target pollutant. In areas where phosphorus is the target pollutant, a lower concentration of fines (8 percent) should be used. A soil phosphorus test using the Mehlich-3 (or equivalent) method is recommended but not required to receive water quality credits. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram. This is enough phosphorus to support plant growth without exporting phosphorus from the cell. It is assumed this mix will not exceed the upper range of recommended values (30 milligrams per kilogram), although at lower concentrations of organic matter a soil test may be needed to confirm there is adequate phosphorus for plant growth.

Mix D

Caution: If phosphorus is a water quality concern for receiving waters, Bioretention Mix D (as well as Mix C) is recommended when using infiltration systems having an underdrain. The following discussion provides general guidelines for Bioretention Mix D. If using or considering Bioretention Mix D, please see specific guidelines for this mix to avoid confusion with Mixes A, B, and C.

Bioretention Soil Mix D soil shall be a mixture of coarse sand, compost and topsoil in proportions which meet the following:

• silt plus clay (combined): 25 to 40 percent, by dry weight
• total sand: 60 to 75 percent, by dry weight
• total coarse and medium sand: minimum of 55 percent of total sand, by dry weight
• fine gravel less than 5 millimeters: up to 12 percent by dry weight (calculated separately from sand/silt/ clay total)
• organic matter content: 2 to 5 percent, percent loss on ignition by dry weight; MnDOT Grade 2 compost (See Specification 3890, page 685) is recommended.
• saturated hydraulic conductivity: 1 to 4 inches per hour ASTM F1815. Note that although this infiltration rate is generally applicable at 85 percent compaction, Standard Proctor ASTM D968, this is an infiltration rate standard and not a compaction standard. Therefore, this infiltration rate may be met at lower levels of compaction.

Suggested mix ratio ranges, by volume, are

• Coarse sand: 50 to 65 percent
• Topsoil: 25 to 35 percent
• Compost (assuming MnDOT Grade 2 compost is being used): 10 to 15 percent. Note this yields an organic matter content of approximately 2 to 5 percent.

Note that the above mix ratios are on a volume basis rather than a weight basis. See specific guidance on these.

A soil phosphorus test using the Mehlich-3 (or equivalent) method is recommended but not required to receive water quality credits. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram. This is enough phosphorus to support plant growth without exporting phosphorus from the cell. It is assumed this mix will not exceed the upper range of recommended values (30 milligrams per kilogram), although at lower concentrations of organic matter a soil test may be needed to confirm there is adequate phosphorus for plant growth.

Media mixes for infiltration practices or modified infiltration practices

The following mixes are acceptable for infiltration practices.

Mix A: Water quality blend

A well blended, homogenous mixture of

• 60 to 70 percent construction sand;
• 15 to 25 percent top soil; and
• 15 to 25 percent organic matter.

Sand: Provide clean construction sand, free of deleterious materials. AASHTO M-6 or ASTM C-33 washed sand.

Top Soil: Sandy loam, loamy sand, or loam texture per USDA textural triangle with less than 5 percent clay content

Organic Matter: MnDOT Grade 2 compost (See Specification 3890, page 685) is recommended.

It is assumed this mix will leach phosphorus. When an underdrain is utilized a soil phosphorus test is needed to receive water quality credits for the portion of stormwater captured by the underdrain. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram when using the Mehlich-3 (or equivalent) test. This is enough phosphorus to support plant growth without exporting phosphorus from the cell.

Mix B: Enhanced filtration blend

A well-blended, homogenous mixture of

• 70 to 85 percent construction sand; and
• 15 to 30 percent organic matter.

Sand: Provide clean construction sand, free of deleterious materials. AASHTO M-6 or ASTM C-33 washed sand.

Top Soil in the mix will help with some nutrient removal, especially nutrients, but extra care must be taken during construction to inspect the soils before installation and to avoid compaction.

Organic Matter: MnDOT Grade 2 compost (See Specification 3890, page 685) is recommended.

It is assumed this mix will leach phosphorus. When an underdrain is utilized a soil phosphorus test is needed to receive water quality credits for the portion of stormwater captured by the underdrain. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram when using the Mehlich-3 (or equivalent) test. This is enough phosphorus to support plant growth without exporting phosphorus from the cell.

Mix C: North Carolina State University water quality blend

Source: North Carolina Department of Environment and Natural Resources, 2009. See Section 12.3.4

This mix is a homogenous soil mix of

• 85 to 88 percent by volume sand (USDA Soil Textural Classification);
• 8 to 12 percent fines by volume (silt and clay, with a maximum clay content of 5% recommended); and
• 3 to 5 percent organic matter by volume (ASTM D 2974 Method C) MnDOT Grade 2 compost (See Specification 3890, page 685) is recommended.

A higher concentration of fines (12 percent) should be reserved for areas where nitrogen is the target pollutant. In areas where phosphorus is the target pollutant, a lower concentration of fines (8 percent) should be used. A soil phosphorus test using the Mehlich-3 (or equivalent) method is recommended but not required to receive water quality credits. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram. This is enough phosphorus to support plant growth without exporting phosphorus from the cell. It is assumed this mix will not exceed the upper range of recommended values (30 milligrams per kilogram), although at lower concentrations of organic matter a soil test may be needed to confirm there is adequate phosphorus for plant growth.

Mix D

Caution: If phosphorus is a water quality concern for receiving waters, Bioretention Mix D (as well as Mix C) is recommended when using infiltration systems having an underdrain. The following discussion provides general guidelines for Bioretention Mix D. If using or considering Bioretention Mix D, please see specific guidelines for this mix to avoid confusion with Mixes A, B, and C.

Bioretention Soil Mix D soil shall be a mixture of coarse sand, compost and topsoil in proportions which meet the following:

• silt plus clay (combined): 25 to 40 percent, by dry weight
• total sand: 60 to 75 percent, by dry weight
• total coarse and medium sand: minimum of 55 percent of total sand, by dry weight
• fine gravel less than 5 millimeters: up to 12 percent by dry weight (calculated separately from sand/silt/ clay total)
• organic matter content: 2 to 5 percent, percent loss on ignition by dry weight; MnDOT Grade 2 compost (See Specification 3890, page 685) is recommended.
• saturated hydraulic conductivity: 1 to 4 inches per hour ASTM F1815. Note that although this infiltration rate is generally applicable at 85 percent compaction, Standard Proctor ASTM D968, this is an infiltration rate standard and not a compaction standard. Therefore, this infiltration rate may be met at lower levels of compaction.

Suggested mix ratio ranges, by volume, are

• Coarse sand: 50 to 65 percent
• Topsoil: 25 to 35 percent
• Compost (assuming MnDOT Grade 2 compost is being used): 10 to 15 percent. Note this yields an organic matter content of approximately 2 to 5 percent.

Note that the above mix ratios are on a volume basis rather than a weight basis. See specific guidance on these.

A soil phosphorus test using the Mehlich-3 (or equivalent) method is recommended but not required to receive water quality credits. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram. This is enough phosphorus to support plant growth without exporting phosphorus from the cell. It is assumed this mix will not exceed the upper range of recommended values (30 milligrams per kilogram), although at lower concentrations of organic matter a soil test may be needed to confirm there is adequate phosphorus for plant growth.

Mix E: MnDOT 3877.2 Type G 'Filter Topsoil Borrow'

A well-blended, homogenous mixture of

• 60 to 80 percent sand meeting gradation requirements of 3126, “Fine Aggregate for Portland Cement Concrete”; and
• 20 to 40 percent compost meeting requirements 3890 Grade 2 Compost.

Provide topsoil borrow containing two blended components of sand and compost for water quality, plant growing medium, and filtration medium with a filtration rate of at least 4 inches per hour [10 centimeters per hour].

See page 672 of MnDOT Standard Specifications for Construction

Mix F: Custom Infiltration Basin Planting Soil

This mix is a homogenous soil mix of

• 75 percent by weight loamy sand (USDA Soil Textural Classification based on grain size); and
• 25 percent by weight MnDOT grade 2 compost (See page 687 of Standard Specifications for Construction, Specification 3890).

Loamy sand as determined by the USDA soil texture classification based on grain size. Loamy sand is defined as soil material that contains at the upper limit 85 to 90 percent sand, and the percentage of silt plus 1.5 times the percentage of clay is not less than 15. At the lower limit it contains not less than 70 to 85 percent sand, and the percentage of silt plus twice the percentage of clay does not exceed 30. In addition, the maximum particle size shall be less than 1-inch.