Construction specifications for bioretention
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Construction specifications for bioretention

This page provides construction details, materials specifications and construction specifications for bioretention systems.

Construction details

Illustration of a cross-section for a bioretention facilities general plan. To access the .dwg file, click here.

CADD based details for bioretention are contained in the Computer-aided design and drafting (CAD/CADD) drawings section. The following details, with specifications, have been created for bioretention systems:

Bioretention Facilities General Plan
Bioretention Facilities Performance Types Cross-Sections
- Infiltration / Recharge Facility
- Filtration / Partial Recharge Facility
- Infiltration / Filtration / Recharge Facility
- Filtration Only Facility

Materials specifications - filter media

This site is currently undergoing final review. For more information, open this link.
The anticipated review period for this page is through March 2014

Filter media depth

Research has shown that minimum bioretention soil media depth needed varies depending on the target pollutant(s).

Minimum bioretention soil media depths recommended to target specific stormwater pollutants. From Hunt et al. (2012) and Hathaway et al., (2011). NOTE: The Construction Stormwater permit requires a 3 foot separation from the bottom of an infiltration practice and bedrock or seasonally saturated soils.
Link to this table

Pollutant	Depth of Treatment with upturned elbow or elevated underdrain	Depth of Treatment without underdrain or with underdrain at bottom	Minimum depth
Total suspended solids (TSS)	Top 2 to 3 inches of bioretention soil media	Top 2 to 3 inches of bioretention soil media	Not applicable for TSS because minimum depth needed for plant survival and growth is greater than minimum depth needed for TSS reduction
Metals	Top 8 inches of bioretention soil media	Top 8 inches of bioretention soil media	Not applicable for metals because minimum depth needed for plant survival and growth is greater than minimum depth needed for metals reduction
Hydrocarbons	3 to 4 inch Mulch layer, top 1 inch of bioretention soil media	3 to 4 inches Mulch layer, top 1 inch of bioretention soil media	Not applicable for hydrocarbons because minimum depth needed for plant survival and growth is greater than minimum depth needed for hydrocarbons reduction
Nitrogen	From top to bottom of bioretention soil media; Internal Water Storage Zone (IWS) improves exfiltration, thereby reducing pollutant load to the receiving stream, and also improves nitrogen removal because the longer retention time allows denitrification to occur underanoxic conditions.	From top to bottom of bioretention soil media	Retention time is important, so deeper media is preferred (3 foot minimum)
Particulate phosphorus	Top 2 to 3 inches of bioretention soil media.	Top 2 to 3 inches of bioretention soil media.	Not applicable for particulate phosphorus because minimum depth needed for plant survival and growth is greater than minimum depth needed for particulate phosphorus reduction
Dissolved phosphorus	From top of media to top of submerged zone. Saturated conditions cause P to not be effectively stored in submerged zone.	From top to bottom of bioretention soil media	Minimum 2 feet, but 3 feet recommended as a conservative value; if IWS is included, keep top of submerged zone at least 1.5 to 2 feet from surface of media
Pathogens	From top of soil to top of submerged zone.	From top to bottom of bioretention soil media	Minimum 2 feet; if IWS is included, keep top of submerged zone at least 2 feet from surface of media
Temperature	From top to bottom of bioretention soil media; Internal Water Storage Zone (IWS) improves exfiltration, thereby reducing volume of warm runoff discharged to the receiving stream, and also improves thermal pollution abatement because the longer retention time allows runoff to cool more before discharge.	From top to bottom of bioretention soil media	Minimum 3 feet, with 4 feet preferred

Performance specifications

The following performance specifications are applicable to all bioretention media.

Growing media must be suitable for supporting vigorous growth of selected plant species.
The pH range (Soil/Water 1:1) is 6.0 to 8.5
Soluble salts (soil/Water 1:2) should not to exceed 500 parts per million
All bioretention growing media must have a field tested infiltration rate between 1 and 8 inches per hour. Growing media with slower infiltration rates could clog over time and may not meet drawdown requirements. Target infiltration rates should be no more than 8 inches per hour to allow for adequate water retention for vegetation as well as adequate retention time for pollutant removal. The following infiltration rates should be achieved if specific pollutants are targeted in a watershed.
- Total suspended solids: Any rate is sufficient, 2 to 6 inches recommended
- Pathogens: Any rate is sufficient, 2 to 6 inches recommended
- Metals: Any rate is sufficient, 2 to 6 inches recommended
- Temperature: slower rates are preferable (less than 2 inches per hour)
- Total nitrogen (TN): 1 to 2 inches per hour, with 1 inch per hour recommended
- Total phosphorus (TP): 2 inches per hour

The following additional bioretention growing media performance specifications are required to receive P reduction credit.

Option A - use bioretention soil with phosphorus content between 12 and 30 mg/kg per Mehlich III test
Option B - include a soil amendment that facilitates adsorption of phosphorus

Cation exchange capacity (CEC) is a measure of a soil's ability to adsorb and exchange cations. A CEC of 10 milliequivalents or greater per gram of soil is recommended.

Guidance for bioretention media composition

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 is recommended. (see also the section on Using Compost as a Soil Amendment

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 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 is recommended. (see also the section on Using Compost as a Soil Amendment

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 test. This is enough phosphorus to support plant growth without exporting phosphorus from the cell.

Mix C: North Carolina State University water quality blend (North Carolina Department of Environment and Natural Resources. 2009)

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); and
3 to 5 percent organic matter by weight (ASTM D 2974 Method C) MnDOT Grade 2 compost 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 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

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

silt plus sand (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 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 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

A soil phosphorus test using the Mehlich-3 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.

Comparison of pros and cons of bioretention soil mixes
Link to this table.

Mix	Composition in original Manual	Proposed updated composition	Pros	Cons
A	55-65% construction sand 10-20% top soil 25-35% organic matter²	60-70% construction sand 15-25% top soil 15-25% organic matter² to receive P credit for water captured by underdrain the P content must be less than 30 mg/kg (ppm) per Mehlich III (or equivalent) test; NOTE a minimum P concentration of 12 mg/kg is recommended for plant growth.	Likely to sorb more dissolved P and metals than mix B because it contains some fines; best for growth of most plants	Likely to leach P; if topsoil exceeds maximum allowed clay content, higher fines content could result in poor hydraulic performance and long drawdown times
B	50-70% construction sand 30-50% organic matter	70-85% construction sand 15-30% organic matter to receive P credit for water captured by underdrain the P content must be less than 30 mg/kg per Mehlich III (or equivalent) test; NOTE a minimum P concentration of 12 mg/kg is recommended for plant growth.	Easy to mix; least likely to clog	Likely to leach P, lack of fines in mix results in less dissolved pollutant removal; harder on most plants than mix A because it dries out very quickly
C	Not in original MN Stormwater Manual	85-88 percent by volume sand and 8 to 12 percent fines by volume, 3 to 5 percent organic matter by volume recommended P content between 12 and 30 mg/kg per Mehlich III (or equivalent) test	Likely to sorb more dissolved P and metals than mix B because it contains some fines; less likely to leach P than mix B because of low P content	Harder on most plants than mix A because it dries out very quickly. Research in Wisconsin indicates that in cold climates, excess of Na ions can promote displacement of Mg and Ca in the soil, which breaks down soil structure and decreases infiltration rate, and can also cause nutrient imbalances¹
D	Not in original MN Stormwater Manual	All components below by dry weight: 60-75% sand Min. 55% total coarse and medium sand as a % of total sand Less than 12% fine gravel less than 5 mm (Calculated separately from sand/silt/ clay total) 2 to 5 % organic matter recommended P content between 12 and 30 mg/kg per Mehlich III (or equivalent) test	Best for pollutant removal, moisture retention, and growth of most plants; less likely to leach P than mix B because of low P content	Harder to find. Research in Wisconsin indicates that in cold climates, excess of Na ions can promote displacement of Mg and Ca in the soil, which breaks down soil structure and decreases infiltration rate, and can also cause nutrient imbalances
E	Not in original manual	60-80% sand meeting gradation requirements of MnDOT 3126, ―Fine Aggregate for Portland Cement Concrete 20-40% MnDOT 3890 Grade 2 Compost 30% organic leaf compost	High infiltration rates, relatively inexpensive	As compost breaks down, nutrients available for plants decreases
F	Not in original manual	75% loamy sand by volume: Upper Limit: 85-90% sand with %Silt + 1.5 times %Clay > 15%. Lower Limit: 70-85% sand with %Silt + 2 times %Clay < 30%. Maximum particle size < 1-inch 25% MnDOT 3890 Grade 2 Compost	Finer particles in loamy sand holds moisture for better plant growth	Lower infiltration rates, requires careful soil placement to avoid compaction, requires custom mixing

¹This problem can be avoided by minimizing salt use. Sodium absorption ratio (SAR) can be tested; if the SAR becomes too high, additions of gypsum (calcium sulfate) can be added to the soil to free the Na and allow it to be leached from the soil (Pitt et al in press).
²MnDOT Grade 2 compost is recommended.

Other media

Several other media are currently being tested. A few examples are listed below.

Wisconsin peat moss replacement (Bannerman, 2013)

The following mix utilizes peat moss instead of compost.

12 percent peat moss*2 percent Imbrium Sorptive®MEDIA*86 percent sand

This mix aims to maximize phosphorus removal in 2 ways:

substituting peat moss for compost, since peat moss has lower phosphorus content than compost and does not leach phosphorus; and
including Sorptive®MEDIA to sorb phosphorus and minimize phosphorus in effluent

Wisconsin layered system

This layered system is designed to minimize phosphorus in bioretention effluent.