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

Soil medium / filter media content

Mix A: Water quality blend

A well blended, homogenous mixture of 55 to 65 percent construction sand: 10 to 20 percent top soil; and 25 to 35 percent organic leaf compost is necessary to provide a soil medium with a high infiltration/filtration capacity.

Sand: Provide clean construction sand, free of deleterious materials. AASHTO M-6 or ASTM C-33 with grain size of 0.02 to 0.04 inches

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

Organic Leaf Compost: (MnDOT Grade 2) 2 (see also the section on Using Compost as a Soil Amendment

Mix B: Enhanced filtration blend

A well-blended, homogenous mixture of 50 to 70 percent construction sand and 30 to 50 percent organic leaf compost is necessary to provide a soil medium with a higher infiltration/filtration capacity.

• Sand: Provide clean construction sand, free of deleterious materials. AASHTO M-6 or ASTM C-33 with grain size of 0.02 to 0.04 inches
• Organic Leaf Compost: Mn/DOT Grade 2
• Topsoil 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.

Soil medium / filter media depth

Field experiments show that pollutant removal is accomplished within the top 30 inches of soil depth with minimal additional removal beyond that depth (Prince George’s County, 2002). Therefore, the recommended depth of the prepared soil is 30 inches. However, if large trees are preferred in the design, a soil depth of 48 to 52 inches should be utilized. The soil depth generally depends upon the root depth of the prescribed vegetation and content of underlying soils.

Gravel Filter Specifications - Underdrain gravel blanket shall be double washed stone, 1 to 1½ inches in size. Pea Gravel shall be washed, river-run, round diameter, ¼ - ½ inch in size.

Mulch Content and Depth - Fresh shredded bark mulch (Mn/DOT Type 6) should be used when possible to maximize nitrogen retention. If aged mulch is used, use the shredded type instead of the “chip” variety to minimize floating action. The mulch layer should not exceed 3 inch in depth. Too much mulch can restrict oxygen flow to roots. In addition, mulch should not be mounded around the base of plants since this encourages damage from pests and diseases.

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 36 mg/kg per Mehlich III test
• Option B - include a soil amendment that facilitates adsorption of phosphorus

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 leaf compost.

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 Leaf Compost: (MnDOT Grade 2) 2 (see also the section on Using Compost as a Soil Amendment

Mix B: Enhanced filtration blend

A well-blended, homogenous mixture of

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

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 Leaf Compost: Mn/DOT Grade 2

Mix C: North Carolina State University bioretention soil media (North Carolina Department of Environment and Natural Resources. 2009)

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)

Higher (12 percent) fines content should be reserved for areas where TN is the target pollutant. In areas where phosphorus is the target pollutant, lower (8 percent) fines should be used.

Bioretention Soil 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
• saturated hydraulic conductivity: 1 to 4 inches per hour
• ASTM F1815 at 85 percent compaction, Standard Proctor ASTM D968
• phosphorus between 12 and 36 parts per million (ppm)
• cation exchange capacity greater than 10 meq/g

Suggested mix ratio ranges are

• Coarse sand: 50 to 65 percent
• Topsoil: 25 to 35 percent
• Compost: 10 to 15 percent

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.

Construction specifications

Given that the construction of bioretention practices incorporates techniques or steps which may be considered non-traditional, it is recommended that the construction specifications include the following format and information:

A. Temporary erosion control

• Install prior to site disturbance
• Protect catch basin/inlet
• It is HIGHLY RECOMMENDED that future bioretention locations not be used as temporary sedimentation basins. If used as temporary sedimentation basins, the bioretention practice should be over excavated a minimum of 18 inches below sedimentation basin grade.

B. Excavation, backfill and grading

• Timing of grading of infiltration practices relative to total site development
• Use of low-impact, earth moving equipment (wide track or marsh track equipment, or light equipment with turf-type tires)
• Do not over-excavate
• Restoration in the event of sediment accumulation during construction of practice
• Alleviate any compacted soil (compaction can be alleviated at the base of the practice by using a primary tilling operation such as a chisel plow, ripper or sub-soiler to a minimum 12 inch depth
• Gravel backfill specifications
• Gravel filter specifications
• Filter fabric specifications

C. Native plants, planting and transplanting

• Site preparation of planting areas
• Timing of native seeding and native planting
• Weed control
• Watering of plant material

D. Construction sequence scheduling

• Temporary construction access
• Location of silt fence installation to protect BMPs and downstream receiving waters
• Removal and storage of excavated material
• Installation of underground utilities
• Rough grading
• Seeding and mulching disturbed areas
• Road construction
• Final grading
• Site stabilization
• Installation of semi-permanent and permanent erosion control measures
• Silt fence removal

E. Construction observation

• Adherence to construction documents
• Verification of physical site conditions
• Erosion control measures installed appropriately

Related pages

• Overview for bioretention
• Types of bioretention
• Design criteria for bioretention
• Construction specifications for bioretention
• Operation and maintenance of bioretention
• Cost-benefit considerations for bioretention
• External resources for bioretention
• References for bioretention
• Requirements, recommendations and information for using bioretention BMPs in the MIDS calculator