Difference between revisions of "Construction specifications for bioretention"
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| + | Research has shown that minimum bioretention soil media depth needed varies depending on the target pollutant(s). | ||
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| + | {{:Minimum bioretention soil media depths recommended to target specific stormwater pollutants}} | ||
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| + | ====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 | ||
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| + | ===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. | ||
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| + | :'''Sand:''': Provide clean construction sand, free of deleterious materials. [http://www.transportation.org/Pages/default.aspx AASHTO M-6] or [http://www.astm.org/ ASTM C-33] washed sand. | ||
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| + | :'''Top Soil''': Sandy loam, loamy sand, or loam texture per USDA textural triangle with less than 5 percent clay content | ||
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| + | :'''Organic Leaf Compost:''' (MnDOT Grade 2) 2 (see also the section on[[Turf| Using Compost as a Soil Amendment]] | ||
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| + | ====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. | ||
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| + | :'''Sand''': Provide clean construction sand, free of deleterious materials. AASHTO M-6 or ASTM C-33 washed sand. | ||
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| + | :'''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. | ||
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| + | :'''Organic Leaf Compost''': Mn/DOT Grade 2 | ||
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==Construction specifications== | ==Construction specifications== | ||
Revision as of 19:59, 9 January 2014
This page provides construction details, materials specifications and construction specifications for bioretention systems.
Contents
Construction details
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: 10-20 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.