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

This page was last modified on 1 November 2018, at 09:33.

Minnesota Pollution Control Agency | 651-296-6300, 800-657-3864 | Assistance | Web site policy