Minimum bioretention soil media depths recommended to target specific stormwater pollutants
Minimum bioretention soil media depths recommended to target specific stormwater pollutants.<
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| Pollutant | Depth of Treatment with upturned elbow or elevated underdrain | Depth of Treatment without underdrain or with underdrain at bottom | Minimum depth |
|---|---|---|---|
| TSS | Top 2-3” of bioretention soil media | Top 2-3” 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” of bioretention soil media | Top 8” 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 | 3-4” Mulch layer, top 1” of bioretention soil media | 3-4” Mulch layer, top 1” 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-ft minimum) |
| Particulate phosphorus | Top 2-3” of bioretention soil media. | Top 2-3” 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 ft, but 3 ft recommended as a conservative value; if IWS is included, keep top of submerged zone at least 1.5-2’ from surface of media |
| Pathogens | From top of soil to top of submerged zone. | From top to bottom of bioretention soil media | Minimum 2’; if IWS is included, keep top of submerged zone at least 2’ 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 ft, with 4 ft preferred |