Phosphorus credits for bioretention systems with an underdrain.
Link to this table

Particulate phosphorus Dissolved phosphorus
Is Media Mix C or D being used or, if using a mix other than C or D, is the media phosphorus content 30 mg/kg or less per the Mehlich 3 (or equivalent) test1?
• If yes, particulate credit = 100% of the particulate fraction (assumed to be 55% of total P)
• If no or unknown, particulate credit = 0%

The assumption of 55 percent particulate phosphorus and 45 percent dissolved phosphorus is likely inaccurate for certain land uses, such as industrial, transportation, and some commercial areas. Studies indicate particulate phosphorus comprises a greater percent of total phosphorus in these land uses. It may therefore be appropriate to modify the above equation with locally derived ratios for particulate and dissolved phosphorus. For more information on fractionation of phosphorus in stormwater runoff, link here.

TP removal credit

• Particulate fraction (55% of TP) * removal rate for that fraction (80% for bioretention and tree trench and 68% for swale main channel) = 0.55 * 0.80 = 0.44 or 44% for bioretention and tree trench (with underdrain) and 0.55 * 0.68 or 36% for swale main channel (with underdrain)
1. Is Media Mix C or D being used or, if using a mix other than C or D, is the media phosphorus content 30 mg/kg or less per the Mehlich 3 (or equivalent) test1?
• If yes, credit as a % (up to a maximum of 20%) = 20 * (depth of media above underdrain, in feet/2)
• If no or unknown, credit = 0%

2. Does the system include approved P-sorbing soil amendments2?

• If yes, additional 40% credit

TP removal credit

• TP removal if dissolved credit is 20% = Dissolved fraction (45%) * removal rate for that fraction (20%) = 0.09 or 9 percent
• Adjust TP removal if depth is less than 2 feet
• Adjust TP removal if dissolved credit is higher due to use of P-sorbing soil amendments

The assumption of 55 percent particulate phosphorus and 45 percent dissolved phosphorus is likely inaccurate for certain land uses, such as industrial, transportation, and some commercial areas. Studies indicate particulate phosphorus comprises a greater percent of total phosphorus in these land uses. It may therefore be appropriate to modify the above equation with locally derived ratios for particulate and dissolved phosphorus. For more information on fractionation of phosphorus in stormwater runoff, link here.

1Other widely accepted soil P tests may be used. Note: a basic conversion of test results may be necessary
2Acceptable P sorption amendments include

• 5% by volume elemental iron filings above IWS or elevated underdrain
• minimum 5% by volume sorptive media above IWS or elevated underdrain
• minimum 5% by weight water treatment residuals (WTR) to a depth of at least 10 cm
• other P sorptive amendments with supporting third party research results showing P reduction for at least 20 year lifespan, P credit commensurate with research results