Phosphorus testing in media

For BMPs including: bioretention, enhanced grass swales, vegetated filter strips, absorbent landscapes, green roofs and, bulk materials including: compost, and topsoil, phosphorus (P) should be measured as extractable phosphorus. Extractable phosphorus is the portion that is easily available to organisms like plants and algae, so the parameter of immediate concern to water quality.

The quantity of extractable phosphorus is determined through acid or base extraction of a sample and testing the concentration in solution by a soil testing laboratory. Commonly used extraction methods on soil samples are the Bray and Kurtz P-1 procedure for non-calcareous soil or the Sodium Bicarbonate (Olsen) method for calcareous soil .

The Olsen method is recommended as the default to use for typical Ontario soils . Calcareous soils are mostly or partly composed of calcium carbonate (i.e., lime or limestone). The Olsen extraction method should be used if the soil contains more than 2% calcium carbonate

For green roof media, the Saturated Media Extract (SME) method should be used . In this extraction procedure, a sample of the media is saturated with deionized water containing a small amount of Pentetic acid (DTPA) to enhance extraction of micro-nutrients . This SME procedure should also be used to measure concentrations of soluble salts and nitrogen in green roof growing media.

Inspections

See above an example of the phosphorous cycle in a bioretention system. The cycle can be altered depending on the specific design configuration of the LID, including the adoption of a saturated zone, type and amouNT of mulch used and sorbitive media included or not.

Construction inspections

If laboratory testing indicates the extractable phosphorus concentration is not within the design or product specification range, notify the supplier, issue a “do not install” order to the construction site supervisor and contact the design professionals and property owner or project manager to determine corrective actions.

Assumption and Verification inspections

For filter media or growing media found through testing to be below the design or product specification range, or Acceptance Criteria range, corrective actions are only needed if problems with vegetation cover, condition or composition (i.e., dominance by weeds) are also detected through visual inspection. Where vegetation cover is poor, unhealthy or dominated by weeds and soil P is lower than the design specification or Acceptance Criteria, schedule investigative work to do further sampling and testing to determine the affected area and depth and decide on corrective actions. Depending on the findings, corrective action could involve amending the soil with compost or other fertilizer.

Amendments to green roof growing media to address P deficiency should be prescribed by the media manufacturer or product vendor. Where soil P concentration is found to be higher than the Acceptance Criteria range, and the BMP drains to a nutrient sensitive receiving water, continuous monitoring during natural or simulated storm events should be undertaken that includes sampling and testing of nutrient concentrations (i.e., Phosphorus and Nitrogen) in sub-drain or surface flows from the BMP to evaluate if the exceedance is negatively impacting effluent quality and if corrective actions are warranted. Corrective action could involve incorporating a soil additive that increases phosphorus retention, or replacement of part or all of the media or topsoil with material that is within the design or product specification.

A thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

Decayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.