Phosphorus

Phosphorus Cycle from Wikimedia Commons

Limiting excess phosphorus

Volume reduction

In many forms of LID practice, the dominant mechanism reducing the loading of phosphorus, is the significant volume reduction of the outflow achieved.

Chemical control

For infiltration practices an 'amendment' or chemically reactive 'additive' can help to retain even more phosphorus. Typically these components would make up 5 to 10% by volume of the filter media mixture. A number of granular amendments have been demonstrated to improve nutrient removal from discharge water in BMPs such as bioretention systems, stormwater planters, absorbent landscapes, sand filters or green roofs.

There are two primary processes involved, chemical precipitation and adsorption. Both mechanisms are ultimately finite, but have been shown in come cases to make significant improvements on the discharged water quality over several years.

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Filter Media Additives

Material	Benefits	Potential concerns
Biochar	Renewable Enhances soil aggregation, water holding capacity and organic carbon content	Currently expensive Energy intensive to produce Some sources say ineffective for phosphorus removal
Bold & GoldTM	Documented total phosphorus removal of up to 71%	Proprietary
Iron filings or Zero valent iron (ZVI)	Proven phosphorus retention Retained phosphorus is stable	May harm plants Removal efficiency declines with increased concentration of incoming phosphorus
Red sand or Iron-enriched sand	Proven phosphorus removal Also removes TSS	Poor orthophosphate removal in hypoxic or anoxic conditions
Smart SpongeTM	Removes phosphorus, as well as TSS, fecal coliform bacteria and heavy metals Non-leaching	1-3 year lifespan, after which the product is removed as solid waste Proprietary
Sorbtive MediaTM	High phosphorus removal efficiency	Proprietary
Water treatment residuals	Waste product reuse	Quality control (capabilities depend on source, treatment methods, storage time)

Phosphorus testing in media

To help ensure LID BMPs sustain healthy vegetation cover while not contributing substantially to nutrient loading of receiving waters, the quantity of extractable (i.e., available) P in the soil component needs to be measured and compared to design specifications or acceptance criteria. 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 P is the portion that is easily available to organisms like plants and algae i.e. is the measure of immediate concern to water quality.

The quantity of extractable P 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.

Inspection

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 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.

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.

The total mass of a pollutant entering a waterbody over a defined time period.

The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration.

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.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

Any form of rain or snow.

The attachment of gas, vapour or dissolved matter onto the surface of solid materials.

Total suspended solids

Watercourses and Lake Ontario, to which Stormwater and Combined Sewer Overflows discharge.

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.

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 constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.