Water quality

==Overview==
Improvements in the quality of stormwater runoff is one of the primary benefits of low impact development strategies. Although some chemical changes occur inside the practices, much of the pollution reduction of low impact development strategies comes from runoff reduction. i.e. diverting excess water through infiltration or evapotranspiration. 
==Nutrients==
Plants (including algae) require three macro nutrients to grow: Nitrogen, potassium, and phosphorus. See Nutrients page for more information about nutrients. Of these three, phosphorus is the often the "growth-limiting" nutrient (i.e., the local environment may have an abundance of nitrogen and potassium, but algae won't develop unless phosphorus is available too).
See Phosphorus page for more information about phosphorus. See Additives page for guidance on filter media additives to enhance retention of phosphorus and other types of pollutants in Bioretention, Bioswale, Stormwater Planter and Stormwater Tree Trench facilities.
===STEP stormwater practice effluent quality monitoring results===
The box plot figures to the right show stormwater effluent quality monitoring results from STEP monitoring projects conducted over a 25-year time period (between 1997 and 2021) at sites within Greater Toronto Area municipalities.
Median concentrations of Total Suspended Solids (TSS) in untreated stormwater and Oil and Grit Separator (OGS) outlet locations were found to exceed the Canadian Water Quality Guideline of 30 mg/L. Median TSS concentrations at stormwater detention pond and chamber system outlet locations were found to be below 30 mg/L, but exceeded the guideline value in 36% of the 231 monitored storm events. Median TSS concentrations at bioretention, permeable pavement and infiltration trench outlet locations were found to be all below 30 mg/L. TSS concentrations in effluent from bioretention was found to exceed 30 mg/L in only 15% of the 301 monitored storm events, and in only 12% of the 296 monitored events for permeable pavements.
Median concentrations of Total Phosphorus (TP) in untreated stormwater and all stormwater treatment facility type outlet locations were found to exceed the Ontario Provincial Water Quality Objective of 0.03 mg/L. Untreated stormwater was found to exceed 0.03 mg/L in between 96 to 98% of the 545 monitored storm events (Asphalt Reference and SWM Inlet data sets). Stormwater detention pond or chamber system effluent was found to exceed 0.03 mg/L during 91% of the 211 monitored events. Effluent from bioretention outlet locations was found to exceeded 0.03 mg/L during 86% of the 355 monitored events. Effluent from permeable pavement outlet locations exceeded 0.03 mg/L during 62% of the 300 monitored storm events.
It is notable that green roof and infiltration trench results are from only one site each, and based on a relatively small number of storm events (24 to 25).
Click on the links below for LID BMP-specific treatment performance sections for more figures and information on water quality treatment performance.
* Bioretention Performance
* Permeable Pavements Performance
==Heavy metals==
==External Links==
*Environment Canada on Phosphorus and Excess Algal Growth 
*Lake Simcoe phosphorus reduction strategy
*Canadian BMP Water Quality Database
==References==