The effectiveness of pollutant removal by landscape BMPs is a function of the following:
- physical, chemical, and biological processes
- the fraction of runoff treated by the BMP
- the nature of the pollutant being removed
Thus, an effective BMP train is one that treats 100% of runoff by physical, chemical, and biological processes. The table below shows relative removal efficiencies of infiltration basins, vegetated filter strips, grass swales, wet ponds, and storm water wetlands for five variables (total suspended solids, total phosphorus, total nitrogen, pesticides, and chemical oxygen demand).
By including as many removal mechanisms as possible, the probability of success for removal of a particular pollutant is increased. These factors should be considered as follows:
- BMPs that use settling and filtering processes are relatively effective at removing sediment and pollutants that are bound to sediment particles.
- Turf buffers are very effective filters that allow drainage of water from the course and, at the same time, effective filtering to improve water quality.
- Turf density, leaf texture, and canopy height are physical factors that restrain soil erosion and sediment loss by dissipating impact energy from rain and irrigation water droplets providing a resistance to surface movement of water over turf.
- Ponds and infiltration BMPs can achieve 60 to 100% removal efficiencies for sediment.
- Infiltration BMPs are capable of similar removal efficiencies for sediment, but are subject to clogging if sediment inputs are excessive.
Wet ponds and extended-detention ponds with shallow marshes have a moderate to high capability for removing both soluble and particulate pollutants because they use settling and biological uptake and degradation of pesticides.
Stormwater pollutant removal efficiencies, urban BMP designs (Sources: Schueler 1987 and NYSDEC, 1993)
|Extended Detention Pond|
|“First flush” runoff volume produced by 1.0 inch storm, detained for 24 hours|
|Runoff volume produced by 1.0 inch storm detained for 24 hours or more with shallow marsh added in bottom stages|
|Permanent pool equal to 0.5 inch of runoff per watershed acre|
|Permanent pool equal to 2.5 times the volume of runoff from the mean storm (0.5 inch)|
|Water Quality Basin|
|Infiltration basin which exfiltrates “first flush” of 0.5 inch runoff/impervious acre|
|25 to 50 foot turf strip|
|100 foot wooded strip|
|25 to 50 foot wooded strip|
|High slopes with check dams|
|*TSS= Total Suspended Solids; TP=Total Phosphorus; TN= Total Nitrogen; Zn=Zinc; Pb=Lead; BOD=Biological Oxygen Demand|