Rain garden used for on-site stormwater management at a commercial property.
Source: LID Center
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Effectively reducing nutrient loadings to water ways is an increasing challenge for stormwater managers. Excess nitrogen and phosphorus in stormwater runoff are a major cause of eutrophication of receiving waters. Nitrogen and phosphorus exist in more than one form in stormwater runoff and require multiple removal mechanisms to achieve significant reductions in total concentration. LID BMPs can be effective at reducing nutrient concentrations in stormwater runoff because many encourage filtration, infiltration, and vegetative uptake.  

1.   Nitrogen

The primary sources of nitrogen in stormwater are fertilizers and atmospheric deposition. Nitrogen is present in the atmosphere in airborne dust particles, as well as in gaseous forms, known as NO_x, which are produced by the combustion of fossil fuels. These compounds are washed out of the atmosphere by precipitation. Most of the nitrogen in runoff is in the form of ammonium or organic compounds.    

Nitrogen can be removed from stormwater by three principal routes:

1. Conversion by bacteria to nitrogen gas;
2. Loss by infiltration; and
3. Uptake by plants.

Microbial nitrogen removal is a three step process requiring a sequence of aerobic and anaerobic conditions. The first step is ammonification, in which organic nitrogen compounds are converted to ammonium, NH₄⁺. The second step is nitrification, in which ammonium, NH₄⁺, is converted to nitrate, NO₃^-. Nitrification requires aerobic conditions. In the third step, known as denitrification, nitrate, NO₃^-, is converted to nitrogen gas, N₂, which then escapes into the atmosphere. Denitrification requires anaerobic conditions.    

In order to decrease total nitrogen, both nitrification and denitrification must take place. As discussed, these processes require very different environmental conditions. Most BMPs are designed to perform the aerobic steps of ammonification and nitrification, but do not provide the conditions necessary for denitrification. Denitrification requires extended storage of stormwater under anaerobic conditions, often for several days after a storm event.   

2.   Phosphorus

The primary sources of phosphorus are fertilizer residues on the ground and suspended in airborne dust particles. Dust particles are washed out of the air by precipitation, and fertilizer residues are washed off the ground by stormwater runoff. Phosphorus is present in two forms: dissolved orthophosphate, PO₄^3-, and insoluble complexes with other elements. Insoluble phosphate complexes can be removed from stormwater by settling. Dissolved orthophosphate can be removed by sorption to filter media or by plant uptake.