Of particular concern in bioretention systems are nitrogen and phosphorus because excess amounts of these nutrients in stormwater cause eutrophication and groundwater contamination. Nitrogen in stormwater can be present in different inorganic (NH4 þ, NO3−, NO2−) and organic forms. Orthophosphate (PO43−) is the most common and bioavailable
form of phosphorus. Phosphorus reactswith iron, calcium, or aluminum,and therefore, can be retained in bioretention systems by precipitation or adsorption to solid particles.