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 (NH4þ, NO3−, NO2−) and organic
forms. Orthophosphate (PO43−) is the most common and bioavailable
form of phosphorus. Phosphorus reacts with iron, calcium, or aluminum,
and therefore, can be retained in bioretention systems by precipitation or
adsorption to solid particles.