Urban stormwater runoff nitrogen composition and fate in bioretention systems

Multiple chemical forms of nitrogen in urban stormwater make its management challenging. Sixteen storm events were monitored and analyzed for total nitrogen (TN), particulate organic nitrogen (PON), nitrate (NO3-N), nitrite (NO2-N), ammonium (NH3-N), and dissolved organic nitrogen (DON) in stormwater runoff and in treated discharge through a conventional bioretention cell. Influent PON can be effectively removed via bioretention sedimentation/filtration, NH3-N by ion exchange/sorption, and NO2-N by oxidation. However, significant DON and NO3-N leached from the bioretention cell, resulting in only 9% net overall TN concentration reduction. Captured PON and vegetation detritus in the bioretention cell can be leached as DON or mineralized into NO3-N. The effluent N is dominated by NO3-N (46%) and DON (42%). Therefore, in addition to creating denitrification conditions for NO3-N, preventing DON leaching is also critical for effective nitrogen removal though bioretention systems. The bioretention cell exhibited a moderate mass load reduction for TN (41%), which mainly results from runoff volume reduction.