Monitoring nitrogen loading and retention in an urban stormwater detention pond

Stormwater detention ponds have become ubiquitous in urbanized areas and have been suggested as potential hotspots of N transformation within urban watersheds. As a result, there is a great deal of interest in their use as structural best management practices to reduce the excessive N export from these watersheds. We conducted continuous monitoring of the influent and effluent N loads of a stormwater detention pond located on the Princeton University campus in Princeton, New Jersey. Our monitoring was conducted during four 21-d periods representing the four seasons of the northeastern United States. Water quality samples were collected and analyzed for nitrate (NO₃^-) during all four monitoring periods. During two of these periods, loads of ammonium (NH₄⁺), dissolved organic N, and particulate N (PN) were measured. Our results show that NO₃^- dominated the influent N load, particularly in dry weather inflows to the detention pond. However, PN, which is often neglected in stormwater quality monitoring, made up as much as 30% of the total load and an even greater fraction during storm events. The results of our monitoring suggest that seasonal variation may play an important role in N retention within the detention pond. Although retention of NO₃^-, the most dominant fraction of N in the influent stormwater, was observed during the summer sampling period, no significant NO₃^- retention was observed during the spring or the two cold-weather sampling periods.