Stormwater detention ponds and the nitrogen cycle in urban watersheds

Stormwater detention ponds have become ubiquitous in urbanized areas and may act as 'hot spots' of nitrogen 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 nitrogen export from these watersheds. This dissertation examines the role of stormwater detention ponds in the nitrogen cycle of urban watersheds. Fieldwork was conducted in the Harry's Brook watershed in Princeton, NJ, USA, in order to obtain data on the spatiotemporal variance of instream nitrogen and how this is influenced by land use and the presence of stormwater detention ponds. Intensive monitoring of the Broadmead Stormwater Detention Pond, located on the Princeton University campus, was conducted in order to consider the relative loading of different forms of nitrogen, the potential effects of seasonality, and the transport of nitrogen within and between storm events.;Fieldwork in Harry's Brook consisted of synoptic baseflow sampling and monitoring of stream nitrogen export during storm events, along with a preliminary survey of detention ponds and other infrastructure in the watershed. The highest baseflow nitrate (NO3-) concentrations were observed at stream sites with old land use that pre-date Best Management Practice (BMP) Requirements. The lowest baseflow NO3- concentrations were consistently observed at the sites in the control subcatchment, where urban development was minimal.;Ammonium (NH4+) was an insignificant contributor to the baseflow nitrogen load of Harry's Brook, and was rarely observed above the detection limit. Dissolved Organic Nitrogen (DON) was also monitored during two synoptic sampling events. The ratio of Dissolved Inorganic Nitrogen (DIN) to DON was highest at the most urbanized sites (>80%) and lowest at the least developed sites. (<10%).;The Harry's Brook storm event sampling results show that discharge, rather than concentration, was the most important factor determining nitrate export with 'hot moments' of NO3- export coinciding with peaks in flow. However, the highest nitrate concentrations were observed on the receding limb of the hydrograph, following peak discharge. During extreme rainstorms, this delayed response may constitute an important secondary hot-moment of nitrate export.;The results of monitoring at the Broadmead Detention Pond show that NO 3- dominated the cumulative influent nitrogen load, particularly during drier periods when the cumulative inflow is dominated by dry-weather interflows. Particulate Nitrogen (PN), which is often neglected in stormwater quality monitoring, also made up a significant fraction (as much as 30%) of the total load, and an even greater fraction during storm events. DON and NH4+ were observed to be less significant contributors to the influent nitrogen load.;The results of this monitoring suggest that seasonal variation may play an important role in nitrogen retention within the detention pond. Particulate Nitrogen was observed to be efficiently retained (84--88%). However, while retention of NO3- was observed during the summer sampling period, no significant NO3- retention was observed during the spring or the two cold-weather sampling periods. This resulted in a 'negative' retention of total nitrogen during the winter sampling period. Further study is necessary to determine the key controls on N retention and incorporate them in detention pond design.