Inorganic nitrogen dynamics in an urban constructed wetland under base-flow and storm-flow conditions

In urban landscapes constructed wetlands are often used to treat wastewater. These systems have been shown to decrease inorganic nitrogen (N) pollution and improve water quality. However, many studies focus just on N removal under base-flow conditions. With climate change altering precipitation patterns, it is becoming increasingly critical to understand how these systems function during storm events in addition to base-flow conditions. Our research focused on N dynamics in an urban constructed wetland located on the State University of New York, Binghamton campus. We conducted a multi-year field research project to examine seasonal and yearly variations in dissolved inorganic nitrogen (DIN) export under naturally variable hydrologic conditions. From 2007 to 2011, during base-flow conditions, we collected weekly to monthly grab samples at the inlet and the outlet of the wetland, except some winter months; corresponding discharge measurements were also recorded. From July 2010 through March 2011, during storm events, we used two ISCO automated samplers to collect water samples and stage measurements at the inlet and outlet weirs. Water samples were analyzed for DIN (NO3-N and NH4-N). Mass balance input and output budgets were calculated by multiplying concentration and discharge values for each sampling event and averaging them to calculate mean monthly fluxes. Under base-flow conditions, DIN concentrations in the wetland were significantly lower at the outlet than the inlet in all sampling years and for the spring, summer, and autumn seasons, but not during the winter. Over the entire 4-year sampling period, 38% of the DIN entering the wetland was removed during base-flow. In contrast, the wetland removed DIN in only two of the seven storm events sampled and overall, DIN-export was 34% greater than the input. Our results show that during base-flow, the constructed wetland was capable of decreasing N loads from urban sources. However, it had a limited ability to lower N during storm events. Thus, if the number of rainfall events increase as expected under future climate change scenarios, the N removal capacity of urban constructed wetlands could be compromised.