Citation:

Newcomer-Johnson, T., S. Kaushal, P. Mayer, AND M. Grese. A Case Study on Nitrogen Uptake and Denitrification in a Restored Urban Stream in Baltimore, Maryland. American Water Works Association Sustainable Water Management Conference, New Orleans, Louisiana, March 19 - 22, 2017.

Impact/Purpose:

Jay Garland coordinated a Special Topic Session (STS) for the American Water Works Association Sustainable Water Management Conference 2017, March 19 – March 22, 2017, in New Orleans, Louisiana. The session title is "US EPA Research to Inform Assessment and Protection of Waters from Water Quality Impacts." The session description is: Staff from the USEPA Office of Research and Development will describe research projects focused on improving the protection of our nation’s water from water quality impacts. Talks will discuss watershed modeling and monitoring for 1) informing nutrient trading, 2) assessing/mitigating harmful algal blooms, 3) informing nutrient thresholds, and 4) both increasing adoption of nutrient management strategies and prioritizing watersheds for their deployment.

Description:

Restoring urban infrastructure and managing the nitrogen cycle represent emerging challenges for urban water quality. We investigated whether stormwater control measures (SCMs), a form of green infrastructure, integrated into restored and degraded urban stream networks can influence watershed nitrogen loads. We hypothesized that hydrologically connected floodplains and SCMs are “hot spots” for nitrogen removal through denitrification because they have ample organic carbon, low dissolved oxygen levels, and extended hydrologic residence times. We tested this hypothesis by comparing nitrogen retention metrics in two urban stream networks (one restored and one urban degraded) that each contain SCMs, and a forested reference watershed at the Baltimore Long-Term Ecological Research site. We used an urban watershed continuum approach which included sampling over both space and time with a combination of: (1) longitudinal reach-scale mass balances of nitrogen and carbon conducted over 2 years during baseflow and storms (n = 24 sampling dates × 15 stream reaches = 360) and (2) 15N push–pull tracer experiments to measure in situ denitrification in SCMs and floodplain features (n = 72). The SCMs consisted of inline wetlands installed below a storm drain outfall at one urban site (restored Spring Branch) and a wetland/wet pond configured in an oxbow design to receive water during high flow events at another highly urbanized site (Gwynns Run). The SCMs significantly decreased total dissolved nitrogen (TDN) concentrations at both sites and significantly increased dissolved organic carbon concentrations at one site. At Spring Branch, TDN retention estimated by mass balance (g/day) was ~150 times higher within the stream network than the SCMs. There were no significant differences between mean in situ denitrification rates between SCMs and hydrologically connected floodplains. Longitudinal N budgets along the stream network showed that hydrologically connected floodplains were important sites for watershed nitrogen retention due to groundwater–surface water interactions. Overall, our results indicate that hydrologic variability can influence nitrogen source/sink dynamics along engineered stream networks. Our analysis also suggests that some major predictors for watershed N retention were: (1) streamwater and groundwater flux through stream restoration or stormwater management controls, (2) hydrologic residence times, and (3) surface area of hydrologically connected features

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