Citation:

Mayer, P., M. Pennino, T. Newcomer-Johnson, AND S. Kaushal. Long-term assessment of floodplain reconnection as a stream restoration approach for managing nitrogen in ground and surface waters. Urban Ecosystems. Springer Science+Business Media B.V, Dordrecht, Netherlands, , s11252-021-01199-z, (2022). https://doi.org/10.1007/s11252-021-01199-z

Impact/Purpose:

Stream restoration is a popular approach for managing nitrogen in degraded, flashy urban streams yet, questions remain over its efficacy to remove significant amounts of nitrogen despite its recommended use in the latest protocol for Chesapeake Bay restoration efforts (“Consensus Recommendations to Improve Protocols 2 and 3 for Defining Stream Restoration Pollutant Removal Credits”). Here, we investigated the long-term effects of geomorphic stream restoration on in-stream nitrogen transport and transformation in an urban stream in the Chesapeake Bay watershed. Our study focused on a stream restored using natural channel design principles to repair damage to the channel and protect surrounding sewer and stormwater infrastructure. We monitored both surface water and ground water in the stream before and after restoration and applied models to estimate flashiness and nitrogen flux. We found that the restoration improved nitrogen uptake and that restoration represents a potential BMP for nutrient management depending on restoration features and approach. However, the channel design show signs of degradation after restoration due to erosion and flashy runoff which may limit the utility and longevity of stream restoration as a BMP. Our data will help guide practitioners identify the most effective stream restoration approaches for nitrogen management in urban ecosystems.

Description:

Stream restoration is a popular approach for managing nitrogen (N) in degraded, flashy urban streams. Here, we investigated the long-term effects of stream restoration involving floodplain reconnection on riparian and in-stream N transport and transformation in an urban stream in the Chesapeake Bay watershed. We examined relationships between hydrology, chemistry, and biology using a Before/After-Control/Impact (BACI) study design to determine how hydrologic flashiness, nitrate (NO3−) concentrations (mg/L), and N flux, both NO3− and total N (kg/yr), changed after the restoration and floodplain hydrologic reconnection to its stream channel. We examined two independent surface water and groundwater data sets (EPA and USGS) collected from 2002–2012 at our study sites in the Minebank Run watershed. Restoration was completed during 2004 and 2005. Afterward, the monthly hydrologic flashiness index, based on mean monthly discharge, decreased over time from 2002 and 2008. However, from 2008–2012 hydrologic flashiness returned to pre-restoration levels. Based on the EPA data set, NO3 − concentration in groundwater and surface water was significantly less after restoration while the control site showed no change. DOC and NO3 − were negatively related before and after restoration suggesting C limitation of N transformations. Long-term trends in surface water NO3 − concentrations based on USGS surface water data showed downward trends after restoration at both the restored and control sites, whereas specific conductance showed no trend. Comparisons of NO3 − concentrations with Cl− concentrations and specific conductance in both ground and surface waters suggested that NO3 − reduction after restoration was not due to dilution or load reductions from the watershed. Modeled NO3 − flux decreased post restoration over time but the rate of decrease was reduced likely due to failure of restoration features that facilitated N transformations. Groundwater NO3 − concentrations varied among stream features suggesting that some engineered features may be functionally better at creating optimal conditions for N retention. However, some  engineered features eroded and failed post restoration thereby reducing efficacy of the stream restoration to reduce  flashiness and NO3 − flux. N management via stream restoration will be most effective where flashiness can be reduced and DOC made available for denitrifiers. Stream restoration may be an important component of holistic watershed management including stormwater management and nutrient source control if stream restoration and floodplain reconnection can be done in a manner to resist the erosive effects of large storm events that can degrade streams to pre-restoration conditions. Long-term evolution of water quality functions in response to degradation of restored stream channels and floodplains from urban stressors and storms over time warrants further study, however. 

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