Abstract
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.
Highlights
Urban streams receive excess nitrogen (N) from multiple sources in the watershed and transport N downstream because supply from the watershed is greater than demand in the stream (Grimm et al 2005; Kaushal et al 2014a, b)
We examined the spatial variability in NO3−, Dissolved organic carbon (DOC), and Cl− concentrations across stream features because we expected that chemical behavior would be driven by spatial and temporal dynamics affected by geomorphic differences
Our studies support the idea that in-stream processes and hydrologic connectivity between the stream channel and subsurface zones may influence N processing in urban streams (Craig et al 2008, Kaushal et al 2008)
Summary
Urban streams receive excess nitrogen (N) from multiple sources in the watershed and transport N downstream because supply from the watershed is greater than demand in the stream (Grimm et al 2005; Kaushal et al 2014a, b). Stream restoration designed to repair and reconnect stream channels, is an increasingly popular approach for managing N in urban streams. Such restoration attempts to improve hydrologic condition favorable for N transformation and denitrification, by reducing flashiness, increasing residence times, and adding organic carbon for denitrifiers (Kaushal et al 2008; Gift et al 2010; Mayer et al 2010b; Duan et al 2019). Our research was intended to fill a gap in long-term studies of restoration, improve our understanding of N behavior in groundwater and surface water of restored streams, and elucidate possible BMPs for N management in urban ecosystems
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