Long-term trends in hydrology and biogeochemistry in a geomorphically restored urban stream.

Few studies have investigated the long-term effects of geomorphic manipulation to restore ecosystem function in highly dynamic urban streams. We present data from a decadal scale, intensive study of Minebank Run, a stream restored in Baltimore, MD, USA in the Chesapeake Bay watershed designed to elucidate the resiliency and reliability of stream restoration as a best management practice for improving water quality. Minebank Run, which has been the subject of over 30 related studies, exhibited Urban Stream Syndrome characteristics prior to restoration including bank incision and flashy hydrology due to impervious surfaces in the watershed. Using a BACI design, we examined relationships between hydrology and biogeochemistry before and after restoration and interpreted these patterns in relation to nutrient uptake in surface and ground water. Nitrate (NO3-) concentration in surface water declined after restoration and converged with NO3- patterns observed at a control site. Ground water NO3- did not decline, suggesting that restoration affected surface water NO3- concentrations but that ground water may remain a reservoir for N. NO3- concentrations and flux were estimated using EGRET (Exploration and Graphics for RivEr Trends) package to run the Weighted Regressions on Time, Discharge and Season (WRTDS) model. Both NO3- concentration and variability, and therefore flux, declined over time after the restoration. Low NO3- concentrations during drought were likely due to subsurface N transformations such as denitrification. Lower stream flow and water table levels were related to longer ground water residence time. Nitrogen (N) transformation was strongly influenced by ground water residence time and carbon (C) quantity and quality. Restoration features including oxbows, bank stabilization techniques, and weirs were variously effective at improving N processing depending on their effects on hydrology. Urban streams remain dynamic despite efforts to control bank erosion and increase static geomorphology. Stream restoration projects that increase C availability to denitrifiers, reduce stream flow velocities, and increase ground water residence are likely to improve N retention and removal in urban streams.

Impact/Purpose

Nitrogen is a leading cause of impairment among streams in the US and globally, causing eutrophication and loss of biodiversity. Stream restoration is a green infrastructure approach often implemented to attenuate anthropogenic nitrogen. However, restoring urban infrastructure and managing the nitrogen cycle represent emerging challenges for urban water quality because of spatial and temporal variability in nitrogen impacts and the variable effects of restoration approaches. Our long-term research involving over 30 related studies elucidates the controlling factors on nitrogen uptake in urban streams and the effects of restoration. Our results indicates that hydrologic variability can influence nitrogen source/sink dynamics along engineered stream networks. 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. Restoration features including oxbows, bank stabilization techniques, and weirs are variously effective at improving N processing depending on their effects on hydrology. Stream restoration projects that increase C availability to denitrifiers, reduce stream flow velocities, and increase ground water residence are likely to improve N retention and removal in urban streams.

Citation

Mayer, P., M. Pennino, T. Newcomer-Johnson, AND S. Kaushal. Long-term trends in hydrology and biogeochemistry in a geomorphically restored urban stream. The 5th Symposium on Urbanization and Stream Ecology (SUSE5), Austin, TX, February 12 - 15, 2020.