BIOGEOCHEMICAL EVALUATION OF RESTORED URBAN STREAMS

Urban surface and subsurface infrastructure change pathways connecting storm waters to streams. These hydrologic changes alter the function and structure of riparian and stream ecosystems by contributing to channel degradation, altering nutrient fluxes, and increasing downstream contaminant export. Geomorphic stream restoration is commonly implemented in response to these problems, and is now a billion-dollar industry. Channel stabilization is often the primary focus of these projects, while restoring ecosystem function and improving water quality are of secondary concern. Considerable uncertainty exists about the impact of restoration strategies on nutrient fluxes, nutrient retention, and hyporheic exchange. Furthermore, seasonal and interannual variation associated with restorations warrant further study. Thorough biogeochemical analyses of restored and unrestored reaches should guide best management practices for site selection, design, implementation, and monitoring of future projects.

Our research investigates stream metabolism and nitrogen dynamics of six urban stream reaches in the greater Baltimore area. Sites represent a gradient of hyporheic connectivity, restoration age, and canopy cover. We use seasonal nitrate tracer injections, coupled with continuous discharge and stream metabolism, to establish relationships between nutrient retention and metabolism, and to quantify the effectiveness of stream restoration on annual nutrient export. In addition, bi-monthly, broad scale, synoptic sampling of nutrients in the streams further quantifies temporal variability in ecosystem function across reaches. Preliminary results indicate high variability in nitrate uptake length across reaches (84-610 m), and variable net nitrate removal, suggesting that geomorphic restoration increases nitrate retention. Further study will establish the biotic and abiotic drivers of change in nutrient retention