NUTRIENT TRENDS AND ORGANIC PROCESSING IN A PIEDMONT URBAN FOREST STREAM: RESULTS OF A MULTI-YEAR EXAMINATION OF WHOLE-WATERSHED RESTORATION (REEDY CREEK, CHARLOTTE, NC)

Headwater streams are critical to the water quality of large river systems and are important candidates for stream restoration. Impaired headwater streams throughout the Piedmont region exhibit incision into clay-rich saprolite. At Reedy Creek, an urban forest stream in Charlotte, NC, whole-watershed restoration in 2017-2019 used the natural channel design method. As part of a multi-year study of water quality, groundwater-surface water interactions, and macroinvertebrate response to stream restoration, we monitored baseflow stream chemistry at 11 sites on Reedy Creek and its tributaries from 2014-2021. We also examined shallow groundwater chemistry at 10 riparian and hillslope wells within the watershed. This study is noteworthy by: (1) the large restoration scope, involving many channels within the 6 km² watershed; and (2) the long monitoring period. Significant water quality changes (by the nonparametric Mann-Whitney test; p<0.05) were observed pre- vs. post-restoration relative to a forested reference watershed. Nitrate concentrations declined at 7 sites; ammonia increased at 8 sites; dissolved organic carbon increased at 6 sites; total dissolved nitrogen decreased at 2 sites; and dissolved oxygen (DO) declined at 2 sites. Lower post-restoration DO was most evident in summer. Groundwater chemistry trends were more subdued, but the significant changes we did observe were consistent with the changes to surface water chemistry. Collectively, these results suggest potential mechanisms for nutrient concentrations to decline in restored streams, which could be applicable to other unrestored urban streams in the Piedmont region. Increased organic processing and more efficient nutrient scavenging post-restoration may be driven by increasing soil-water interaction, increased hydrologic retention, and/or increased instream processing linked to decreased riparian shading.