Evaluating the Efficacy of Whole-Watershed Stream Restoration: Implications for Groundwater Storage, Water Quality, and Suspended Solids in a Piedmont Urban Forest Watershed
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Abstract
The quality and quantity of water in degraded watersheds (i.e.: watersheds undergoing changing land-use, urbanization, agricultural use) may be impaired compared to undisturbed forested watersheds. To enhance the ecosystem function of impaired watersheds, restoration projects have been implemented on a variety of land-use types and at a variety of spatial scales. The 6 km2 Reedy Creek urban forested headwater watershed located in Charlotte, North Carolina, has undergone extensive, whole-watershed stream restoration to offset ecosystem degradation caused by historical agricultural use and urban development within the watershed. Streams within the Reedy Creek watershed exhibited channel straightening, disconnection with surrounding floodplains, channel widening, and channel incision. Here, three research topics were investigated with the aim of understanding the efficacy of using restoration to meet stated goals and to reestablish ecological and hydrological functioning within the watershed: 1) if whole-watershed stream restoration was successful at increasing recharge to the thick unsaturated zone and ultimately raising groundwater levels by creating more shallow streams; 2) if the whole-watershed stream restoration approach used in the Reedy Creek watershed in which stream channel morphology and characteristics were altered has changed N retention and seasonal flux; and 3) whether stream restoration has influenced TSS flux within the watershed at both baseflow and stormflow.Study 1 To examine groundwater storage, groundwater levels were measured continuously from January 2016 – March 2021 in five well transects throughout the watershed. Post-restoration, groundwater levels in ten wells increased a total of 1.6 m, on average, due to channel relocation. Post-restoration, "flashy" groundwater behavior ended, suggesting that fast pathways were saturated as the water table rose. Stream water levels rose, on average, ~0.3 m at the five studied transects. Both elevated stream water and groundwater levels were attributed to the physical raising of stream channels. Overall, the post-restoration watershed has increased groundwater storage. Study 2 To examine watershed nitrogen dynamics within the restored watershed, weekly baseflow water samples were monitored for nitrate (NO3-), ammonium (NH4+), total dissolved nitrogen (TDN), and dissolved organic nitrogen (DON) (January 2016 – March 2021) and seasonal ash-free dry mass and canopy cover data were monitored at the watershed outlet. Data indicate that percent ash free dry mass (AFDM) increased from a pre-restoration average of 1.6% to a post-restoration average of 2.8% and canopy cover decreased from a pre-restoration average of 56 % to a post-restoration average of 50 %. Nitrate concentrations decreased from decreased from a pre-restoration average of 0.29 mg N/L to a post-restoration average of 0.08 mg N/L while nitrate flux also decreased from a pre-restoration average of 12.86 mg N/s to a post-restoration average of 5.13 mg N/s. TDN concentrations decreased from a pre-restoration average of 0.43 mg N/L to a post-restoration average of 0.32 mg N/L while flux decreased from a pre-restoration average of 16.96 mg N/s to a post-restoration average of 16.94 mg N/s. DON concentration increased from a pre-restoration average of 0.15 mg N/L to a post-restoration average of 0.21 mg N/L while flux increased from a pre-restoration average of 5.50 mg N/s to a post-restoration average of 977 mg N/s. Here, a significant relationship existed between watershed characteristic changes (i.e.: AFDM and canopy cover) and ammonium and DON flux. Overall, stream restoration was associated with nitrate and TDN concentrations and flux and increasing DON and ammonium concentrations and flux. Study 3 To examine changes in baseflow and stormflow TSS concentrations and flux due to changes in channel morphology, bed features, channel depth, and a shift from sand/gravel to clay dominated bank and bed (i.e.: soil cohesion would increase requiring higher shear stress to dislodge and move clay particles), baseflow surface water samples were collected weekly and analyzed for TSS from 2015– 2021 while stormflow samples were collected throughout the study period. TSS concentration and flux were compared before and after restoration at the watershed outlet. Baseflow TSS concentrations decreased from a pre-restoration average of 20 mg/L to a post-restoration average of 7 mg/L while flux decreased from a pre-restoration average of 48.8 kg/ha/year to a post-restoration average of 23.3 kg/ha/year. Stormflow TSS concentrations decreased from a pre-restoration average of 331 mg/L to a post-restoration average of 67.2 mg/L while per storm TSS flux g decreased from a pre-restoration average of 1.9 x 106 g to a post-restoration average of 1.2 x 106 g. Overall, stream restoration led to a decrease in TSS at both baseflow and stormflow which may be an indication of reduced erosion within the channel and a reduction in turbulent flow. These findings have implications for developing stream restoration plans for degraded watersheds, such as understanding the changes to groundwater storage, watershed-scale hydrologic retention, drought resiliency, and stormwater management. Furthermore, findings may be an indication of increased hyporheic exchange potential in the channel leading to increased denitrification in the post-restoration period, increase in organic nitrogen source for the stream, and changes in temperature and dissolved oxygen levels altering nitrification potential. Finally, findings may be an indication of increased stream bed and bank stability with decreased TSS in the post-restoration period. Overall the stream restoration approach used within the Reedy Creek headwater watershed was successful at meeting stated restoration goals. Here, stream restoration was successful at meeting the stated restoration goals of enhancing water quality and reducing water pollution within the watershed. Moreover, this restoration project was also successful at increasing groundwater storage – which was not a stated goal of restoration but one that is directly related to restoration at the watershed scale. Overall, the Reedy Creek restoration project was effective at meeting stated goals.