SHALLOW GROUNDWATER RESPONSE TO WHOLE-WATERSHED RESTORATION OF URBAN FOREST STREAMS

Small headwater streams are important for stormwater management, flood control, nutrient transformation and retention, and biodiversity. However, urbanization results in altered hydrology and increased erosion, which impairs physical and biogeochemical functions. These problems are exacerbated when headwater streams become incised, severing connections to the floodplain, which limits their ability to exchange nutrients and reduces groundwater-surface water interactions (GWSWIs). With restoration's ability to alter environmental parameters and resulting water quality, urban headwater streams are good candidates for restoration. The Reedy Creek Restoration Project, completed in 2019, reconstructed 7 urban streams in the 6km² Reedy Creek watershed in the headwaters of the Pee Dee River system. Restoration followed the natural channel design approach. Riparian groundwater levels (WLs) have been measured continuously since 2013 in 5 wells. Hillslope and floodplain groundwater levels have been measured since 2016 in 5 additional wells. Baseflow stream chemistry has been analyzed since 2016 in 11 tributary streams. Objectives of this study are to quantify: (1) interannual variability in WLs for each tributary, (2) seasonal variations in WLs, and (3) how WLs have changed from pre (Pre-R) to post (Post-R) restoration.

Pre-R WLs fluctuated on average 85 cm interannually. On average Pre-R WLs fluctuated seasonally about 100 cm between the fall/winter and spring/summer periods. After restoration, average WLs increased from Pre-R WLs of 340 cm below the land surface to Post-R WLs of 210 cm below the land surface as a result of channel relocation and the new channel being much shallower than the Pre-R deeply incised channel. The riparian WLs tended to rise first after restoration, with one site’s WLs rising 102 cm in 8 weeks, followed by a rise in the corresponding hillslope WLs 2 months later; this may also be due to channel relocation. Overall, the Post-R watershed has increased groundwater storage leading to a gentler hydraulic gradient from the upland to channel and a newly established potential for flooding. As the Post-R period continues, we are examining weekly baseflow stream chemistry and seasonal groundwater chemistry to determine the effect, if any, on water quality contributed by the newly saturated soil and saprolite since water tables have risen.