HYPORHEIC EXCHANGE AND NUTRIENT UPTAKE IN A FORESTED AND URBAN STREAM IN THE SOUTHERN APPALACHIANS

Hyporheic exchange (HE) controls stream water quality by regulating biogeochemical processes, ecosystem functioning, and nutrient dynamics. The objective of this study was to better understand and quantify the extent of urban impact on HE and how that affects stream nutrient uptake. Hyporheic exchange and nutrient uptake were studied through tracer injection experiments in an urban stream, Boone Creek, and a forested stream, Winkler Creek in the Southern Appalachian Mountains. In this study, two sets of metrics were evaluated including transient storage and nutrient uptake metrics. The average dimensionless transient storage metrics F_med is defined as the fraction of the median travel time through a 200-m reach that is due to transient storage. For Winkler Creek the F_med value was found to be 2.7-fold greater than that for Boone Creek. With regard to nutrient uptake metrics, Boone Creek was found to have an uptake length 10-fold longer than Winkler Creek; while, Winkler Creek was found to have an uptake velocity over 14 times faster than Boone Creek. The results show a greater extent of HE and higher nutrient uptake in Winkler Creek than Boone Creek, which indicates that urbanization can deteriorate stream ecosystem functions by reducing HE and nutrient retention capacity. As a result, nutrient export may increase downstream and create eutrophication problems. Thus, hyporheic restoration is crucial and has to be taken into account in restoring the ecosystems of urban streams.