Abstract

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 Appalachians, USA. In this study, two sets of metrics were evaluated including transient storage and nutrient uptake metrics. The average dimensionless transient storage metrics F_med, the fraction of the median travel time through a 200-m reach that is due to transient storage, of Winkler Creek was found to be 3-fold greater than that of Boone Creek. With regard to nutrient uptake metrics, Boone Creek was found to have an uptake length 13-fold longer and an uptake velocity 16.7 times slower than Winkler Creek. The results show a greater extent of HE and higher nutrient uptake in the forested stream than the urban stream, which indicate that urbanization can deteriorate stream ecosystem functions by reducing HE and nutrient retention capacity. As a result, extra amounts of nutrients might export downstream and create a eutrophication problem. Thus, hyporheic restoration is crucial and has to be taken into account in restoring the ecosystems of urban streams.

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