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Geomorphic stream restoration as an approach for reducing nutrients in degraded urban watersheds
Citation:
MAYER, P. M. Geomorphic stream restoration as an approach for reducing nutrients in degraded urban watersheds. Presented at Ozark Prairie SETAC Meeting, Gray Summit, MO, July 14 - 16, 2008.
Impact/Purpose:
To determine if stream restoration improves the nutrient removal capacity of streams.
Description:
Elevated nitrate levels in streams and groundwater pose human and ecological threats. Stream restoration may improve the nutrient removal capacity of streams, yet few studies have investigated the effectiveness of restoration as a nutrient BMP despite significant national efforts to restore streams in attempts to improve water quality. Minebank Run, a geomorphically degraded urban stream in Baltimore MD, was restored in 2005 using various channel reconstruction techniques to stabilize banks and reconnect the flood plain. We examined this stream before and after restoration to measure and identify limits to microbial denitrification, a natural process occurring in soils and groundwater that removes nitrate by transforming to a biologically inactive gas form. We found that denitrification at Minebank Run occurred under anaerobic conditions (e.g. saturated soils) and where adequate supplies of carbon were available to microbes. Our studies also revealed higher denitrification activity in zones with greater organic matter and higher denitrification rates in low-bank, reconnected floodplain regions where ground water residence times were longer. Furthermore, the redox environment of the subsurface was closely linked to ground water tables. Consequently, hyporeheic nitrate concentration was lower when stream flow decreased and mean water table elevation subsequently dropped. Overall, nitrate concentrations in the hyporheic zone of this degraded stream were strongly influenced by carbon availability, stream flow, and ground water topography. Stream restoration efforts that increase carbon availability to denitrifiers and which reduce stream flow velocities and/or increase ground water residence time are likely to improve the nutrient uptake capacity of urban stream channels. If implemented appropriately, geomorphic stream restoration may represent a nutrient BMP in urban watersheds.