Abstract: Session B  9:10 am (BACK)

Examining Denitrification Driven by Hyporheic Exchange in Restored Streams: Examining N Removal Rates Recommended by Chesapeake Bay Expert Panel

Angela Gardner Allen
Wildlands Engineering
Raleigh, NC

As mitigation laws begin to solidify across the country, stream restoration has become an important tool in offsetting deleterious effects development has on lotic ecosystems. The recent final draft of “Recommendations of the Expert Panel to Define Removal Rates for Individual Stream Restoration Projects” (Bergs et al. 2013) has outlined a measure quantifying instream nutrient removal in future stream restoration projects within the Chesapeake Bay. The expert panel has simplified the complex biogeochemical processes that take place in the hyporheic zone to a single rate of denitrification, which enables practitioners to assign a mass rate of N removed per reach per day. While this method may be the simplest to use, it begs the question of the process being too simplified.

The functional role of the hyporheic zone has been documented for several decades, however the implications on instream restoration projects were not reported on until 2006 (Kasahara and Hill 2006). Since then, the research has greatly expanded to indicate how geomorphic features, stream structures, and water chemistry interact in natural and restored stream reaches to benefit nutrient removal. The vertical hydraulic gradient greatly affects zones of upwelling and downwelling were nitrification and denitrification occur (Knust and Warwick 2009, Saenger and Zanke 2009, Anderson et al. 2005). The change in hydraulic gradient resulting from the addition of constructed riffles can account for 70-83% of the total hydraulic flux in a reach (Kasahara and Hill 2007). The addition of meanders can increase hyporheic exchange by 56-53% and heterogeneity of geomorphic features can increase exchange by 17-32%.  Hyporheic exchange and denitrification rates are also affected by stream and groundwater flow characteristics, sediment composition, spacing of geomorphic features, the presence of nitrifying and denitrifying bacteria, riparian conditions, and water chemistry.

This paper reviews the literature to date on N removal rates driven by hyporheic exchange in restored streams, provides research based design recommendations to maximize potential for nutrient removal, including varying hydraulic gradients, harvesting of hyporheic material, sizing of structures, and developing biogeochemical hot spots. It also examines whether one removal rate is appropriate considering how biogeochemical processes are affected by design choices.