State Water Resources Research Institute Program

Project Id: 2010AR252B
Title: Denitrification, Internal N Cycling, and N Retention in River Impoundment Reservoirs
Project Type: Research
Start Date: 3/01/2010
End Date: 2/28/2011
Congressional District: 3
Focus Categories: Geochemical Processes, Nutrients, Sediments
Keywords: mass balance, nitrogen transformation, nutrient retention/removal, sediment
Principal Investigator: Scott, Jefferson Thad (University of Arkansas)
Federal Funds: $ 20,579
Non-Federal Matching Funds: $ 41,160
Abstract: The use of synthetic nitrogenous fertilizers by humans may be increasing nitrogen (N) export from the Mississippi River and expanding the seasonal hypoxic zone in the Gulf of Mexico. However, only a fraction of land applied N fertilizer may actually be exported to coastal systems. Mass balance studies have shown that river impoundment reservoirs can retain significant quantities of their N loads and may be important N sinks in the landscape. However, few studies have directly assessed the mechanisms controlling reservoir N retention. Of the potential fates, only denitrification can remove N from these systems permanently.

In this study we will examine the role of sediment denitrification in N retention in four northwest Arkansas reservoirs. Sediment denitrification rates will be quantified using continuous-flow, intact-core experiments coupled with membrane inlet mass spectrometry (MIMS) across multiple seasons. Sediment cores will also be sampled for nitrate (NO3-) and ammonium (NH4+) flux rates, sediment oxygen demand, organic carbon content, pH, and temperature. A denitrification predictive model will be developed from these parameters using multiple-regression, which will be used to scale denitrification rates to the whole ecosystem. Ecosystem-scale N flux estimates will also be derived using stream gauge, streamflow, and stream chemistry information from inflowing tributaries and reservoir elevation, water release, and water chemistry from reservoirs.

Simultaneously measuring whole-system denitrification and N retention will allow us to estimate the relative importance of permanent N removal and characterize conditions that would favor this N fate. A potential outcome of the study will be recommendations for reservoir management that may enhance permanent N removal from reservoir waters. These recommendations could be incorporated into reservoir management plans in order to maximize denitrification and thereby decrease N export to downstream coastal environments.

Progress/Completion Report, 2010, PDF

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