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This article in JEQ

  1. Vol. 39 No. 3, p. 810-823
     
    Received: Jan 9, 2009


    * Corresponding author(s): mayer.paul@epa.gov
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doi:10.2134/jeq2009.0012

Nitrogen Dynamics at the Groundwater–Surface Water Interface of a Degraded Urban Stream

  1. Paul M. Mayer *a,
  2. Peter M. Groffmanb,
  3. Elise A. Strizad and
  4. Sujay S. Kaushalc
  1. a U.S. Environmental Protection Agency, National Risk Management Research Lab., Ground Water and Ecosystems Restoration Division, Ada, OK 74820
    b Cary Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545
    d current address: U.S. Nuclear Regulatory Commission, MS T7-E18, 11545 Rockville Pike, Rockville, MD 20852. Assigned to Associate Editor Christopher Green
    c Univ. of Maryland, Center for Environmental Science, Chesapeake Biological Lab., 1 Williams St., P.O. Box 38, Solomons, MD 20688

Abstract

Few studies have quantified the impact of urbanization on the biogeochemistry of streams at the groundwater–surface water interface, a zone that may be critical for managing nitrogen transformations. We investigated the groundwater ecosystem of Minebank Run, a geomorphically degraded urban stream near Baltimore, Maryland in the Chesapeake Bay watershed. Our objectives were to identify the spatial and temporal extent of chemical, microbial, and hydrological factors known to influence denitrification, a microbial process that removes nitrate nitrogen (NO3 ). Measurements of denitrification enzyme activity confirmed that subsurface sediments at Minebank Run, especially those with high concentrations of organic carbon, have the capacity to denitrify NO3 Levels of NO3 in groundwater were lower where more dissolved organic carbon (DOC) was available, suggesting that denitrification and removal of NO3 in groundwater were limited by DOC availability. Groundwater NO3 was highest when groundwater levels were highest, which, in turn, corresponded to high oxidation–reduction potential (ORP), indicative of high groundwater–surface water exchange. Stream flow patterns controlled stream bank and bed infiltration and, subsequently, dictated groundwater levels. Declines in water levels likely increased subsurface mixing, which led to low ORP conditions that sustained NO3 removal via denitrification. The groundwater–surface water interface is a zone of active nitrogen transformation. Management efforts that increase DOC availability to denitrifiers, reduce stream-flow velocity and flashiness, and increase groundwater residence time will likely improve the nitrogen removal capacity of urban stream channels.

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