title: Transport and fate of nitrate at the ground-water/surface-water interface.
abstract: Although numerous studies of hyporheic exchange and denitrification have been conducted in pristine, high-gradient streams, few studies of this type have been conducted in nutrient-rich, low-gradient streams. This is a particularly important subject given the interest in nitrogen (N) inputs to the Gulf of Mexico and other eutrophic aquatic systems. A combination of hydrologic, mineralogical, chemical, dissolved gas, and isotopic data were used to determine the processes controlling transport and fate of NO(3)(-) in streambeds at five sites across the USA. Water samples were collected from streambeds at depths ranging from 0.3 to 3 m at three to five points across the stream and in two to five separate transects. Residence times of water ranging from 0.28 to 34.7 d m(-1) in the streambeds of N-rich watersheds played an important role in allowing denitrification to decrease NO(3)(-) concentrations. Where potential electron donors were limited and residence times were short, denitrification was limited. Consequently, in spite of reducing conditions at some sites, NO(3)(-) was transported into the stream. At two of the five study sites, NO(3)(-) in surface water infiltrated the streambeds and concentrations decreased, supporting current models that NO(3)(-) would be retained in N-rich streams. At the other three study sites, hydrogeologic controls limited or prevented infiltration of surface water into the streambed, and ground-water discharge contributed to NO(3)(-) loads. Our results also show that in these low hydrologic-gradient systems, storm and other high-flow events can be important factors for increasing surface-water movement into streambeds.
authors.affiliation: U.S. Geological Survey, 413 National Center, Reston, VA 20192, USA. lpuckett@usgs.gov
authors.forename: Larry J
authors.initials: LJ
authors.lastname: Puckett
authors.forename: Celia
authors.initials: C
authors.lastname: Zamora
authors.forename: Hedeff
authors.initials: H
authors.lastname: Essaid
authors.forename: John T
authors.initials: JT
authors.lastname: Wilson
authors.forename: Henry M
authors.initials: HM
authors.lastname: Johnson
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Brayton
authors.forename: Jason R
authors.initials: JR
authors.lastname: Vogel
mesh_terms: D005618:Fresh Water / Q000737:chemistry*; D009566:Nitrates / Q000737:chemistry*; D014481:United States
chemical_list: D009566:Nitrates