IDENTIFICATION OF SPRING AND EARLY SUMMER
  PATHWAYS AND SOURCES OF NITRATE-NITROGEN
  DELIVERY IN THE BLUE EARTH RIVER WATERSHED IN
  SOUTH-CENTRAL MINNESOTA
  
  Joe Magner1, Steve Komor, and Greg Payne2 
  1Minnesota Pollution Control Agency, St. Paul, Mn 55155
   2U. S. Geological Survey, Moundsview, Mn 55112
  
  The four year Minnesota River Assessment Project identified the Blue
  Earth River (BER) as delivering over half of the nitrate-nitrogen (nitrate-N)
  load to the lower Minnesota River. Recent study has indicated that
  agricultural activities in the Mississippi River basin are believed to
  contribute to the development of the zone of hypoxia in the Gulf of
  Mexico.
  Nitrate-N concentrations greater than 10 ppm have been routinely reported
  by the city of Mankato water utility in the BER at Mankato in the spring
  and early summer for nearly a decade. Above normal precipitation years,
  i.e., 1993, have resulted in lower concentrations of nitrate-N, however, a
  year like 1990 which followed several below normal precipitation years
  resulted in elevated nitrate-N concentrations (>30 ppm). Increased nitrate-N
  concentrations have been linked to increases in stream-flow particularly
  following the spring snow melt, soil thaw and spring rainfall. Agricultural
  land in the BER watershed has been extensively drained in the past half
  century to improve crop production.
  Surface and subsurface pathways of spring and early summer runoff were
  characterized by the collection of: stream-flow, field water quality
  parameters, major ion chemistry and the stable isotopic ratios of hydrogen
  and oxygen. Data were initially collected near Mankato in major tributaries
  of the BER in 1991. Throughout the spring runoff of 1994 - 1996 smaller
  watershed scales, representative of land uses which serve as sources of
  nitrogen, were sampled and analyzed to assess relative contributions.
  Several years of nitrate-N and pathway data collection indicates that the
  largest source of nitrate-N leaches from an integrative soil nitrogen pool
  into subsurface tiles under corn and soybean fields. Subsurface drainage
  tiles have become the predominant pathway of water transport in the BER.
  The linkage of climate, drainage and fertilization appear to influence the
  amount of nitrate-N found in the tributaries of the BER.
  
  Keywords: nitrate-nitrogen, Blue Earth River watershed, isotopic tracers,
  tile drainage
  
  
  1)Joe Magner, MPCA, 520 Lafayette Rd, St. Paul, Mn 55155 (612) 296-9268, FAX (612) 296-8683, joseph.magner@pca.state.mn.us
  2) poster, or platform
  3) The presenting author is a professional hydrologist.