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Nitrate Retention in Riparian Ground Water at Natural and Elevated Nitrate Levels in North Central Minnesota
Author(s) -
Duff John H.,
Jackman Alan P.,
Triska Frank J.,
Sheibley Richard W.,
Avanzino Ronald J.
Publication year - 2007
Publication title -
journal of environmental quality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.888
H-Index - 171
eISSN - 1537-2537
pISSN - 0047-2425
DOI - 10.2134/jeq2006.0019
Subject(s) - riparian zone , hydrology (agriculture) , transect , nitrate , alluvium , environmental science , groundwater , groundwater recharge , riparian buffer , surface runoff , geology , ecology , geomorphology , aquifer , oceanography , geotechnical engineering , habitat , biology
The relationship between local ground water flows and NO 3 − transport to the channel was examined in three well transects from a natural, wooded riparian zone adjacent to the Shingobee River, MN. The hillslope ground water originated as recharge from intermittently grazed pasture up slope of the site. In the hillslope transect perpendicular to the stream, ground water NO 3 − concentrations decreased from ∼3 mg N L −1 beneath the ridge (80 m from the channel) to 0.01 to 1.0 mg N L −1 at wells 1 to 3 m from the channel. The Cl − concentrations and NO 3 /Cl ratios decreased toward the channel indicating NO 3 − dilution and biotic retention. In the bankside well transect parallel to the stream, two distinct ground water environments were observed: an alluvial environment upstream of a relict beaver dam influenced by stream water and a hillslope environment downstream of the relict beaver dam. Nitrate was elevated to levels representative of agricultural runoff in a third well transect located ∼5 m from the stream to assess the effectiveness of the riparian zone as a NO 3 − sink. Subsurface NO 3 − injections revealed transport of up to 15 mg N L −1 was nearly conservative in the alluvial riparian environment. Addition of glucose stimulated dissolved oxygen uptake and promoted NO 3 − retention under both background and elevated NO 3 − levels in summer and winter. Disappearance of added NO 3 − was followed by transient NO 2 − formation and, in the presence of C 2 H 2 , by N 2 O formation, demonstrating potential denitrification. Under current land use, most NO 3 − associated with local ground water is biotically retained or diluted before reaching the channel. However, elevating NO 3 − levels through agricultural cultivation would likely result in increased NO 3 − transport to the channel.