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U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

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Geochemistry of Nitrogen in a Farmed Watershed Near Princeton, Minnesota

by

Richard B. Wanty (U.S. Geological Survey, M.S. 916, Denver Federal Center, Denver, CO 80225), Michele L. Tuttle (U.S. Geological Survey, M.S. 916, Denver Federal Center, Denver, CO 80225), Matthew K. Landon (U.S. Geological Survey, 2280 Woodale Dr., Mounds View, MN 55112), Geoffrey N. Delin (U.S. Geological Survey, 2280 Woodale Dr., Mounds View, MN 55112), and J.K. Böhlke (U.S. Geological Survey, M.S. 431 National Center, Reston, VA 22092)

Abstract

A broad interagency research program is being conducted near Princeton, Minnesota, to investigate possible advantages of various modifications to existing farming techniques with the goal of decreasing contamination of ground-water and surface-water supplies by agricultural chemicals. Most of the program is focused on a 160-acre test farm, with the intention that the results from the test farm be applied to other farms in the area. To facilitate extending the results to a broader region, a study was conducted to investigate regional effects of agricultural activity on nitrogen in ground waters and surface waters of the Battle Brook watershed. Shallow ground water has received the greatest inputs of nitrate-nitrogen. Concentrations of nitrate-nitrogen (nitrate-N) exceeding 10 milligrams per liter (mg/L) as N are common near the water table. Nitrate-N concentrations decreased rapidly with depth (deeper than about 8 to 10 meters) in the saturated zone throughout the watershed, possibly because of mixing with uncontaminated waters or because of denitrification. Extensive wetlands in the area are fed by the discharge of shallow ground water, yet nitrate-N was not detected in any wetland waters. Biological activity in the wetland and its sediments is capable of decreasing nitrate-N concentrations. Monthly to bimonthly sampling of a piezometer near the water table at the Princeton test farm during July, 1991 to August, 1992, shows that nitrate-N concentrations varied between 13 and 25 mg/L in response to seasonal recharge events. The magnitude of the variation decreased with depth; about 2 meters below the water table, nitrate-N concentrations ranged from 12 to 17 mg/L.

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