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Transport and Fate of Nitrate in a Glacial Outwash Aquifer in Relation to Ground Water Age, Land Use Practices, and Redox Processes

^a U.S. Geological Survey, 413 National Center, Reston, VA 20192
^b U.S. Geological Survey, 2280 Woodale Road, Mounds View, MN 55112

View larger version (45K): [in a new window]   Fig. 1. Location of the study area north of the Otter Tail River.

View larger version (16K): [in a new window]   Fig. 2. (a) Annual fertilizer N applications in Otter Tail county since 1945; (b) cumulative N applications during the same period.

View larger version (33K): [in a new window]   Fig. 3. Cross-section of the study transect showing land use, piezometer nest locations, screened intervals, stratigraphy, and approximate locations of MODFLOW layers.

View larger version (31K): [in a new window]   Fig. 4. Ground water flow lines and travel times along the study transect based on the MODFLOW/MODPATH simulation.

View larger version (31K): [in a new window]   Fig. 5. Apparent ground water recharge dates as determined from chlorofluorocarbon (CFC) concentrations measured in ground water samples and generalized age distribution along the transect based on the MODFLOW/MODPATH simulation.

View larger version (15K): [in a new window]   Fig. 6. Argon and N2 gas concentrations for samples collected during 1994, 1995, and 1997 and their theoretical values. Values to the right of the air-saturated water equilibrium line represent excess nitrogen concentrations. Horizontal tick marks on the equilibrium line indicate Ar and N2 equilibrium concentrations from 0 to 20°C in 2° increments.

View larger version (27K): [in a new window]   Fig. 7. Distribution of excess N2 along the transect.

View larger version (26K): [in a new window]   Fig. 8. Concentrations of NO-3 along the transect.

View larger version (53K): [in a new window]   Fig. 9. Distibution of (a) Cl- and (b) SO2-4 along the transect.

View larger version (40K): [in a new window]   Fig. 10. The generalized redox state of the aquifer along the transect.

View larger version (27K): [in a new window]   Fig. 11. Distribution of reconstructed (original) concentrations of NO-3 calculated as the sum of excess N2 and NO-3 measured along the transect.