D.R. Goebel, G.G. Mayer, S.A. Bassingthwaite 1
A prairie wetland complex near Sarles, North Dakota, has been instrumented to determine the hydrogeology and geochemistry of wetlands in an agricultural setting. The field site is located in a lowrelief till plain consisting of many small, isolated depressions and a larger, more connected wetland complex. Till is present in variable thickness and overlies the Pierre Shale.
Nitrate contamination of shallow groundwater ( > 10 mg/L N) occurs in wells peripheral to the tilled fields and not in wells within pastureland. Nitrate concentrations in shallow groundwater range from 25 mg/L N in samples near tilled fields and values of < 1 mg/L N below pastureland. Nitrate was detected once (17 mg/L N) in wetland waters when underlying sediments were frozen. Nitrate was not detected in surface water from the wetlands during any of the other sampling events, even after replenishment of the wetland basins in 1993. Spring runoff (1995) into the wetlands showed nitrate concentrations ranging from < 1 to 27 mg/L N. Soil sampling for NO3N in past cropping years indicates a potential carryover of 20 to 450 kg ha-1 nitrate in fields peripheral to the wetlands. The denitrification capacity of saturated wetland sediments can be qualified by the variation of nitrate between runoff and surface waters.
Local groundwater flow originates in the elevated till landscape and discharges to the main wetland. Shallow groundwater at the site is predominantly a sodium sulfate type, with dissolved solids as high as 26,000 mg/L. Highest salinity and dissolved organic carbon (DOC) occur in wells downgradient from the wetlands. Based on the salinity of wetland waters, runoff from agricultural lands makes up a significant portion of the water budget. Spatial distribution of nitrates in shallow groundwater indicates that the nitrate pollution potential is associated with land use.
