Shallow ground-water quality beneath cropland in the Red River of the North Basin, Minnesota and North Dakota, 1993-95

During 1993-95, the agriculture on two sandy, surficial aquifers in the Red River of the North Basin affected the quality of shallow ground water in each aquifer differently. The Sheyenne Delta aquifer, in the western part of the basin, had land-use, hydrogeological, and rainfall characteristics that allowed few agricultural chemicals to reach or remain in the shallow ground water. The Otter Tail outwash aquifer, in the eastern part of the basin, had characteristics that caused significant amounts of nutrients and pesticides to reach and remain in the shallow ground water. Shallow ground water from both aquifers is dominated by calcium, magnesium, and bicarbonate ions. During the respective sampling periods, water from the Sheyenne Delta aquifer was mostly anoxic and water from the Otter Tail outwash aquifer had a median dissolved oxygen concentration of 3.6 mg/L (milligrams per liter). The median nitrate concentration was 0.03 mg/L as nitrogen (mg/L-N) in shallow ground water from the Sheyenne Delta aquifer and 6.1 mg/L-N in that from the Otter Tail outwash aquifer. Of 18 herbicides and 4 insecticides commonly used in the aquifer areas and for which analyses were done, 5 herbicides and 1 herbicide metabolite were detected in the shallow ground water from the Sheyenne Delta aquifer and 8 herbicides and 2 metabolites were detected in that from the Otter Tail outwash aquifer. The total herbicide concentration median was less than the detection limit in shallow ground water from the Sheyenne Delta aquifer and 0.023 μg/L (micorgrams per liter) in that from the Otter Tail outwash aquifer. Triazine herbicides were the most commonly detected herbicides and were detected at the highest concentrations in the shallow ground water from both study areas. One sample from the Sheyenne Delta aquifer contained a high concentration of picloram. Agricultural chemicals in both aquifers were stratified vertically and their concentration correlated inversely with ground-water age. The highest concentrations of these chemicals and the youngest ground-water ages were at the water table. Concentrations decreased and age increased with water-table depth. Nitrate concentration varied seasonally over one-half an order of magnitude, though concentrations only repeated seasonally in some shallow ground water.

Land-use factors that increased nitrate and herbicide concentrations were greater tilled area, chemical application, irrigation, and cropland contiguity. Hydrogeological factors that increased these concentrations were a deeper watertable (higher oxygen concentration and less organic carbon), larger grain-size and degree of sorting of aquifer material (shorter time in the soil zone and aquifer), and fewer sulfur-containing minerals (lignite and pyrite) composing the aquifer. High rainfall, just before sampling of the Sheyenne Delta aquifer, contributed to the relatively low nitrate and pesticide concentrations in the shallow ground water of this aquifer by raising the water table higher into the soil zone, increasing ponded water (increasing biodegradation), preventing some chemical application (flooded fields), and leaching and then displacing nitrate-rich water downward, beneath new recharge. The shallow ground-water quality measured beneath cropland in these land-use study areas covers a large range. The land-use, hydrogeological, and rainfall factors controlling this quality also control shallow ground-water quality in other surficial aquifers in the Red River of the North Basin. Although not used for drinking water, 43% of the shallow ground water from the Otter Tail outwash aquifer was above the U.S. Environmental Protection Agency's nitrate maximum contaminant level of 10 mg/L-N, reducing its potential uses. These high nitrate concentrations do not threaten the Otter Tail outwash aquifer's surface-water bodies with eutrophication however, because significant denitrification occurs beneath riparian wetlands before ground water discharges to surface waters.