Data for the areal and vertical distribution of dissolved nitrate and related compounds in water from the South Platte River alluvial aquifer were used to determine the origin and fate of high nitrate concentrations in water from a part of the aquifer underlying an area of intensive irrigated agriculture between Platteville and Greeley, Colorado. Nitrate concentrations at the water table varied areally from < 0.5 to 47 milligrams per liter as nitrogen, and average nitrate concentrations were higher in aerobic water underlying agricultural fields (22.6 ± 12.3 milligrams per liter) than in anaerobic water underlying the river (6.2 ± 4.2 mg/L). There was no apparent relation between nitrate concentration and depth. Values of delta15-N for dissolved nitrate in aerobic water underlying fields (11.0 ± 2.3 per mil) were consistent with an animal-waste source for the nitrate. Heavier (delta15-N values for dissolved nitrate in anaerobic water underlying the river (17.6 ± 2.1 per mil) and the lower nitrate concentrations in water underlying the river indicate that microbial denitrification in the anaerobic part of the aquifer lowered nitrate concentrations, leaving the residual nitrate enriched in delta15-N prior to groundwater discharge to the river. Further evidence for microbial denitrification in the aquifer included a buildup of N2 and N2O gases, both products of denitrification, in anaerobic water from the aquifer. These results may have important consequences for agricultural nitrate-management practices and aquatic biological assessments in the study area.
1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225