Distribution of nitrate and orthophosphate in selected streams in Central Nebraska

The Central Nebraska Basins is one of 60 study units in the National Water-Quality Assessment Program of the U.S. Geological Survey. The study unit includes the Platte River and two major tributaries, the Loup and Elkhorn Rivers. Agriculture is the predominant land use in the study unit, with only eight urbanized communities exceeding a population of 10,000. Water samples were collected from selected streams in the study unit during 1993- 1995. The data were used to assess the distribution of nitrogen compounds and phosphorus in the streams and to relate the concentrations of these constituents to environmental settings. This article focuses on dissolved nitrate and orthophosphate. Dissolved nitrate concentrations were highest (90th percentiles were less than 7.0 milligrams per liter as nitrogen) in areas with extensive cropland and pasture, where chemical fertilizers are intensively applied. Synoptic measurements conducted in March and August 1994 indicate that relatively little residual fertilizer, as nitrate, applied during a single crop-growing season enters streams. Dissolved nitrate concentrations showed a seasonal pattern, being highest during winter months and lowest during the late spring and summer. Dissolved orthophosphate concentrations tended to be low across the study unit, 90 percent of all analyses did not exceed 1.7 milligrams per liter as phosphorus.