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WATER-QUALITY ASSESSMENT OF THE SOUTH PLATTE RIVER BASIN, COLORADO, NEBRASKA, AND WYOMING--ENVIRONMENTAL SETTING AND WATER QUALITY OF FIXED SITES, 1993-95

National Water-Quality Assessment (NAWQA) Program
South Platte River Basin Study

by David W. Litke and Robert A. Kimbrough 1

ABSTRACT

In 1991, the U.S. Geological Survey (USGS) began full-scale implementation of the National Water-Quality Assessment (NAWQA) Program, with the objectives of describing the status and trends in the quality of a large, representative part of the Nation's surface-water and groundwater resources and of providing a sound scientific understanding of the primary natural and human factors affecting the quality of these resources. The South Platte River Basin was among the first 20 NAWQA study units selected for study under the full-scale implementation plan. Twelve fixed sites were established on streams in the South Platte River Basin to provide data to support the objectives of the NAWQA study. The sites were selected to represent forest land use (three sites), agricultural land use (three sites), urban land use (three sites), mixed urban/agricultural land use (two sites), and rangeland land use (one site). This report describes the environmental setting and hydrologic conditions at the sites, summarizes monthly data collected at the sites, discusses the effects of land use on concentrations, evaluates historical trends in concentrations at selected sites, and presents estimates of stream loads for selected constituents.

The 12 fixed sites were sampled monthly from March 1993 to September 1995. Constituents determined from monthly sampling included field measurements and concentrations of major constituents, organic carbon, nutrients, and suspended sediment. Concentration data were compared to stream standards as points of reference for water quality. There were occasional occurrences of constituent concentrations exceeding chronic stream-segment standards for sulfate, iron, nitrite, and un-ionized ammonia. However, there were more common exceedances of chronic standards for manganese at one forest site, two urban sites, and on mixed urban/agricultural site. The U.S. Environmental Protection Agency (USEPA) recommended limit for total phosphorus concentrations in streams (0.1 milligrams per liter) for the control of eutrophication was exceeded at all urban, agricultural, and mixed urban/agricultural sites and at one forest site; the minimum measured phosphorus concentration at most of these sites also exceeded the recommended limit.

Land use was a statistically significant classifying variable for all of the water-quality constituents. For some constituents (specific conductance, water temperature, dissolved solids, and sodium), each land use was significantly different from all others. For other constituents (for example, nitrite plus nitrate), there were as few as two significant classes. Water quality generally was best (significantly smaller median concentrations) for rangeland and forest land use. Mixed urban/agricultural land use had the poorest water quality (significantly larger median concentrations) among the land uses sampled, which was attributed to the combined inputs of urban and agricultural land use.

Seasonal Kendall trend tests on combined USGS and USEPA data for 1963-96 detected downward trends in salinity at sites along the South Platte River, whereas nitrate concentrations had upward trends. Streamflow had upward trends at sites along the river, but the salinity and nitrate concentration trends still were significant after being adjusted for variability in streamflow.

Daily loads were estimated at the fixed sites for dissolved solids, total nitrogen, total phosphorus, and suspended sediment for March 1993 to September 1995. Loads were largest at South Platte River near Kersey, Colorado, where streamflow also was largest; mean annual loads at this site for water years 1994-95 were 810,000 tons of dissolved solids, 510,000 tons of suspended sediment, 880 tons of total phosphorus, and 6,400 tons of total nitrogen. The downstream pattern of mean annual loads along the South Platte River was similar for the four constituents; loads increased from Denver to Kersey, Colorado, and decreased from Kersey to North Platte, Nebraska.

1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225

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