Water Quality Assessment of the South Platte River Basin, Colorado , Nebraska, and Wyoming

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Water - Quality Assessment of the South Platte River Basin, Colorado, Nebraska, and Wyoming -- Analysis of Available Nutrient, Suspended- Sediment, and Pesticide Data, Water Years 1980 -92 By Kevin F. Dennehy, David W. Litke, Peter B. McMahon, Janet S. Heiny, ano'Cathy M. Tate ABSTRACT In 1991, a water - quality investigation of the South Platte River Basin was initiated as part of the U.S. Geological Survey's National Water - Quality Assessment (NAWQA) program. One of the first tasks of the assessment was a compilation, screening, and interpretation of available nutrient, suspended- sediment, and pesticide data collected from surface- and ground -water sites in the basin. The data used in the analysis were from water years 1980 -92. The analysis of existing water - quality data provides a perspective on recent water - quality conditions in the South Platte River Basin, evaluations of the strengths and weaknesses of available data, and implications for water -qual- ity issues and future study priorities and design. Most of the data used in the analysis were retrieved from the U.S. Geological Survey National Water Information System and U.S. Environmental Protection Agency Storage and Retrieval data bases. Data collected from three local agencies not contained in either data base were also used in the analysis. A total of 3,484 samples from 54 surface -water sites and 107 wells were used in the analysis. The quantity of data available from these sites and wells varied considerably with respect to constituent sampled. The areal distribution of nutrient samples collected from surface -water and ground -water sites were sufficient in number and areal distribu- tion to describe current water - quality conditions throughout the basin, but data were not sufficient to analyze factors and processes affecting water quality. However, suspended- sediment and pesti- cide data were sparse in their distribution with respect to time, space, and flow regime, and were sufficient only to provide a preliminary description of conditions in the basin. Data indicate that nutrient concentrations (nitrogen and phosphorus) in surface -water sam- ples were elevated downstream of point source discharges, especially wastewater treatment plants, in urban and mixed urban and agricultural areas. Concentrations of dissolved nitrite plus nitrate were substantially higher during the winter (low -flow) season than at other times of the year. Only two surface -water samples equaled or exceeded the U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level (MCL) (10 milligrams per liter (mg/L) as nitro- gen). The composition of the nitrogen load was mostly as organic nitrogen plus ammonia in the urban areas, whereas nitrite plus nitrate was most of the load in the downstream agricultural areas. Nitrogen concentrations in ground -water samples were generally highest in samples from the allu- vial aquifer collected from wells completed in agricultural areas. Forty -six percent of the dis- solved nitrite plus nitrate analyses from wells completed in the alluvium underlying agricultural areas equaled or exceeded the MCL for nitrate. Generally, suspended- sediment concentra- tions were larger at the downstream sites in the basin. Water- management practices, especially in the lower basin, affect and control the sediment transport along the river. Suspended sediment concentrations varied by month, with the highest loads being transported during snowmelt runoff. Most pesticide concentrations were less than laboratory reporting levels. The pesticides with the highest percent detections in surface water among the six land uses studied were atra- zine in agricultural areas and picloram in mixed agricultural and urban land use. Only one surface - water site, located in the mixed agricultural and urban land -use area, had a pesticide (parathion) concentration that exceeded a water - quality ABSTRACT