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

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criteria. All but one of the detectable concentra- tion of pesticides in ground -water samples occurred in alluvial wells, and all detections were from agricultural land -use areas. None of the pes- ticide concentrations in ground water exceeded State and Federal water - quality criteria. INTRODUCTION In 1991, the U.S. Geological Survey (USGS) began a full -scale National Water - Quality Assessment ( NAWQA) program. The long -term goals of the NAWQA program are to describe the status and trends in the quality of a large part of the Nation's surface- and ground -water resources and to provide a sound, scien- tific understanding of the primary natural and human factors affecting the quality of these resources. In meeting these goals, the program will produce a wealth of water - quality information that will be useful to policy makers and managers at the national, State, and local levels. A major design feature of the NAWQA program is the integration of water - quality information at differ- ent areal scales. The principal building blocks of the program are the study -unit investigations on which the national -level assessments are based (Jones and Sylvester, 1992). The 60 study -unit investigations that make up the program are hydrologic systems that include parts of the Nation's major river basins and aquifer systems. These study units include areas ranging from about 600 mil (Oahu Study Unit) to about 67,000 mil (Central High Plains Study Unit) and incor- porate about 60 to 70 percent of the Nation's water use and population served by public water supply. In 1991, the South Platte River Basin was among the first 20 NAWQA study units selected for study. A diverse group of local, State, and Federal agen- cies and organizations have collected water - quality data throughout the South Platte River Basin for a vari- ety of purposes. One of the first activities to be under- taken by this study was a compilation, screening, and analysis of available water - quality data. This prelimi- nary water - quality assessment will help establish prior- ities and help formulate plans for the intensive data collection activities. Purpose and Scope This report presents the analysis of available nutrient, suspended- sediment, and pesticide informa- tion for the South Platte River Basin in Colorado, Nebraska, and Wyoming for water years 1980 -92. More specifically, the purposes of this report are to: (1) Describe recent water - quality conditions for these selected constituents to the extent possible in the South Platte River Basin; (2) provide information on strengths and weaknesses of the available data; and (3) assess implications of findings to relevant water - quality issues in the basin and to future study priorities and design. A total of 3,484 samples from 54 surface -water sites and 107 wells were used in this analysis. Nutrient constituents examined included: total nitrogen, total organic nitrogen plus ammonia (for surface -water sites), dissolved organic nitrogen plus ammonia (for ground -water sites), dissolved ammonia, dissolved nitrite, dissolved nitrite plus nitrate, dissolved phos- phorus, dissolved orthophosphate, and total phospho- rus. All nitrogen species are reported as nitrogen, and all phosphorus species are reported as phosphorus. Suspended- sediment constituents examined included suspended- sediment concentration and suspended - sediment particle size distribution. Pesticide constitu- ents examined included these compound classes: car - bamates, chlorophenoxy acid herbicides, organochlorine and organophosphorus insecticides, and triazine and other nitrogen- containing herbicides. Acknowledgments The authors wish to thank the various Federal and State agencies and members of the South Platte River Basin Liaison Committee for their cooperation in providing information and data that were used in pre- paring this report. Specifically, the authors are grateful to the U.S. Environmental Protection Agency, Region VIII; Colorado Department of Health; Denver Regional Council of Governments; Denver Water Department; Metro Wastewater Reclamation District; and Northern Front Range Water Quality Planning Association for their assistance during the preparation of this report. We would also like to thank James E. Paschal (U.S. Geological Survey) for compiling the database for this report and Sharon L. Qi (U.S. Geolog- ical Survey) for preparing the report graphics and tables. DESCRIPTION OF THE SOUTH PLATTE RIVER BASIN This section gives a brief synopsis of the natural and anthropogenic features in the South Platte River Basin that may affect water quality. For a more detailed description of the environmental setting and its implications for water quality, the reader may refer to Dennehy and others (1993). 2 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