Water quality at basic fixed sites in the upper Colorado River basin National Water-Quality Assessment study unit, October 1995-September 1998

1 Water Quality at Basic Fixed Sites in the Upper Colorado River Basin National Water-Quality Assessment Study Unit, October 1995–September 1998 By Norman E. Spahr, Robert W. Boulger, and Richard J. Szmajter Abstract The Upper Colorado River Basin study unit of the U.S. Geological Survey National Water-Quality Assessment Program consists of the Colorado River watershed upstream from near the Colorado-Utah State line. The basin is about equally divided between the Southern Rocky Mountains and the Colorado Plateau physiographic provinces. Data were collected at pairs of indicator sites for mining, increasing urban development, and agricultural land use. Reference basic fixed sites were established in each physiographic province to provide baseline or background information in areas where anthropogenic influences are minimal. Water-quality data collection began at three of the sites in water year 1995. Full implementation of data collection at the 14-site network began in October 1996 and continued through September 1998. Six hundred and sixty waterquality samples were collected at the network sites. Snowmelt runoff dominates the hydrology in most of the basin, but water management for irrigation, storage, and transmountain diversions substantially changes annual runoff characteristics in some areas. Streamflow during water years 1995 and 1997 was generally greater than long-term average conditions. During water year 1996, streamflow also was above average at many sites but not to the extent as seen during 1995 or 1997. Water year 1998 streamflows typically were near or slightly below the long-term average. Extreme low-flow conditions generally did not occur at the sites during the data-collection period. Dissolved nitrate and total phosphorus concentrations at the background site within the Southern Rocky Mountain physiographic province typically were low (hundreths of milligrams per liter). Concentrations in areas of urban development and areas in the lower parts of the basin generally were in the tenths of milligrams per liter and in some agricultural areas were in the milligram per liter range. Median dissolved-solids concentrations at sites in the Southern Rocky Mountains were typically less than 200 milligrams per liter. Small tributaries in the Colorado Plateau and agricultural areas had dissolvedsolids concentrations in the thousands of milligrams per liter range. Trace-element concentrations were high, at times, in areas of mining land use. Median zinc concentration for the French Gulch near Breckenridge site was 2,700 micrograms per liter. Comparison of measured concentrations to Colorado State instream standards showed that concentrations of dissolved oxygen, pH, nitrate, and ammonia were within instream standards at all sites. Concentrations of cadmium and zinc at the site on French Gulch (a mining-affected site) often were greater than the State instream standard. INTRODUCTION Investigation began on the Upper Colorado River Basin study unit (UCOL) of the U.S. Geological Survey (USGS) National Water-Quality Assessment 2 Water Quality at Basic Fixed Sites in the Upper Colorado River Basin National Water-Quality Assessment Study Unit, October 1995–September 1998 (NAWQA) Program in 1994. The long-term goals of the NAWQA Program are to describe the current water-quality conditions for a large part of the Nation’s freshwater streams and rivers, to describe how water quality is changing over time, and to improve understanding of the primary natural and human factors that affect water-quality conditions (Leahy and others, 1990). The UCOL encompasses 17,843 square miles in two major physiographic provinces, the Southern Rocky Mountains and the Colorado Plateau. The environmental setting and description of the UCOL were presented by Apodaca and others (1996). A network of surface-water sites (fig. 1) was established in the UCOL as part of the occurrence and distribution phase of the NAWQA design. This network is composed of the basic fixed sites, the basic building blocks of the NAWQA Program, and is described by Spahr and others (1996). The occurrence and distribution phase of NAWQA characterizes the broad-scale geographic and seasonal distributions of water-quality conditions in relation to major point and nonpoint contaminant sources and natural or background conditions (Gilliom Upper Colorado River Basin study unit