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<br />Although many trace-element core-sediment <br />concentrations exceeded the Canadian probable <br />effect level for freshwater lakes, under current <br />limnological conditions, the high core-sediment <br />concentrations do not adversely affect water <br />quality in Dillon Reservoir. The trace-element <br />concentrations in the reservoir water column meet <br />the standards established by the Colorado Water <br />Quality Control Commission. <br /> <br />INTRODUCTION <br /> <br />Dillon Reservoir is located in the Upper <br />Colorado River study unit (UCOL) of the <br />U.S. Geological Survey (USGS) National Water- <br />Quality Assessment (NAWQA) Program. The UCOL <br />was one 59 study units selected for assessment of <br />current water-quality conditions, the change in water- <br />quality over time, and the primary natural and human <br />factors that affect water quality. Investigation began on <br />the UCOL in 1994. <br />As well as providing drinking-water storage for <br />the city of Denver, Dillon Reservoir is of recreational <br />and esthetic value to both residents and visitors. The <br />reservoir's drainage area occupies the southern part of <br />Summit County. Since the reservoir was constructed in <br />1963, the drainage area has undergone significant <br />changes. The region has had a large increase in both <br />permanent and tourist populations and continues to be <br />affected by a long history of mining activity. <br />A common goal of water-quality monitoring is <br />the identification of trends. In many cases, however, <br />the data record is inadequate to effectively identify a <br />change over time. The usefulness of a data record for <br />trend detection may be limited by low sampling <br />frequencies, a short period of data collection, and <br />small numbers of variables analyzed. The use of sedi- <br />ment cores is one manner in which a missing historical <br />data record may be partly reconstructed (Van Metre <br />and Callender, 1996, 1997). In an effort to assess <br />changes in water quality, sediment cores were <br />collected at seven locations in Dillon Reservoir. <br /> <br />Purpose and Scope <br /> <br />This report describes long-term trends in <br />water quality for the Dillon Reservoir watershed by <br />analyzing concentrations of pesticides, polychlori- <br />nated biphenyls (PCBs), polycyclic aromatic <br /> <br />hydrocarbons (PAHs), and trace elements in sediment <br />cores taken from Dillon Reservoir. Loads of trace <br />elements entering and leaving Dillon Reservoir <br />were determined for 1997 and 1998 and are related <br />to results from the sediment cores. Differences in <br />trace-element loading among the three primary tribu- <br />taries to Dillon Reservoir also are investigated. <br /> <br />Acknowledgments <br /> <br />The authors thank the many individuals <br />within the USGS who contributed to this study. <br />Special thanks are extended to Bob Boulger for his <br />help with water-quality sampling; Jeffrey Deacon <br />and Lori Apodaca, who gathered the streambed- <br />sediment samples; Ted Callender for trace-element <br />analysis of sediment cores; and Sonya Jones for <br />doing the sediment coring. The authors also thank <br />Mary Kidd for editorial review of this report, <br />Joy Monson for layout, and Sharon P. Clendening <br />for producing the illustrations. <br /> <br />Description of Study Area <br /> <br />The 335-square-mile area that drains to <br />Dillon Reservoir (the study area, fig. I) ranges in <br />elevation from 14,270 feet above sea level to the <br />reservoir water level of about 9,0 I 5 feet. Construction <br />of the dam was completed and the storage pool filled <br />in 1963. During water year 1998, average surface area <br />of the reservoir was 3, 140 acres, average capacity was <br />246,777 acre-feet, and the residence time of water <br />was about I year. Dillon Reservoir is fed primarily by <br />three streams- Tenmile Creek, the Snake River, and <br />the Blue River. The smallest of the three main streams <br />draining into Dillon Reservoir, the Snake River, <br />contributes approximately one-fourth of the flow into <br />the reservoir. The remaining flow is divided almost <br />equally between Tenmile Creek and the Blue River. <br />Runoff and discharge from the incorporated towns <br />of Blue River, Breckenridge, Dillon, Frisco, and <br />Montezuma drain to Dillon Reservoir. Only one <br />major center of population within Summit County, <br />Silverthorne, is downstream from the reservoir. Four <br />ski resorts also lie within the reservoir drainage area: <br />Arapahoe Basin, Breckenridge, Copper Mountain, <br />and Keystone. <br />Between 1970 and 1980, Summit County <br />reported the largest growth rate of any county in <br />the United States, a 232-percent population increase <br /> <br />2 Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County, Colorado <br /> <br />