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Introduction 5 11 71 ? ? N EXPLANATION Unconsolidated surficial deposits of Quaternary age Modern alluvium Gravels and alluvium Pre-BUII Lake age gravel and alluvium Glacial drift of Pinedale and Bull 4U Lake Glaciations or Pre-Bull Lake age Igneous rocks of Tertiary and(or) Cretaceous age Laramide inslrusivc rocks ¦ Ash-flow tuff of main volcanic sequence Sedimentary rocks of Tertiary age ? Troublesome Formation-Siltstone, sandstone, and conglomerate ¦ Coalmont Formation-Sandstone, shale. coal beds, and conglomerate Sedimentary rocks of Cretaceous age ? Windy Gap Member of the Middle ? Park Formation Pierre Shale undivided . LAWJ Colorado Group Sedimentary rocks of Cretaceous and Jurassic age Dakota and Morrison Formations Sedimentary rocks, broadly classified ? Mesozoic rocks-Mainly early Cretaceous. Jurassic. and Triassic age Igneous and metamorphic rocks of Precambrian age ® Granitic rock of 1,400 Mega-annum ? ¦ ¦ Granitic rock of 1,700 Mega-annum Base modified from U.S. Geological Survey County map series 1:50,000, 1978 Grand County sheets 2 and 4 Biotite gneiss, schist. and migmatite Felsic and hornblende gneisses, either separate or interlayered 0 1 2 3 4 5 MILES 11 1 i I I I I I 1 0 1 2 3 4 5 KILOMETERS Figure 3. Generalized geology of the Fraser River watershed (Green, 1992; Tweto, 1979). Water from the alluvial and Troublesome Formation aquifers is used for residential and municipal purposes. The population of Grand County in 2000 was 12,442, an increase of more than 56 percent since 1990 and more than 300 percent since 1970 (U.S. Census Bureau, 2002). In addi- tion, the county receives large seasonal influxes of visitors related to recreational and tourism-related activities, which add to the demand on water resources of the area. Three wastewater- treatment facilities (fig. I ) and public water supplies that service the larger population centers are located upstream from Taber- rash. The wastewater facilities serve the Winter Park ski area and the towns of Winter Park and Fraser and discharge to the Fraser River. Discharge from the facilities is highest in winter and early spring because of recreational activities. Outside of Fraser, well water is used for water supply and ISDS's are used for post-use treatment of sewage. Acknowledgments The authors thank the Grand County Board of County Commissioners for their support throughout the study described in this report. We also thank the individuals, businesses, and agencies that permitted access to their wells for ground-water sampling. The authors acknowledge the efforts of Lori Apo- daca, Rick Arnold, Steve Char, Jason Gurdak, and C. Michelle Smith of the USGS in collecting the ground-water-quality data presented in this report, and Jim Martinez and Clay Thompson