Hydrogeology and Simulated Effects of Urban Development on Water Resources of Spanish Springs Valley, Washoe County, West-Central Nevada By David L. Berger, Wyn C. Ross, Carl E. Thodal, and Armando R. Robledo U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 96-4297 Prepared in cooperation with the NEVADA DIVISION OF WATER RESOURCES Carson City, Nevada 1997 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY GORDON P. EATON, Director Any use of trade names in this publication is for descriptive purposes only and does not constitute endorsement by the U.S. Government For additional information Copies of this report can be write to: purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Branch of Information Services 333 West Nye Lane, Room 203 Box 25286 Carson City, NV 89706-0866 Denver, CO 80225-0286 email: [email protected] CONTENTS Abstract............................................................................................................................................ 1 Introduction............................................................................................~^ 1 Purpose and Scope .................................................................................................................................................... 2 Location and General Features of Study Area .......................................................................................................... 2 Urban Development .................................................................................................................................................. 4 Previous Investigations ............................................................................................................................................. 4 U.S. Geological Survey Site Designations ................................................................................................................ 4 Acknowledgments..................................................................................................................................................... 6 Methods Used in this Study .................................................................................................................................................. 6 Collection of Hydrogeologic Data ............................................................................................................................ 6 Geochemical Techniques........................................................................................................................................... 11 Geophysical Methods ................................................................................................................................................ 13 Hydrologic Models.................................................................................................................................................... 14 Hydrogeologic Framework ................................................................................................................................................... 14 Geologic History and Structural Setting ................................................................................................................... 15 General Character of Hydrogeologic Units ............................................................................................................... 15 Consolidated Rock.......................................................................................................................................... 15 Basin Fill........................................................................................................................................................ 17 Areal Extent and Thickness ............................................................................................................... 21 Hydraulic Properties........................................................................................................................... 21 Hydrologic Setting ................................................................................................................................................................ 26 Climate and Vegetation ............................................................................................................................................. 27 Surface-Water Flow System ...................................................................................................................................... 32 Sources and Movement of Ground Water ................................................................................................................. 34 Chemical Composition of Ground Water .................................................................................................................. 42 Ground-Water Recharge and Discharge ............................................................................................................................... 42 Recharge from Precipitation ...................................................................................................................................... 45 Infiltration of Imported Surface Water ...................................................................................................................... 45 Discharge by Evapotranspiration .............................................................................................................................. 47 Subsurface Flow to Adjacent Basins ......................................................................................................................... 48 Ground-Water Development ..................................................................................................................................... 48 Evaluation of the Ground-Water Budget and Yield, 1994 ........................................................................................ 50 Simulation of Ground-Water Flow ....................................................................................................................................... 51 Mathematical Basis ................................................................................................................................................... 51 General Features of Flow Model............................................................................................................................... 52 Selection of Time Periods and Assigned Recharge and Discharge ............................................................... 52 Boundary Conditions ..................................................................................................................................... 54 Initial Conditions and Aquifer Properties ...................................................................................................... 58 Simulation of Transient Conditions, 1979-94 ........................................................................................................... 61 Calibration and Results .................................................................................................................................. 61 Simulated Ground-Water Budget .................................................................................................................. 65 Sensitivity Analysis ....................................................................................................................................... 68 Limitations of Flow Model............................................................................................................................ 68 Simulation of Responses to Hypothetical Ground-Water Development Scenarios .................................................. 69 Selection of Scenarios ................................................................................................................................... 69 Discussion of Results ..................................................................................................................................... 70 Summary and Conclusions ................................................................................................................................................... 70 References Cited ...................................................................................................................._^ 77 CONTENTS III FIGURES 1-6. Maps of the Spanish Springs Valley area, west-central Nevada, showing: 1. Location and general features........................................................................................................................ 3 2. Parceled land with public-water systems and domestic wells and general area of agricultural land use in 1979 and 1994........................................................................................................................................ 5 3. Wells, precipitation stations, surface-water gages, and water-chemistry sampling sites.............................. 7 4. Generalized geologic units, structural features, and seismic-refraction lines............................................... 16 5. Total-intensity magnetic field........................................................................................................................ 19 6. Complete Bouguer gravity field.................................................................................................................... 20 7-9. Graphs showing