USGS science for a changing world Q. I Hydrogeology, Water Quality, Water Budgets, to and Simulated Responses to Hydrologic Changes in o o30 CD o Santa Rosa and San Simeon Creek Ground-Water Basins, CD < San Luis Obispo County, California 30 C I m 30 cCO o mCT CO CO o CD U.S. GEOLOGICAL SURVEY CO-D Water-Resources Investigations O O Report 98-4061 O >o o 30 CO O CO 30 30 CO Prepared in cooperation with the SAN LUIS OBISPO COUNTY FLOOD CONTROL AND WATER CONSERVATION DISTRICT Hydrogeology, Water Quality, Water Budgets, and Simulated Responses to Hydrologic Changes in Santa Rosa and San Simeon Creek Ground-Water Basins, San Luis Obispo County, California By Eugene B.Yates and Kathryn M. Van Konyenburg U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 98-4061 Prepared in cooperation with the SAN LUIS OBISPO COUNTY FLOOD CONTROL AND WATER CONSERVATION DISTRICT ON O Sacramento, California 1998 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Thomas J. Casadevall, Acting Director The use of firm, trade, and brand names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey. For additional information write to: Copies of this report can be purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Information Services Placer Hall, Suite 2012 Box 25286 6000 J Street Federal Center Sacramento, CA 95819-6129 Denver, CO 80225 CONTENTS Abstract................................................................................................................................................................................. 1 Introduction ............................................................................................................................................................^ 2 Purpose and Scope....................................................................................................................................................... 2 Description of Study Area .......................................................................................................................................... 4 Acknowledgments ...................................................................................................................................................... 5 Hydrogeology .............................................................................. 6 Regional Geology and Geologic History .................................................................................................................... 6 Ground-Water Basins .................................................................................................................................................. 15 Geologic Units ............................................................................................................................................................ 15 Bedrock ............................................................................................................................................................. 15 Basin Fill........................................................................................................................................................... 17 Water Levels .................................................................................................... 18 Transmissivity and Storage Coefficient ...................................................................................................................... 29 Water Quality ........................................................................................................................................................................ 31 Surface-Water Quality ................................................................................................................................................ 31 Ground-Water Quality ................................................................................................................................................ 35 Identification of Subareas ................................................................................................................................. 35 Salinity .............................................................................................................................................................. 38 Sources of Salinity .........:........................................................................................................................ 38 Seawater Intrusion .................................................................................................................................. 40 Chemistry of Brackish Water .................................................................................................................. 42 Factors Affecting Use ....................................................................................................................................... 43 Seasonal and Long-Term Trends ...................................................................................................................... 44 Water Budgets ............................................................................................................................................^ 45 Rainfall ......................................................................................................................... 47 Streamflow ............................................................................................................................................................^ 48 Flow Entering Ground-Water Basins ................................................................................................................ 48 Gains and Losses within Ground-Water Basins ............................................................................................... 50 Areal Recharge ........................................................................................................................................................... 52 Evaporative Demand ......................................................................................................................................... 52 Land Use ........................................................................................................................................................^ 55 Soil-Moisture Budget ....................................................................................................................................... 56 Subsurface Inflow and Outflow ................................................................................................................................... 61 Transpiration by Phreatophytes .................................................................................................................................. 63 Water Use....................................................................................................................................... 64 Agricultural ....................................................................................................................................................... 64 Municipal and Industrial................................................................................................................................... 65 Overview of Budgets............................................................................................................................................. 67 Digital Simulation of Hydrologic System ............................................................................................................................ 67 Model Design .............................................................................. 68 Model Calibration ....................................................................................................................................................... 71 Model Results ................................................................................................................................................. 73 Water Levels ..................................................................................................................................................... 73 Flows............................................................................................................................................. 73 Sensitivity Analysis..................................................................................................................................................... 74 Limitations of Model .................................................................................................................................................. 77 Simulated Responses to Hydrologic Changes ...................................................................................................................... 78 Effects of Agricultural and Municipal Pumpage ........................................................................................................ 78 Contents III Occurrence and Effects of Drought............................................................................................................................ 82 Single Long Dry Season .................................................................................................................................. 82 Dry Wells................................................................................................................................................