Montana Bureau of Mines and Geology Open-File Report 638 2013 HYDROGEOLOGIC INVESTIGATION OF THE LOWER BEAVERHEAD STUDY AREA BEAVERHEAD COUNTY, MONTANA GROUNDWATER MODELING REPORT Julie A. Butler and Ginette Abdo Montana Bureau of Mines and Geology Ground Water Investigations Program HYDROGEOLOGIC INVESTIGATION OF THE LOWER BEAVERHEAD STUDY AREA BEAVERHEAD COUNTY, MONTANA GROUNDWATER MODELING REPORT Julie A. Butler and Ginette Abdo Montana Bureau of Mines and Geology Ground Water Investigations Program Montana Bureau of Mines and Geology Open-File Report 638 2013 Montana Bureau of Mines and Geology Open-File Report 638 TABLE OF CONTENTS Introduction ......................................................................................................................................................................... 1 Report Purpose ............................................................................................................................................................ 1 General Setting ............................................................................................................................................................. 1 Climate ............................................................................................................................................................................. 1 Physiography ................................................................................................................................................................. 3 Man-Made Hydrologic Features ............................................................................................................................ 4 Model Objectives ......................................................................................................................................................... 4 Conceptual Model ............................................................................................................................................................... 4 Geologic Framework .................................................................................................................................................. 5 Hydrogeologic Units ................................................................................................................................................... 7 Alluvial Aquifer ..................................................................................................................................................... 8 Tertiary Sediment Aquifer................................................................................................................................ 10 Volcanic Rock Aquifer ........................................................................................................................................ 12 Groundwater Flow System ...................................................................................................................................... 13 Vertical Gradients ................................................................................................................................................ 16 Hydrologic Boundaries ............................................................................................................................................. 18 Aquifer Properties ...................................................................................................................................................... 18 Tertiary Sediment Aquifer Test ...................................................................................................................... 18 Volcanic Rock Aquifer Test ............................................................................................................................... 19 Alluvial Aquifer Tests ......................................................................................................................................... 21 Summary of Hydraulic Properties ................................................................................................................ 21 Sources and Sinks ........................................................................................................................................................ 21 Groundwater Budget ................................................................................................................................................. 21 Computer Code .................................................................................................................................................................... 24 Groundwater Flow Model Construction ................................................................................................................... 25 Model Grid ...................................................................................................................................................................... 25 Hydraulic Parameters ................................................................................................................................................ 27 Boundary Conditions ................................................................................................................................................. 27 Model Borders ....................................................................................................................................................... 27 Surface-Water Bodies ......................................................................................................................................... 27 Irrigation Field Recharge .................................................................................................................................. 28 Well Withdrawals ................................................................................................................................................. 28 Calibration ............................................................................................................................................................................. 29 Selection of Calibration Targets ............................................................................................................................ 29 Steady-State Calibration ........................................................................................................................................... 30 Transient Calibration ................................................................................................................................................. 33 Sensitivity Analysis ..................................................................................................................................................... 36 Model Veriication ....................................................................................................................................................... 36 Predictive Simulations ...................................................................................................................................................... 37 Scenario 1 ....................................................................................................................................................................... 38 Scenario 2 ....................................................................................................................................................................... 38 Scenario 3 ....................................................................................................................................................................... 39 Scenario 4 ....................................................................................................................................................................... 41 Canal Seepage Scenarios .......................................................................................................................................... 41 Scenarios 5 and 6 ................................................................................................................................................. 41 Scenario 7 ................................................................................................................................................................ 47 Summary and Conclusions ............................................................................................................................................. 48 v Butler and Abdo, 2013 Assumptions and Limitations ...........................................................................................................................48 Model Predictions ..................................................................................................................................................50 Recommendations .................................................................................................................................................52 References ........................................................................................................................................................................ 52 Appendix A. Model File Index ................................................................................................................................... 55 Appendix B. Estimating Irrigation Recharge in the Groundwater Budget ............................................61