March 2017 Montana Bureau of Mines and Geology Open-File Report 688 HYDROGEOLOGIC INVESTIGATION OF THE BOULDER VALLEY, JEFFERSON COUNTY, MONTANA GROUNDWATER MODELING REPORT Julie A. Butler1 and Andrew L. Bobst Montana Bureau of Mines and Geology Ground Water Investigations Program 1Currently employed by the Massachusetts Department of Environmental Protection, Boston, MA Cover photo by Andrew L. Bobst, 2011 HYDROGEOLOGIC INVESTIGATION OF THE BOULDER VALLEY, JEFFERSON COUNTY, MONTANA GROUNDWATER MODELING REPORT March 2017 Julie A. Butler1 and Andrew L. Bobst Montana Bureau of Mines and Geology Ground Water Investigations Program 1Currently employed by the Massachusetts Department of Environmental Protection, Boston, MA Montana Bureau of Mines and Geology Open-File Report 688 Montana Bureau of Mines and Geology Open-File Report 688 TABLE OF CONTENTS Preface ....................................................................................................................................................1 Abstract ..................................................................................................................................................1 Introduction ............................................................................................................................................3 Background ......................................................................................................................................3 Purpose and Scope ...........................................................................................................................3 Model Objectives .............................................................................................................................3 Previous Investigations ....................................................................................................................3 Physiography....................................................................................................................................6 Climate .............................................................................................................................................6 Vegetation ........................................................................................................................................6 Water-Development Infrastructure...................................................................................................6 Conceptual Model ..................................................................................................................................8 Geologic Framework .......................................................................................................................8 Hydrogeologic Setting ...................................................................................................................11 Groundwater Flow System ............................................................................................................13 Hydrologic Boundaries ..................................................................................................................13 Aquifer Properties ..........................................................................................................................13 Sources and Sinks ..........................................................................................................................13 Groundwater Budget ......................................................................................................................13 Irrigation Recharge (Ir) ............................................................................................................17 Upland Recharge (Ur) ..............................................................................................................18 Canal Leakage (Cl) ..................................................................................................................20 Groundwater Inflow/Outflow (Agi and Agd) ...........................................................................20 Pumping-Well Withdrawals (Pw) ............................................................................................22 Evapotranspiration by Riparian Vegetation (Etr) .....................................................................23 Changes in Storage (Ds) ..........................................................................................................23 Net River Gain (Riv) ................................................................................................................23 Budget Summary .....................................................................................................................24 Groundwater Flow Model Construction ..............................................................................................24 Computer Code ..............................................................................................................................24 Spatial Discretization .....................................................................................................................24 Temporal Discretization .................................................................................................................27 Hydraulic Parameters .....................................................................................................................27 Boundary Conditions .....................................................................................................................27 Active Grid Border ..................................................................................................................27 Internal Sources and Sinks .......................................................................................................28 v Butler and Bobst, 2017 Streams .....................................................................................................................................28 Calibration ............................................................................................................................................33 Selection of Calibration Targets .....................................................................................................33 Observation Grouping and Weighting .....................................................................................35 Steady-State Calibration ................................................................................................................35 Methods ...................................................................................................................................35 Results ......................................................................................................................................37 Transient Calibration ......................................................................................................................42 Methods....................................................................................................................................42 Aquifer Storage Properties .......................................................................................................43 Transient Calibration Targets ...................................................................................................43 Results ......................................................................................................................................43 Sensitivity Analysis ..............................................................................................................................49 Methods..........................................................................................................................................49 Results ............................................................................................................................................51 Predictive Simulations .........................................................................................................................54 Scenario 1 .......................................................................................................................................59 Scenario 2 .......................................................................................................................................59 Scenario 3 .......................................................................................................................................59 Scenario 4 .......................................................................................................................................64 Scenarios Summary ......................................................................................................................64 Summary and Conclusions ..................................................................................................................65 Assumptions and Limitations ........................................................................................................65 Model Predictions ..........................................................................................................................65 Recommendations ..........................................................................................................................65 Acknowledgments ................................................................................................................................70