Regional Background Information and Assessment Methodology Adapted From: “Hydrogeologic Assessment of the Principal Aquifers in the Upper Musselshell River Basin, Montana in Support of Source Water Assessment and Delineation Reports for Public Water Systems” by Lisa O’Connell, Montana Bureau of Mines and Geology and Bill O’Connell, Montana Rural Water Systems, Inc. June 2003. Table of Contents 1. Background....................................................................................................................................... 5 1.1 Purpose........................................................................................................................................ 5 1.2 Geographic Description .............................................................................................................. 6 1.3 Land Use ..................................................................................................................................... 6 2. Geology of Major Aquifers............................................................................................................... 7 2.1 Alluvium ..................................................................................................................................... 7 2.2 Terrace Gravels........................................................................................................................... 7 2.3 Bearpaw Shale ............................................................................................................................ 9 2.4 Judith River................................................................................................................................. 9 2.5 Claggett....................................................................................................................................... 9 2.6 Eagle ........................................................................................................................................... 9 2.7 Colorado Shale.......................................................................................................................... 10 2.8 Kootenai.................................................................................................................................... 10 2.9 Belt............................................................................................................................................ 10 3. Geologic Structures......................................................................................................................... 10 3.1 Woman’s Pocket Anticline ....................................................................................................... 11 3.2 Shawmut Anticline.................................................................................................................... 11 3.3 Wheatland Syncline .................................................................................................................. 11 3.4 Big Snowy Mountains............................................................................................................... 11 3.5 Little Belt Mountains ................................................................................................................ 11 3.6 Crazy Mountains....................................................................................................................... 12 3.7 Castle Mountains ...................................................................................................................... 12 3.8 Big Elk Dome ........................................................................................................................... 12 3.9 Lake Basin Fault Zone.............................................................................................................. 12 3.10 Little Elk Dome....................................................................................................................... 12 3.11 Haymaker Anticline/Dome ..................................................................................................... 13 3.12 Daisy Dean Anticline/Dome................................................................................................... 13 3.13 Gordon Butte........................................................................................................................... 13 4. Methods........................................................................................................................................... 13 4.1 Well Inventories........................................................................................................................ 13 4.2 Picking Geologic Formations For Wells................................................................................... 14 4.3 Ground Water Flow Direction and Hydraulic Gradient ........................................................... 14 4.4 Delineating Inventory Regions ................................................................................................. 14 4.5 Surface Water Delineations ...................................................................................................... 16 4.6 Inventory Method...................................................................................................................... 16 4.7 Control Zone Delineation ......................................................................................................... 18 5. Principal Aquifers and Ground Water Flow For Water Sources Used For Public Water Supplies............................................................................................................................................... 18 5.1 Alluvium ................................................................................................................................... 18 5.2 Judith River Formation ............................................................................................................. 19 5.3 Claggett Shale ........................................................................................................................... 19 5.4 Eagle Formation........................................................................................................................ 19 5.6 Belt............................................................................................................................................ 20 5.7 Other ......................................................................................................................................... 20 6. Surface Water................................................................................................................................. 21 6.1 USGS Gaging Stations.............................................................................................................. 21 6.1.1 USGS Harlowton ............................................................................................................... 21 6.1.2 USGS near Shawmut ......................................................................................................... 21 6.1.3 USGS near Lavina ............................................................................................................. 21 6.2 Surface Water Chemistry.......................................................................................................... 21 6.2.1 USGS Harlowton ............................................................................................................... 21 6.2.2 USGS Near Shawmut ........................................................................................................ 22 6.2.3 USGS Near Lavina ............................................................................................................ 22 7. Other Water Resources in the Watershed ...................................................................................... 22 8. Tritium Results................................................................................................................................ 23 9. Conclusions..................................................................................................................................... 24 References........................................................................................................................................... 25 Glossary .............................................................................................................................................. 26 List of Appendices .............................................................................................................................. 28 1. Background The Upper Musselshell is an area with very limited geological and hydrogeological information and water quality data. The impact of land use on ground water and surface water and the level of connection between them also needed to be investigated. Due to these unknowns, the Upper Musselshell watershed was chosen as the focus of this project. Fifteen public water supplies were identified in the watershed (See Appendix I.PWS Location Map). The 15 public water supplies initially identified in the Upper Musselshell watershed were sampled and inventoried. In 1996, Congress amended the Safe Drinking Water Act requiring the delineation of the recharge area and assessing potential contaminant sources for all public water sources (PWS). Since there are over 2000 public water sources in Montana, the delineated recharge areas