science for a changing world ofc In cooperation with the RAVALLI COUNTY BOARD OF COMMISSIONERS and the BITTERROOT CONSERVATION DISTRICT Hydrogeology and Aquifer Sensitivity of the Bitterroot Valley, Ravalli County, Montana Water-Resources Investigations Report 99-4219 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior U.S. Geological Survey Hydrogeology and Aquifer Sensitivity of the Bitterroot Valley, Ravalli County, Montana By David W. Briar and DeAnn M. Dutton Water-Resources Investigations Report 99-4219 In cooperation with the RAVALLI COUNTY BOARD OF COMMISSIONERS and the BITTERROOT CONSERVATION DISTRICT U.S. Department of the Interior BRUCE BABBITT, Secretary U.S. Geological Survey Charles G. Groat, Director Any use of trade, product, or firm name in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government Helena, Montana February 2000 For additional information write to: District Chief U.S. Geological Survey 3162 Bozeman Avenue Helena, MT 59601-6456 Copies of this report may be purchased from: U.S. Geological Survey Branch of Information Services Box 25286 Denver, CO 80225-0286 CONTENTS Page Abstract.................................................................................................................................................................................. 1 Introduction......................................................................................^^ 2 Purpose and scope...................................................................................................................................................... 2 Previous investigations.............................................................................................................................................. 4 Location-numbering system....................................................................................................................................... 4 Acknowledgments............................................................................................_^ 4 Description of study area....................................................................................................................................................... 5 Physiography and drainage........................................................................................................................................ 5 Climate....................................................................................................................................................................... 6 Delineation of focus areas.......................................................................................................................................... 6 Hydrogeology........................................................................................................................................................................ 10 General geology......................................................................................................................................................^ 10 Hydraulic characteristics of basin-fill deposits.......................................................................................................... 12 Ground-water occurrence and movement.................................................................................................................. 14 Ground-water quality................................................................................................................................................. 21 Aquifer sensitivity................................................................................................................................................................^ 33 Summary and conclusions..................................................................................................................................................... 35 Selected refeiraces......................................................................................._ 37 Supplemental data.................................................................................................................................................................. 40 ILLUSTRATIONS Figure 1. Map showing location of study area within the Bitterroot River drainage basin, Ravalli County, Montana... 3 2. Diagram showing location-numbering system.................................................................................................. 5 3. Map showing average annual precipitation in Ravalli County......................................................................... 7 4. Graph showing monthly precipitation for the study period and the mean monthly precipitation for the 1961-90 period of record at Hamilton, Montana......................................................................................... 8 5. Map showing locations of focus areas, inventoried wells, and surface-water sites, and corresponding availability of water-quality analyses .......................................................................................................... 9 6. Map showing generalized surficial geology in the study area.......................................................................... 11 7. Boxplots showing range of static-water levels, yields, and specific capacities of wells based on geologic unit and location............................................................................................................................ 13 8-10. Maps showing altitude of the water-level surface and location of monitoring wells in the: 8. Eightmile area, 1995 ................................................................................................................................ 16 9. Hamilton West area, 1995........................................................................................................................ 17 10. Hamilton Heights area, 1995 ................................................................................................................... 18 11. Map showing location of sediments, focus areas, and drainage areas upgradient from sediment and focus areas............................................................................................................................................. 20 12. Boxplots showing specific conductance in well water grouped by location and geologic unit........................ 22 13. Boxplots showing nitrate concentration in well water grouped by location and geologic unit........................ 24 14. Graph showing nitrate concentration in well water versus depth to top of well casing open interval.............. 25 15-17. Maps showing distribution of nitrate concentration in well water from the: 15. Eightmile area, 1994-96........................................................................................................................... 27 16. Hamilton West area, 1994-96................................................................................................................... 28 17. Hamilton Heights area, 1994-96.............................................................................................................. 29 18. Graphs showing monthly variation in nitrate concentration in well water....................................................... 30 19. Modified trilinear diagram showing percentages of major ions in well water from the three focus areas....... 31 20. Graph showing population of Ravalli County by year, 1970-97 ...................................................................... 34 21. Hydrographs of water levels in wells of the monitoring network..................................................................... 69 CONTENTS iii TABLES Page Table 1. Total annual precipitation in selected areas....................................................................................................... 19 2. Location and description of surface-water sites and one spring........................................................................ 32 3. Summary of specific conductance, nitrate concentration, well density, average precipitation, and percentage of wells completed in Tertiary alluvial-fan deposits in the three focus areas............................... 35 4. Records of selected wells................................................................................................................................... 41 5. Records of water levels in wells ........................................................................................................................ 54 6. Physical properties and major-ion concentrations for water from wells............................................................ 84 7. Trace-element and radon concentrations for water from wells.......................................................................... 106 8. Physical properties and major-ion concentrations for water from surface-water sites and one spring.............. 110 9. Trace-element concentrations for water from surface-water sites..................................................................... Ill 10. Physical properties and major-ion concentrations for deionized-water field blanks......................................... 112 11. Trace-element concentrations for deionized-water field blanks........................................................................ 113 12. Drinking-water regulations and guidelines for public water supply.................................................................