U.S. Department of the Interior COPY Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 99-4291 i; ' 1 , -Water table . ' " N - Prepared in cooperation with the DEPARTMENT OF HAWAIIAN HOME LANDS USGS science for a changing world Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii EyDelwynS. Oki U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 99-4291 Prepared in cooperation with the DEPARTMENT OF HAWAIIAN HOME LANDS Honolulu, Hawaii 2000 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Charles G. Groat, 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 Branch of Information Services 677 Ala Moana Blvd., Suite 415 Box 25286 Honolulu, HI 96813 Denver, CO 80225-0286 CONTENTS Abstract.......................................................................................... 1 Introduction....................................................................................... 1 Purpose and Scope ............................................................................ 4 Well-Numbering System........................................................................ 4 Description of Study Site ....................................................................... 9 Physical Setting......................................................................... 9 Land Use.............................................................................. 9 Climate ............................................................................... 9 Geology.......................................................................................... 9 West Molokai Volcano ......................................................................... 14 East Molokai Volcano.......................................................................... 16 Coastal Deposits.............................................................................. 16 Kualapuu Area ............................................................................... 16 Hydraulic Conductivity of the Rocks ................................................................... 16 Lava Flows .................................................................................. 17 Dikes....................................................................................... 17 Weathering .................................................................................. 17 Coastal Deposits.............................................................................. 17 Regional Ground-Water Flow System................................................................... 17 Ground-Water Recharge ........................................................................ 18 Ground-Water Discharge to Streams .............................................................. 18 Ground-Water Levels .......................................................................... 20 Freshwater Lens .............................................................................. 20 Dike-Impounded Ground Water.................................................................. 23 Ground-Water Flow System in the Kualapuu Area......................................................... 23 Ground-Water Levels .......................................................................... 23 Depth to Saline Water.......................................................................... 24 Chloride Concentration......................................................................... 24 Vertical Profiles of Chloride Concentration in Well 0801-01............................................ 24 Ground-Water Withdrawals........................................................................... 28 Numerical Simulation to Determine Areas of Possible Future Ground-Water Development......................... 31 Model Description ............................................................................ 31 Model Mesh ........................................................................... 31 Boundary Conditions .................................................................... 31 Hydraulic-Conductivity Zones............................................................. 32 Recharge.............................................................................. 32 Model Results................................................................................ 32 Withdrawal at the March 1999 Allocation Rate................................................ 36 Withdrawal of 0.5 Mgal/d from a Single Site.................................................. 36 Withdrawal of 1.0 Mgal/d from a Single Site.................................................. 36 Model Limitations ............................................................................ 40 Contents iii Additional Criteria for Deep Monitor Well Site Selection ................................................... 44 Ground-Water Level........................................................................... 44 Ground-Surface Altitude ....................................................................... 44 Land Classification, Ownership, and Accessibility ................................................... 44 Geology .................................................................................... 44 Locations of Existing Production Wells............................................................ 45 Historical Ground-Water Quality Information....................................................... 45 Site Selection for a Deep Monitor Well ................................................................. 45 Summary and Conclusions ........................................................................... 47 References Cited................................................................................... 48 FIGURES 1-2. Maps showing: 1. Geographic features, island of Molokai, Hawaii, and location of Kualapuu study area .............. 2 2. Hawaii Commission on Water Resource Management aquifer-system boundaries, Molokai, Hawaii ... 3 3. Schematic cross section of the regional ground-water flow system in the Kualapuu area, Molokai, Hawaii.. 5 4-7. Maps showing: 4. Locations of selected wells, Molokai, Hawaii .............................................. 8 5. State land-use districts for 1995, Molokai, Hawaii .......................................... 10 6. Land ownership in the Kualapuu area, Molokai, Hawaii ...................................... 11 7. Mean annual rainfall, Molokai, Hawaii ................................................... 12 8. Chart showing annual rainfall at Kualapuu and Kaunakakai rain gages, Molokai, Hawaii ............... 13 9-11. Maps showing: 9. Generalized surficial geology of Molokai, Hawaii ........................................... 15 10. Surface-water gaging stations, Molokai, Hawaii ............................................ 19 11. Arithmetic mean of measured water levels for the period 1938-97, water-table altitude from resistivity measurements, and depth to saline water body determined from a time-domain electromagnetic survey, Molokai, Hawaii .............................................:.. 21 12-15. Charts showing: 12. Chloride concentration of water samples from different depths in Department of Hawaiian Home Lands well 1 (0801-01), and diagram of well-casing depth and type for wells 0801-01 to -03, Molokai, Hawaii ................................................................... 25 13. Estimated pumping rates at Department of Hawaiian Home Lands well 2 (0801-02) and Department of Water Supply well 0801-03 during May 1995, Molokai, Hawaii............................ 26 14. Chloride concentration of water samples from Department of Hawaiian Home Lands well 2 (0801-02) during May 1995, Molokai, Hawaii .................................................... 27 15. Monthly or annual mean pumpage from selected wells, Molokai, Hawaii ........................ 29 16-25. Maps showing: 16. Ground-water discharge zones used in the ground-water flow model, Molokai, Hawaii ............. 33 17. Horizontal hydraulic-conductivity zones used in the ground-water flow model, Molokai, Hawaii...... 34 18. Average annual recharge used in the ground-water flow model and nodes with simulated 1999-allocated ground-water withdrawal, Molokai, Hawaii ................................. 35 iv Site Selection for a Deep Monitor Well, Kualapuu, Molokai, Hawaii 19. Model-calculated steady-state water levels at well 0901-01 (model node 3094) caused by withdrawing 0.5 million gallons per day, above the March 1999 total allocated withdrawal rate, from the Kualapuu area (outside of the modeled dike complex), Molokai, Hawaii ................ 37 20. Model-calculated steady-state water levels at wells 0801-01 to -03 (model node 3142) caused by withdrawing 0.5 million gallons per day, above the March 1999 total allocated withdrawal rate, from the Kualapuu area (outside of the modeled dike complex), Molokai, Hawaii ................ 38 21. Model-calculated zone where withdrawing 0.5 million gallons per day from a single site does not cause the steady-state water level at the withdrawal site to drop below 10 feet above sea level, and model-calculated zone where withdrawing 0.5 million gallons per day from a single