GROUND-WATER AVAILABILITY FROM THE HAWI AQUIFER IN THE KOHALA AREA, HAWAII By Mark R. Underwood William Meyer, and William R. Souza U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 95-4113 Prepared in cooperation with the DEPARTMENT OF WATER SUPPLY COUNTY OF HAWAII Honolulu, Hawaii 1995 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Director Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government For sale by the U.S. Geological Survey Earth Science Information Center Open-File Reports Section Box 25286, MS 517 Denver Federal Center Denver, CO 80225 For additional information write to: District Chief U.S. Geological Survey 677 Ala Moana Blvd., Suite 415 Honolulu, HI 96813 CONTENTS Abstract ............................................................... 1 Introduction ............................................................ 2 Scope............................................................ 2 Previous investigations.............................................. 6 Description of study area .................................................. 8 Geology............................................................... 10 Ground-water occurrence ................................................. 13 Ground-water development ......................................... 17 Ground-water quality .............................................. 19 Vertical extent of freshwater......................................... 23 Hydraulic conductivity of the basal aquifer ................................... 26 Numerical simulation of the basal aquifer .................................... 32 Description of the model. ........................................... 32 Calibration of the steady-state model.................................. 36 Response of the basal aquifer to ground-water pumpage................... 44 Summary and conclusions ................................................ 55 References cited ........................................................ 56 in FIGURES 1. Map showing Hawaiian islands and aquifer systems in the Kohala area, island of Hawaii............................................... 3 2. Generalized section of Kohala Mountain showing basal aquifer and direction of ground-water movement. .............................. 4 3-7. Maps showing: 3. Mean annual rainfall, Kohala area, island of Hawaii. ..................... 5 4. Streams, ditches, and tunnels in the Kohala area, island of Hawaii .......... 7 5. Surficial geology of the Kohala area, island of Hawaii................... 11 6. Ground-water recharge, Kohala area, island of Hawaii. .................. 14 7. Observed ground-water levels in the Hawi basal aquifer and test-well sites, Kohala area, island of Hawaii. .............................. 15 8. Graph showing water levels measured in well 7347-02 and shaft 7652-01, Kohala area, island of Hawaii ................................... 16 9. Map showing locations of existing and historic wells, existing tunnels, and test holes in the Kohala area, island of Hawaii ...................... 18 10. Graph showing chloride concentrations and depth for wells D and I, Kohala area, island of Hawaii ................................... 24 11-12. Maps showing: 11. Aquifer-test values for horizontal hydraulic conductivity, Kohala area, island of Hawaii.............................................. 27 12. Model grid and boundaries used in the numerical simulation, Kohala area, island of Hawaii ................................... 33 13-14. Graphs showing: 13. Model-calculated water levels at test wells E and H, and values of hydraulic connection, K'/m', between the basal aquifer and the ocean, Kohala area, island of Hawaii ................................... 37 14. Model-calculated water levels and observed water levels for various distributions of hydraulic conductivity (K) and the hydraulic connection between aquifer and ocean (K'/m'), Kohala area, island of Hawaii....... 39 IV FIGURES-Continued 15-24. Maps showing: 15. Distribution of horizontal hydraulic conductivity and hydraulic connection between the aquifer and ocean as modeled, Kohala area, island of Hawaii ................................... 40 16. Model-calculated and observed ground-water levels, Kohala area, island of Hawaii.............................................. 42 17. Model-calculated depth of the freshwater-saltwater interface, Kohala area, island of Hawaii ................................... 43 18. Pumping rates used for the two simulated pumping scenarios, Kohala area, island of Hawaii ................................... 45 19. Model-calculated drawdown for pumping scenario 1, Kohala area, island of Hawaii.............................................. 46 20. Model-calculated drawdown for pumping scenario 2, Kohala area, island of Hawaii.............................................. 47 21. Depth of model-calculated fresh water-salt water interface for pumping scenario 1, Kohala area, island of Hawaii.......................... 49 22. Depth of model-calculated freshwater-saltwater interface for pumping scenario 2, Kohala area, island of Hawaii.......................... 50 23. Model-calculated ground-water levels for pumping scenario 1, Kohala area, island of Hawaii ................................... 53 24. Model-calculated ground-water levels for pumping scenario 2, Kohala area, island of Hawaii ................................... 54 TABLES 1. Concentrations of common ions and other water-quality characteristics of water samples from the Hawi basal aquifer, Kohala area, island of Hawaii .......... 20 2. Concentrations of trace metals and organic compounds in water samples from the Hawi basal aquifer, Kohala area, island of Hawaii. .......................... 21 3. Summary of characteristics of wells used for exploration and aquifer testing of the Hawi basal aquifer, Kohala area, island of Hawaii. ........................ 28 4. Summary of aquifer-test data and results, Kohala area, island of Hawaii ................ 31 5. Steady-state ground-water budget (existing pumpage) for the numerical model, Kohala area, island of Hawaii. ....................................... 41 6. Water levels and interface locations at simulated pumping sites, Kohala area, island of Hawaii ........................................................ 51 7. Steady-state ground-water budget (existing plus simulated pumpage) for the numerical model, Kohala area, island of Hawaii ............................... 51 Conversion Factors Multiply By To obtain acre 4,047 square meter foot (ft) 0.3048 meter foot per mile (ft/mi) 0.1894 meter per kilometer foot per day (ft/d) 0.3048 meter per day foot per day per foot (ft/d)/ft 1 meter per day per meter cubic foot per second (ft3/s) 0.02832 cubic meter per second gallon per minute (gal/min) 0.06308 liter per second million gallons per day (Mgal/d) 0.04381 cubic meter per second mile (mi) 1.609 kilometer ij square mile (mi ) 2.590 square kilometer inch (in.) 25.4 millimeter inch per year (in/yr) 25.4 millimeter per year Temperature is given in degrees Celsius (°Q, which can be converted to degrees Fahrenheit (°F) by using the equation: °F = (1.8 x°C) +32 Abbreviations used in water quality descriptions: |LLg/L, micrograms per liter ppm, parts per million o mg/L, milligrams per liter gm/cm , grams per cubic centimeter |LiS/cm, microsiemens per centimeter at 25° Celsius VI GROUND-WATER AVAILABILITY FROM THE HAWI AQUIFER IN THE KOHALA AREA, HAWAII by Mark R. Underwood, William Meyer, and William R. Souza ABSTRACT A ground-water study consisting of test-well drilling, aquifer tests, and numerical simulation was done to investigate ground-water availability in the basal part of the Hawi aquifer between the western drainage divide of Pololu Valley and Upolu Point in Kohala, Hawaii. The test well drilling provided information on geology, water levels, water quality, vertical extent of the freshwater, and the thickness of the freshwater-saltwater transition zone in that aquifer. A total of 12 test wells were drilled at eight locations. Aquifer tests were done at five locations to estimate the hydraulic conductivity of the aquifer. Using information on the distribution of recharge, vertical extent of freshwater, hydraulic conductivity, and geometry of the basal aquifer, a numerical model was used to simulate the movement of water into, through, and out of the basal aquifer, and the effect of additional pumping on the water levels in the aquifer. Results of the modeling indicate that ground-water withdrawal of 20 million gallons per day above the existing withdrawal of 0.6 million gallons per day from the basal aquifer is hydrologically feasible, but that spacing, depth, and pumping rates of individual wells are important. If pumping is concentrated, the likelihood of saltwater intrusion is increased. The additional withdrawal of 20 million gallons per day would result in a reduction of ground-water discharge to the ocean by an amount equal to pumpage. Although model-calculated declines in water-level outside the area of pumping are small, pumping could cause some reduction of streamflow near the mouth of Pololu Stream. INTRODUCTION The Kohala area of the island of Hawaii is the northwest peninsula of the island (fig. 1). The area is dominated by the asymmetrical, elongated Kohala Mountain. Kohala Mountain is an extinct volcanic dome reaching an altitude of 5,605 ft and is the oldest of the five volcanoes forming the island.