n U.S. Department of the Interior Ground Water in the Southern Lihue Basin, Kauai, Hawaii U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 98-4031 ;\iiuiiaiict Volcanb t crater. * Prepared in cooperation with the COUNTY OF KAUAI DEPARTMENT OF WATER uses science for a changing world Ground Water in the Southern Lihue Basin, Kauai, Hawaii By Scot K. Izuka and Stephen B. Gingerich U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 98-4031 Prepared in cooperation with the COUNTY OF KAUAI DEPARTMENT OF WATER Honolulu, Hawaii 1998 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Thomas J. Casadevall, Acting 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 Abstract........................................................................................ 1 Introduction..................................................................................... 1 Purpose and Scope .......................................................................... 3 Acknowledgments .......................................................................... 3 Physiographic Setting ............................................................................. 3 Geology........................................................................................ 5 Hydraulic Properties of the Rocks.................................................................... 8 Movement of Ground Water ........................................................................ 10 Ground-Water Recharge...................................................................... 12 Ground-Water Discharge to Streams ............................................................ 12 Ground-Water Withdrawal .................................................................... 20 Ground-Water Discharge to the Ocean........................................................... 21 Ground-Water Levels ........................................................................ 23 Conceptual Model of Ground-Water Occurrence and Movement. ........................................... 30 Numerical Model of Ground-Water Movement ......................................................... 32 Model Structure and Input Data................................................................ 33 Hydraulic Conductivities, Water Levels, Ground-Water Discharge, and Interface Depth of the Best-Fit Numerical Model. ................................................................ 40 Sensitivity of the Model to Parameter Variability .................................................. 45 Implications of the Numerical Model for Ground-Water Occurrence ................................... 49 Conclusions..................................................................................... 51 References Cited ................................................................................. 51 Appendix....................................................................................... 54 Figures 1-3. Maps showing: 1. Location of the southern Lihue Basin, Kauai, Hawaii......................................... 2 2. Rainfall distribution in the southern Lihue Basin, Kauai, Hawaii................................ 4 3. Geology of the Lihue Basin, Kauai, Hawaii ................................................ 6 4. Block diagram showing the structure and stratigraphy of the southern Lihue Basin, Kauai ............... 7 5. Map showing locations of wells and well fields in southeastern Kauai, Hawaii ........................ 9 6. Generalized section through the southern Lihue Basin, Kauai, Hawaii showing the relation between freshwater and saltwater and the direction of movement within the freshwater body under pristine conditions and under pumping conditions ................................................... 11 7-8. Maps showing: 7. Recharge in the southern Lihue Basin, Kauai, Hawaii ........................................ 13 8. Locations of continuous stream-gaging stations in southeastern Kauai, Hawaii..................... 14 9-10. Graphs of: 9. Rainfall and hydrographs for selected continuous stream-gaging stations in southeastern Kauai, Hawaii. 16 Contents iii 10. Total discharge and base flow at continuous stream-gaging stations on streams in southeastern Kauai, Hawaii..................................................................... 17 11. Map showing ground-water discharge measured by instantaneous discharge measurements on selected streams in southeastern Kauai, Hawaii ..................................................... 19 12-13 Graphs showing: 12. Water levels compared with depth of drilling for selected wells in the A, Haiku and Puhi; and B, Kilohana and Lihue Town well fields, Kauai, Hawaii...................................... 24 13. Correlation of abrupt water-level declines with drillers' logs from selected wells in the southern Lihue Basin, Kauai, Hawaii .......................................................... 26 14. Diagram showing correlation of abrupt water-level declines in the southern Lihue Basin, Kauai, Hawaii ... 27 15. Generalized water-table map and profile for data from 1910 through 1996 for the southern Lihue Basin, Kauai, Hawaii................................................................... 29 16-21. Diagrams showing: 16. Ground-water flow in the southern Lihue Basin, Kauai, Hawaii ................................ 31 17. Discretization grid, boundaries, streams, pumping wells, and monitoring points used in the numerical ground-water model of the southern Lihue Basin, Kauai, Hawaii .................... 34 18. Distribution of geology used in the numerical ground-water model of the southern Lihue Basin, Kauai, Hawaii..................................................................... 36 19. Distribution of recharge in the numerical ground-water model of the southern Lihue Basin, Kauai, Hawaii. .................................................................... 38 20. Distribution of simulated head in the upper layer and lower layer of the numerical ground- water model of the southern Lihue Basin, Kauai, Hawaii................................... 42 21. Simulated elevation of freshwater/saltwater interface in the upper layer and lower layer of the numerical ground-water model of the southern Lihue Basin, Kauai, Hawaii. ................... 46 22-23. Graphs showing: 22. Variation of simulated head with streambed and ocean-floor leakance in the numerical ground- water model of the southern Lihue Basin, Kauai, Hawaii ................................... 48 23. Comparison of observed water levels compared with simulated water levels for selected values of horizontal hydraulic conductivities (Kh) in the Koloa Volcanics, southern Lihue Basin, Kauai, Hawaii..................................................................... 50 Tables 1. Summary of base-flow analyses for selected streams in southeastern Kauai, Hawaii ........................ 15 2. Reported withdrawals from wells in southeastern Kauai, Hawaii ....................................... 22 3. Well water-level information used to construct the generalized water-table map, southern Lihue Basin, Kauai, Hawaii............................................................................. 28 4. Summary of selected characteristics and input data of the numerical model of the southern Lihue Basin, Kauai, Hawaii............................................................................. 33 5. Withdrawal rates in simulated pumping wells used in the numerical ground-water model of the southern Lihue Basin, Kauai, Hawaii .................................................................. 40 6. Hydraulic conductivities used in the best-fit numerical ground-water model of the southern Lihue Basin, Kauai, Hawaii............, ................................................................ 41 7. Observed and simulated water levels from the upper layer of the numerical ground-water model of the southern Lihue Basin, Kauai, Hawaii........................................................... 44 8. Simulated ground-water discharge to streams, rivers, and the ocean in the best-fit numerical ground-water model of the southern Lihue Basin, Kauai Hawaii................................................. 44 iv Ground Water in the Southern Lihue Basin, Kauai, Hawaii Ground Water in the Southern Lihue Basin, Kauai, Hawaii By Scot K. Izuka and Stephen B. Gingerich Abstract percent of the ground water discharging from the aquifer in the southern Lihue Basin seeps to A multi-phased study of ground-water streams; the remainder seeps directly to the ocean resources, including well drilling, aquifer tests, or is withdrawn by wells. analysis of ground-water discharge, and numerical ground-water modeling, indicates that the rocks of INTRODUCTION the southern Lihue Basin, Kauai, have permeabili­ ties that are much lower than in most other areas of Kauai is the fourth-largest island (553 mi2) in the ground-water development in the Hawaiian tropical, North Pacific Hawaiian Archipelago (fig. 1). islands. The regional hydraulic conductivity of the Kauai's population was about 51,000 in 1990; about 20 Koloa Volcanics, which dominates fresh ground- percent of the population