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Numerical Simulation of Ground-Water Withdrawals in the Southern Lihue Basin, Kauai, Hawaii

U.S. Geological Survey Water-Resources Investigations Report 01-4200

Prepared in cooperation with the County of Kauai Department of Water

 

By Scot K. Izuka and Delwyn S. Oki

The full report is available in pdf.  Links to the pdf.

Abstract

Numerical simulations indicate that groundwater withdrawals from the Hanamaulu and Puhi areas of the southern Lihue Basin will result in a decline in water levels and reductions in base flows of streams near proposed new water-supply wells. Most of the changes will be attained within 10 to 20 years of the start of pumping. Except for areas such as Puhi and Kilohana, the freshwater lens in most inland areas of the southern Lihue Basin is thick and model simulations indicate that changes in water level and the position of the freshwater-saltwater interface in response to pumping will be small relative to the present thickness of the freshwater lens. Effects of the proposed withdrawals on streamflow depend on withdrawal rate and proximity of the wells to streams. Placing pumped wells away from streams with low base flow and toward streams with high base flow can reduce the relative effect on individual streams.

 

Simulation of the 0.42-million-gallon-per-day increase in withdrawal projected for 2000 indicates that the resulting changes in water levels and interface position, relative to conditions prior to the withdrawal increase, will be small, and that stream base flow will be reduced by less than 10 percent. Simulation of the 0.83-million-gallon-per-day withdrawal projected for 2010 indicates further thinning of the freshwater lens in the Puhi area, where the lens already may be thin, as well as baseflow reduction in Nawiliwili Stream. Simulation of an alternative distribution of the 0.83- million-gallon-per-day withdrawal indicates that the effects can be reduced by shifting most of the new withdrawal to the Hanamaulu area where the freshwater lens is thicker and stream base flows are greater.

 

Simulation of the 1.16-million-gallon-per-day increase in withdrawal projected for 2020 indicates that if withdrawal is distributed only among Hanamaulu wells 1, 3, and 4, and Puhi well 5A, further thinning of the already-thin freshwater lens in the Puhi area would occur. Such a distribution would also exceed the maximum draft recommended by the water-systems standards used in Hawaii. Another simulation in which part of the 1.16 million gallons per day was distributed among three additional hypothetical wells in the Hanamaulu area showed that the pumping effects could be shifted from the Puhi area to the Hanamaulu area, where the freshwater lens is thicker, but that base flow in Hanamaulu Stream may decrease by as much as 16 percent.

 

CONTENTS

Abstract

Introduction

Purpose and Scope

Acknowledgments

Setting

Geology

Hydraulic Properties of the Rocks

Ground-Water Occurrence and Movement

Effects Of Ground-Water Withdrawals

Numerical Model Simulation of the Effects of Ground-Water Withdrawals

Original Numerical Mode

Uncertainties in Recharge

Model Limitations

Steady-State Simulations of Proposed Withdrawals

Effects of the 2000 (0.42 Mgal/d) Projection

Effects of the 2010 (0.83 Mgal/d) Projection

Effects of the 2020 (1.16 Mgal/d) Projection

Implications of the Steady-State Simulations

Rate of Development of the Effects of the Proposed Ground-Water Withdrawals

Transient Simulations of the 2000 (0.42 Mgal/d) Projection

Transient Simulations of the 2010 (0.83 Mgal/d, Alternative Distribution) Projection

Transient Simulations of the 2020 (1.16 Mgal/d, Alternative Distribution) Projection

Implications of the Transient Simulations

Summary and Conclusions

References Cited

Appendix


 

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