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Groundwater Recharge and Availability in Eastern Molokai

Project Chief: Delwyn S. Oki
Project Period: October 2008 through December 2015
Cooperator: Maui County Department of Water Supply, Office of Hawaiian Affairs, and Hawaii Department of Hawaiian Home Lands
Location: Island of Molokai

Problem

The Maui County Department of Water Supply (DWS), State Department of Hawaiian Home Lands (DHHL), Hawaii Department of Agriculture (DOA), and private land owners mostly develop groundwater resources on the Island of Molokai from a few selected areas. Groundwater pumped from some existing production wells indicate rising salinity, perhaps as a result of the declining water levels and rising brackish-water transition zone caused by the cumulative effect of withdrawals. Any new groundwater development must be approved by the State Commission on Water Resource Management (CWRM), which designated the Island of Molokai as a Ground Water Management Area in 1992. With this designation, the State was authorized to protect the groundwater resources of Molokai by managing groundwater withdrawals from the aquifer through a permitting process.

A numerical groundwater-flow model of Molokai that was developed by the USGS about 10 years ago continues to be used to simulate changes in water levels and coastal discharge that would occur under different withdrawal scenarios of interest to water purveyors. The existing model, however, is not capable of simulating the effects of withdrawals on salinity in the aquifer. Water purveyors and various stakeholders are now interested in considering several other alternative groundwater-withdrawal scenarios and the effects of withdrawals on salinity and long-term sustainability of the resource.

Objectives

The main objective of this study is to estimate the hydrologic effects of additional groundwater-withdrawal scenarios on (1) salinity and water levels near existing wells and (2) coastal discharge.

Relevance and Benefits

The results from this study are necessary for the proper management of the groundwater in the State. The study is consistent with the USGS Science Strategy to provide citizens, communities, natural-resource managers, and policymakers with a clearer knowledge of the status of their water resources. The numerical model developed as part of this study will represent the best available tool that can be used to (1) effectively manage groundwater withdrawals from central and eastern Molokai and (2) develop long-range plans for future development of resources.

Approach

To meet the objective of this study, the USGS will (1) provide updated estimates of groundwater recharge from infiltration of rainfall and irrigation water for the Island of Molokai, (2) conduct two synoptic water-level surveys, (3) quantify the hydraulic properties of the aquifer, (4) incorporate information from items 1-3 to develop a refined numerical groundwater model capable of simulating three-dimensional flow and transport in the study area, and (5) simulate up to three sets of proposed withdrawal scenarios (each set to consist of no more than 6 individual pumping distributions), to be determined in discussion with the various water purveyors and stakeholders. Results from this study will be published in the USGS Scientific Investigations Report series and made available on the internet.

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Page Last Modified: Wednesday, 18-Jan-2017 18:52:08 EST