Numerical Simulation of the Hydrologic Effects of Redistributed and Additional Ground-Water Withdrawal, Island of Molokai, Hawaii

Because of increased demand for water associated with a growing population, projected increases in demand over the next few decades, and rising salinity of the water pumped from some existing wells, the County of Maui Department of Water Supply (DWS) is currently (2006) considering drilling additional wells to replace or supplement existing wells on the Island of Molokai, Hawaii. Redistributed and additional ground-water withdrawals will affect ground-water levels, discharge of ground water to the nearshore environment, and, possibly, salinity of the water pumped from existing wells.

For this study, an existing numerical ground-water-flow model was used to estimate water-level and coastal-discharge changes, relative to 2005 base-case conditions, caused by withdrawals in the area between Kualapuu and Ualapue on Molokai. For most of the scenarios tested, total withdrawals were either equal to or 0.28 million gallons per day greater than those in the 2005 base case. Model results indicate that a redistribution of withdrawals causes a corresponding redistribution of water levels and coastal discharge. Water levels rise and coastal discharge increases near sites of reduced withdrawal, whereas water levels decline and coastal discharge decreases near sites of increased withdrawal. The magnitude and areal extent of hydrologic changes caused by a redistribution of withdrawals increase with larger changes in withdrawal rates. Simulated water-level changes are greatest at withdrawal sites and decrease outward with distance elsewhere. Simulated water-level declines at proposed withdrawal sites generally were less than 0.5 feet. The low-permeability dike complex of East Molokai Volcano impedes the spread of water-level changes to perennial streams in the northeastern part of the island, and discharge to these streams in the dike complex therefore is unaffected by the proposed withdrawals.

Simulated coastal-discharge changes generally are greatest immediately downgradient from sites of withdrawal change. Simulated coastal-discharge reductions generally are less than 30,000 gallons per day (and everywhere less than 75,000 gallons per day) within model elements for scenarios that exclude the Hawaii Department of Hawaiian Home Lands reservation (2.905 million gallons per day). (Model elements cover discrete 1,640-feet by 1,640-feet square areas.) Simulated coastal-discharge reductions generally represent less than 5 percent change relative to 2005 base-case conditions. Simulated discharge to some fishponds and springs increased in response to decreased withdrawal at upgradient sites, and simulated discharge to other fishponds and springs decreased in response to increased withdrawal. Simulated water-level declines associated with the Hawaii Department of Hawaiian Home Lands reservation were as much as 4 feet at three arbitrarily selected withdrawal sites, and simulated reductions in coastal discharge between Umipaa and Kamiloloa along the south coast exceeded 200,000 gallons per day from several model elements.