The aquifer in the Pearl Harbor area of southern Oahu is the most heavily used aquifer in the State of Hawaii, producing more than 200 million gallons per day during the 1970s when sugarcane was actively cultivated, and more recently, about 100 million gallons per day during 2000. The aquifer has been divided by the State into three hydrologically connected management systems: the Ewa-Kunia system in the west, the Waipahu-Waiawa system in the middle, and the Waimalu system in the east. During some periods, reported withdrawals from the Waimalu management system have exceeded the State's sustainable-yield estimate for this system of 45 million gallons per day. This has led to concern regarding potential saltwater-intrusion effects associated with long-term withdrawals from the Waimalu management system.
In the Waimalu management system, water levels and salinity may be influenced by valley-fill barriers associated with existing stream valleys. Valley-fill barriers are formed by low-permeability valley-fill deposits and weathered volcanic rocks beneath the valley-fill deposits, and impede lateral ground-water movement. The State's sustainable-yield estimates for the Pearl Harbor area do not account for the hydrologic effects of these barriers.
The objectives of this study are to (1) obtain a better understanding of the hydrologic effects of valley-fill barriers in the Pearl Harbor area, Oahu, Hawaii, (2) determine the possible effects of valley-fill barriers on water levels and salinity in the Pearl Harbor area using a three-dimensional, density-dependent ground-water flow model, and (3) estimate the effects of redistributing existing withdrawals on the freshwater resource.
A numerical finite-element ground-water model capable of simulating variable-density flow was developed to meet the study objectives. The model mainly used published estimates for the permeability, storage, and dispersivity values, and simulated water levels and salinity profiles that generally were in agreement with measured water levels and salinity profiles from representative wells in the modeled area. Model sensitivity analyses of valley-fill-barriers indicated that simulated water levels and salinity can be affected by the depth and length of the hypothesized valley-fill barriers.
The model constructed for this study was used to simulate the hydrologic effects of redistributing withdrawals in the Pearl Harbor area by reducing withdrawals in the eastern part of the Waimalu area and increasing withdrawals farther to the west by an equal amount. Simulation results from selected scenarios of redistributed withdrawal indicate that: (1) redistributing withdrawal from Halawa Shaft (2354-01) or the Kalauao Wells (2355-09 to -14), near the eastern part of the Waimalu management system, to Pearl City III (2557-03), near the western part of the Waimalu management system, results in a thickening of the freshwater zone east of Waimalu Stream and a thinning of the freshwater zone west of Waimalu Stream; (2) redistributing withdrawal from Halawa Shaft to Pearl City III results in greater thickening of the freshwater in the eastern part of the Waimalu management system relative to redistributing an equal amount of withdrawal from the Kalauao Wells to Pearl City III; (3) the extent of freshwater thickening in the eastern part of the Waimalu management system caused by reducing withdrawal from the area is directly related to the amount of the reduction; (4) the zone where freshwater thickens in response to reducing withdrawal from a well is greatest downgradient from the well, between the well and the shore; and (5) valley-fill barriers can potentially reduce the zone where freshwater thickness increases in response to reduced withdrawals.
The numerical model developed for this study simulates regional water levels and salinity and may not accurately simulate salinity of water pumped from individual wells. Salinity of water pumped by a well may be controlled by local heterogeneities in the aquifer that are not represented in the model. The model has several other limitations for predictive purposes because of the various assumptions used and possible uncertainties in input data (for example, recharge, withdrawals, boundary conditions, and parameter values). Model reliability can be enhanced as our understanding of ground-water recharge, the distribution of model parameter values, and the geometry of the valley-fill barriers improves, and as numerical modeling technology improves.
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Suggested citation:
Oki, D.S., 2005, Numerical Simulation of the Effects of Low-Permeability Valley-Fill Barriers and the Redistribution of Ground-Water Withdrawals in the Pearl Harbor Area, Oahu, Hawaii: U.S. Geological Survey Scientific Investigations Report 2005-5253, 111 p.
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