Modeling the effects of pumping, barometric pressure, and ocean tides on groundwater levels in northern Oahu, Hawaii

Ground-water levels in northern Oahu, Hawaii, vary in response to both human-induced and natural factors. Water-level responses to (1) human-induced ground-water withdrawals, (2) barometric-pressure variations, and (3) ocean tides are modeled in this study to obtain a better understanding of the ground-water flow system in the Mokuleia, Waialua, and Kawailoa ground-water areas of northern Oahu.; A five-layer model using the computer code SHARP, which treats freshwater and saltwater as immiscible fluids separated by a sharp interface, was developed to simulate ground-water level drawdown and recovery effects caused by pumping. Measured water levels in the Koolau aquifer of northern Oahu were simulated with reasonable success using a horizontal hydraulic conductivity of 2,290 m/d, a vertical hydraulic conductivity of 22.9 m/d, and a specific yield of 0.05 to 0.1.; In the Waianae aquifer of northern Oahu, ground-water levels measured in wells are affected by variations in barometric pressure. Use of a constant barometric efficiency model fails to reproduce both the high- and low-frequency variations in water levels. Two models that account for (1) surface loading of the aquifer by barometric pressure, (2) vertical flow in the saturated zone to the water table, and (3) air flow in the unsaturated zone are developed and are able to represent the overall measured water-level response in a well in the unconfined part of the Waianae aquifer in northern Oahu. On the basis of model results, the ratio of vertical hydraulic conductivity to specific storage is estimated to be about {dollar}9.29 times 10sp3{dollar} m{dollar}sp2{dollar}/d.; Tidal efficiencies and phases for wells in the Waialua and Kawailoa ground-water areas of the Koolau aquifer were compared to a theoretical, one-dimensional analytical solution that accounts for decreasing efficiencies and increasing phase lags with distance from the ocean. For both the O{dollar}sb1{dollar} and M{dollar}sb2{dollar} constituents, the measured efficiencies and phases are in reasonable agreement with the theoretical solution. For the Koolau aquifer of northern Oahu, the ratio of aquifer transmissivity to specific yield is estimated to be about {dollar}2 times 10sp7{dollar} m{dollar}sp2{dollar}/d.