Hydraulic Parameter Estimation Using Aquifer Tests, Specific Capacity, Ocean Tides, and Wave Setup for Hawai'i Aquifers

Abstract

The islands of Hawaii face increasing ground-water demands due to population growth in the last decades. Analytical and numerical models are essential tools for managing sustainable ground-water resources. The models require estimates of hydraulic properties, such as hydraulic conductivity and storage parameters. Four methods were evaluated to estimate hydraulic properties for basalts on the island of Maui. First, unconventional step-drawdown tests were evaluated. The results compare favorably with those from classical aquifer tests with a correlation of 0.81. Hydraulic conductivity is log-normally distributed and ranges from 1 to 2,500 m/d with a geometric mean of 276 m/d and a median of 370 m/d. The second approach developed a simplified parameter-estimation scheme through an empirical relationship between specific capacity and hydraulic parameters that utilized Hawaii's state well database. For Maui's basalts, the analysis yields a geometric-mean and median hydraulic conductivity of 423 and 493 m/d, respectively. Results from aquifer tests and specific-capacity relationships were used to generate island-wide hydraulic-conductivity maps using kriging. The maps are expected to be of great benefit in absence of site-specific field assessments. In the third approach, ocean-tide responses in the central Maui aquifer were used to estimate an effective hydraulic diffusivity of 2.3 x 10^7 m^2/d. The position of the study area necessitated refining the existing analytical solution that considers asynchronous and asymmetric tidal influence from two sides in an aquifer. Finally, measured ground-water responses to wave setup were used to estimate hydraulic parameters. Setup responses were significant as far as 5 km inland and dominated barometric-pressure effects during times of energetic swell events. The effective diffusivity estimated from setup was 2.3 x 10^7 m^2/d, matching that based on tides. Additionally, simple numerical ground-water flow models were developed to assess the accuracy of results from analytical solutions for step-drawdown tests, dual-tides and wave setup, and to evaluate sediment-damping effects on tidal propagation. The estimated mean hydraulic conductivities of the four methods range between 300 and 500 m/d for basalts in Maui. The results of different methods are consistent among each other and match previous estimates for basalts.