National Institute for Water Resources, Water Resources Research Institute Program
The Hawaii aquifers supply water to 1.36 million residents, diverse industries, and a large segment of the U.S. military in the Pacific. With the increase of residential population on the islands, drinking water resources remain limited and are therefore susceptible to impacts from human activity and climate change (Izuka 2013). Decisions related to future infrastructure development and alternate sources of freshwater, including desalinization, depend on the long- term sustainability of the groundwater resources in Hawaii’s aquifers. Numerical groundwater models are becoming standard tools for sustainable development and optimal resource management (e.g., Oki 2005, Gingerich 2008). Identifying relevant components in the hydrogeologic framework and quantifying hydraulic properties for volcanic rock aquifers helps to constrain input parameters for models used in ongoing U.S. Geological Survey (USGS) groundwater-availability and groundwater-resources studies in Hawaii (Izuka 2013). Additionally, evaluating hydraulic properties for coastal sediment aquifers can be beneficial for studies assessing the effects of sea-level rise (e.g., groundwater inundation), including groundwater height in coastal inundation scenarios (Rotzoll and Fletcher 2013). Ocean-tide signal attenuate through coastal aquifers based on the aquifer’s regional hydraulic diffusivity (Jacob 1950, Ferris 1951). Thus, analyzing tidal amplitude decay with distance from the coastline allows estimating regional-scale hydraulic properties and identifying the importance of different hydrologic units (Rotzoll et al. 2013). A few localized studies of tidally influenced groundwater levels in Hawaii exist (e.g., Dale 1974, Gingerich 1995, Oki, 1997, Rotzoll et al. 2008), but a statewide compilation and comparison is lacking. In this study, aquifer properties were examined in a regional context with respect to the hydrogeologic framework of each island. Hydrologic data exist for some areas in Hawaii, but there has been no effort to synthesize the existing separate efforts into a comprehensive framework for the entire Hawaiian Islands region. The results help to better understand the groundwater flow processes in Hawaii aquifers and facilitate the development of regional numerical groundwater flow and transport models. Water managers and hydrologists benefit from a more detailed characterization of the regional groundwater-flow properties for various purposes (e.g., monitoring, management, and research). The scope of work covers the large-scale estimation of hydraulic properties of volcanic- rock and coastal-sediment aquifers from tidal attenuation by compiling available data for the state of Hawaii.