This USGS fact sheet (FS 126‑00, 2000) authored by Stephen B. Gingerich and Delwyn S. Oki provides a comprehensive overview of Hawaii’s groundwater resources. It explains that groundwater supplies about 99 % of domestic water and roughly half of all freshwater use in the state, yet only a small fraction of rainfall (≈3 %) is captured as pumped groundwater. The document describes the hydrogeologic setting of the islands, focusing on volcanic‑rock aquifers whose permeability varies with lava type, weathering, and dike intrusion. Two principal groundwater settings are detailed: freshwater‑lens systems that float on seawater and dike‑impounded systems where low‑permeability dikes compartmentalize water. Island‑by‑island water‑level data illustrate where thin lenses, vertically extensive lenses, or dike‑impounded reservoirs occur. Groundwater discharge mechanisms—including diffuse coastal seepage, springs, and shaft extraction—are linked to the Ghyben‑Herzberg principle for estimating lens thickness. The sheet identifies the main limits to groundwater availability: saltwater intrusion, reduced discharge to streams/ocean, and lowered water levels caused by pumping. It discusses ecological and water‑resource consequences of over‑pumping and provides references for further technical detail.
Topics
Groundwater as Hawaii’s Primary Freshwater Resource
Quantifies groundwater’s contribution to domestic, agricultural, commercial, and industrial water use and places it in the context of Hawaii’s rainfall and surface‑water dynamics.
Volcanic‑Rock Aquifers: Permeability, Lithology, and Confining Units
Describes how lava flows, dikes, and pyroclastic deposits create variable permeability, the effect of weathering, and the role of sedimentary caprock as a confining layer.
Freshwater‑Lens vs. Dike‑Impounded Groundwater Systems
Defines the two major groundwater settings in Hawaii, their structural characteristics, and how they interact where they are adjacent.
Island‑Specific Freshwater Lens Occurrence and Well‑Level Mapping
Presents water‑level thresholds (< 50 ft vs. > 50 ft above sea level) used to map thin lenses, vertically extensive lenses, and perched systems across Kauai, Oahu, Molokai, Lanai, Maui, and the Island of Hawaii.
Groundwater Discharge Pathways: Springs, Streams, and Ocean
Explains how groundwater exits the system via diffuse coastal seepage, springs, stream valleys, and direct ocean outflow, and the ecological importance of these flows.
Saltwater Intrusion and the Ghyben‑Herzberg Principle
Details the physics of freshwater‑saltwater interfaces, the use of the Ghyben‑Herzberg relation to estimate lens thickness, and its limitations in low‑permeability settings.
Impact of Pumping on Groundwater Levels and Ecosystems
Describes how high‑rate extraction lowers water tables, induces saltwater intrusion, reduces spring and stream flow, and can shrink wetlands, affecting habitats and water supply.
Key Limitations to Groundwater Availability in Hawaii
Summarizes the three primary constraints—saltwater intrusion, reduced discharge, and lowered water levels—and the factors influencing each.