How Much Groundwater Can Hawaii Use? Limits Set by the Consequences of Pumping

How groundwater occurs and moves in the volcanic aquifers of Maui, one of Hawaii's basaltic shield-volcano islands. Background photograph of Mount Waiʻaleʻale, Kauaʻi, by Chui Ling Cheng, U.S. Geological Survey. USGS figure.

The aquifers

The Hawaiian Islands were built by basaltic shield volcanoes, which formed aquifers that supply hundreds of millions of gallons of fresh water a day to residents and industries. Groundwater flowing out to streams and the coast also sustains traditional practices and ecosystems. In these aquifers fresh water sits alongside salt water and drains naturally to streams and the sea.

How much fresh water the aquifers can yield is limited by the limits placed on the consequences of pumping — falling water tables, rising salt water, and less groundwater reaching streams, springs and the ocean — and those consequences will change as climate and land use change recharge. This fact sheet summarizes the Hawaiʻi Volcanic Aquifer Study (HVAS) (full report).

Three groundwater settings

Most of Hawaii's groundwater falls into one of three settings, each responding differently to pumping.

Principal groundwater settings on Kauaʻi, Oʻahu and Maui. USGS figure.

Freshwater lens. Stacks of thin lava flows make highly permeable aquifers in which a lens of fresh water floats on denser salt water, flowing from inland toward the coast. Where thick coastal sediments form a low-permeability caprock, the lens is thicker; some water emerges at springs where the caprock thins out above sea level, and some seeps through it.

Freshwater lens with caprock, before and during pumping. USGS figure.

Dike-impounded. Dikes — near-vertical sheets of low-permeability volcanic rock — cut through the lava flows and wall off compartments where groundwater can pile up to high altitudes. Water passes from compartment to compartment, and where erosion has breached them, much of it drains to streams and springs.

Dike-impounded groundwater, before and during pumping. USGS figure.

Thickly saturated. Where the whole aquifer has low permeability, it fills deeply with fresh water. Streams drain some of it, keeping the water table below most of the land, so most natural discharge happens above sea level at streams. Salt water probably lies under the fresh water near the coast; whether it extends farther inland is unknown.

Thickly saturated setting, before and during pumping. USGS figure.

Some groundwater fits none of the three and isn't fully understood — such as the Schofield high-level groundwater in central Oʻahu, in a highly permeable aquifer yet with water levels hundreds of feet higher than a freshwater lens would have.

Every withdrawal has consequences

Any amount of pumping has effects. Limits on them — to protect stream ecosystems and traditional practices, or the productivity of existing wells — cap how much groundwater is available, so knowing the consequences is the key to knowing the supply. Since Hawaii's first well was drilled in 1879, withdrawals from Kauaʻi, Oʻahu and Maui have grown to nearly 400 million gallons a day, and with virtually all drinking water from aquifers, they're expected to grow with the population.

The HVAS used groundwater models of the three islands — which between them cover all the principal settings — to measure the effects of past and future pumping:

• less water to streams and springs, especially in dike-impounded and thickly saturated settings, and in freshwater lenses with caprock;
• less water to the ocean, especially from freshwater lenses and wells near the coast;
• falling groundwater levels, especially in low-permeability aquifers or where pumping is heavy;
• rising salt water, especially in heavily developed freshwater lenses;
• shifting flows between parts of an island, so pumping in one area affects supply in another.

Modeled effects of groundwater withdrawals on Oʻahu's aquifers, 2001–2010. USGS figure.

Recharge can help or hurt

Changes in land use or climate change recharge, which can ease or worsen the effects of pumping, depending on the setting and on how much and where recharge changes.

• More recharge offsets pumping; in Hawaii it has come from crop irrigation and surface-water reservoirs.
• Less recharge makes things worse; it has come from shifts in agriculture and nonnative forest replacing native forest. HVAS projections using forecasts of lower rainfall (Elison Timm and others, 2015) point to substantial decreases in future recharge.

Kauaʻi: groundwater levels changed with land use from 1870 to 2010 — falling where recharge dropped, rising near leaky agricultural reservoirs. USGS figure.

Oʻahu: possible effects of projected recharge changes for 2041–2070 under representative concentration pathway 8.5. USGS figure.

Setting acceptable limits comes first

The study shows the consequences of pumping can be measured with models. The essential next step is to decide how much water-table decline, saltwater rise and loss of discharge is acceptable — usually involving many stakeholders and weighing human and environmental health, traditional and customary practices, economic growth and legal rights. With limits set, models can calculate how much water can be pumped within them, helping balance protection against the need for water.

Further reading: Volcanic aquifers of Hawaiʻi — Hydrogeology, water budgets, and conceptual models (SIR 2015-5164).

Sources

Based on Scot K. Izuka and Kolja Rotzoll, "Availability of groundwater from the volcanic aquifers of the Hawaiian Islands," U.S. Geological Survey Fact Sheet 2023–3010, U.S. Geological Survey; a work of the United States government in the public domain. Its figures, which the import did not carry, are taken from the fact sheet PDF. The source spells the rainfall study's first author both "Elison Timm" and "Ellison Timm"; the journal citation's spelling is used.