Volcanic aquifers of Hawaiʻi—Contributions to assessing groundwater availability on Kauaʻi, Oʻahu, and Maui

The volcanic aquifers of the Hawaiian Islands supply water to 1.46 million residents, diverse industries, and a large component of the U.S. military in the Pacific. Groundwater also supplies fresh water that supports ecosystems in streams and near the coast. Hawaii’s aquifers are remarkably productive given their small size, but the capacity of the islands to store fresh groundwater is limited because each island is surrounded by seawater, and salt water underlies much of the fresh groundwater. The amount of fresh groundwater available for human use from Hawai‘i’s volcanic aquifers is constrained by the consequences of groundwater withdrawal. Restrictions placed on these consequences can translate to limitations on groundwater availability. Changes in recharge resulting from changes in land cover or climate can alter the effect of withdrawals.

This study uses numerical models of the volcanic aquifers of the islands of Kaua‘i, O‘ahu, and Maui to quantify the consequences of historical and plausible future withdrawals and changes in recharge. The study compares the results of model simulations of multiple scenarios of historical and projected future withdrawal and recharge. Results of the simulations using the groundwater models of the islands of Kaua‘i, O‘ahu, and Maui have implications for other islands in Hawai‘i.

Since the first modern water well was drilled in Hawai‘i in 1879, total groundwater withdrawals on Kaua‘i, O‘ahu, and Maui have risen to nearly 400 million gallons per day. Model simulations indicate that these withdrawals have caused reductions in groundwater discharge to streams and springs, reductions in groundwater discharge to the ocean, changes in subsurface flow between sectors within an island, lowering of groundwater levels, and rise of the interface between fresh water and salt water in the aquifers. Future increases in withdrawals will increase the severity of the consequences. Changes in recharge can alter the effect of withdrawals—increases in recharge can offset the consequences of withdrawals, whereas decreases in recharge can exacerbate the effects of withdrawals.

This study quantifies the consequences of withdrawals for past and plausible future circumstances. The models can be used to test other circumstances. Limits placed on the consequences of withdrawals—such as restrictions to protect stream or coastal ecosystems that rely on groundwater discharge and limitations on water-level decline and rise of the freshwater-saltwater interface to protect the productivity of existing wells—can translate to limits on groundwater availability from Hawai‘i’s volcanic aquifers. Setting acceptable limits to the consequences of groundwater withdrawal is also a critical part of assessing groundwater availability. Once these limits are set, numerical models can be used to quantify the amount of water that can be withdrawn within those limits and thereby inform management decisions that seek to balance the need to limit the consequences of groundwater withdrawals with the need to develop water for human use.