MODELING GROUNDWATER AVAILABILITY IN THE VOLCANIC-ISLAND AQUIFERS OF HAWAI‘I

Construction of numerical groundwater models to assess freshwater availability in Hawai‘i’s aquifers faces multiple challenges owing to the islands’ shield-volcano origins and oceanic setting. At the scale of island-wide models, Hawai‘i’s volcanic aquifers chiefly consist of hydrogeologic units that have relatively homogeneous hydraulic properties for thousands of meters vertically, yet contrast sharply with hydraulic properties of horizontally adjacent units. Some units are formed by thin high-permeability lava flows, some are formed by thick low-permeability lava flows, and some are intruded by low-permeability volcanic dikes or capped by semiconfining sediments. Many of Hawai‘i’s aquifers also contain freshwater and saltwater; the coupled movement of freshwater and saltwater in response to groundwater withdrawals affects fresh groundwater availability. To make model assessment of fresh groundwater availability in Hawai‘i tractable, we constructed models that focused on the quantification of three consequences of groundwater withdrawals: (1) water-table depression, (2) reduction in groundwater discharge to streams and the ocean, and (3) saltwater rise. Restrictions placed on these consequences—such as restrictions to protect ecosystems or the productivity of existing wells—can limit groundwater availability. Our model simulations quantified the consequences of historical withdrawals and projected future withdrawal increases. Model results show that the degree to which the consequences bear on fresh groundwater availability vary with hydrogeology, and that changes in recharge can exacerbate or ameliorate the consequences. These model-simulation results inform management decisions that seek to limit the consequences of groundwater withdrawals while allowing the development of water for human use.