IMPACTS OF GROUNDWATER WITHDRAWALS IN HAWAI‘I

The availability of fresh groundwater for human use is limited by whether the impacts of withdrawals are deemed acceptable by community stakeholders or water-resource managers. Quantifying the island-wide hydrologic impacts of withdrawal—saltwater intrusion, water-table decline, and reduction of groundwater discharge to streams, nearshore environments and downgradient groundwater bodies—is thus important to assess fresh groundwater availability. Groundwater-flow models of Kaua‘i, O‘ahu, and Maui were constructed using MODFLOW 2005 with the Seawater-Intrusion Package (SWI2). Consistent model construction, calibration, and analysis were streamlined using Python scripts. Results of simulating historical withdrawals from Hawai‘i’s volcanic aquifers show that the types and magnitudes of impacts vary among hydrogeologic settings. In high-permeability freshwater-lens aquifers, saltwater intrusion and reductions in coastal groundwater discharge have been the principal consequences of withdrawals. In dike-impounded groundwater and thickly saturated low-permeability aquifers, reduced groundwater discharge to streams, water-table decline, or reduced flows to adjacent freshwater-lens aquifers can be unacceptable consequences that limit groundwater availability. The numerical models are used to quantify and delineate the spatial distribution of these impacts for the three islands. The models can also be used to examine how anticipated changes in groundwater recharge and withdrawals will affect groundwater availability in the future.