SPECIAL REPORT SR-2022-05
SUBSURFACE HYDROGEOLOGY OF NU‘UANU TEST HOLES:
AN ASSESSMENT OF STORMWATER HARVESTING POTENTIAL, O‘AHU, HAWAI‘I
Michael Knight, Brytne K. Okuhata, and Aly I. El-Kadi
May 2022, xii+125 pp.
ABSTRACT
Groundwater is the primary source of fresh potable water for residents of Hawai‘i. Groundwater resources, however, are limited to the islands’ finite capacity and are sensitive to anthropogenic and climate change impacts. It is therefore important to properly manage Hawai‘i’s groundwater resources and implement sustainable practices, such as Aquifer Storage and Recovery, which involves injecting storm-water into the unsaturated aquifer. Nu‘uanu Valley, located within Honolulu, O‘ahu, Hawai‘i, has been identified as a promising location for Aquifer Storage and Recovery, but further information regarding the area’s hydrogeology is still needed. This study therefore uses exploratory drilling to improve our understanding of Nu‘uanu Valley’s vadose zone, which comprises interbedded Nu‘uanu basalt flows and Honolulu Volcanics, underlying older alluvium, and the Ko‘olau Volcanics. Two test holes were drilled in central Nu‘uanu Valley, near Nu‘uanu Reservoir No. 1, to depths of 145 and 517 feet below ground surface, respectively, where continuous rock core samples were collected, boxed, cataloged, and described. The mineralogy, density, major and trace element geochemistry, and hydraulic conductivity were further assessed for select rock samples. Based on all drilling and lab analyses, the local area is defined by high-plasticity clay alluvium, at least eight layers of Nu‘uanu alkalic basalt, older alluvium, and Ko‘olau basalt. These results are in relatively good agreement with electrical resistivity tomography survey results, which suggest layers of alternating low-resistivity and high-resistivity units. These different subsurface units vary in degrees of weathering, porosity, and permeability, all of which will affect groundwater flow and transport. Findings from this study therefore increase our understanding of how the Nu‘uanu and Kalihi aquifers will respond to storm-water injection. Future studies should aim to perform more in situ tests to best understand the permeability of each hydrogeologic unit. Future groundwater modeling efforts would also benefit from a detailed, anisotropic model that can accurately simulate flow and transport of each hydrogeologic unit.