Integrated Hydrologic Analysis of Wastewater Contaminant Flows from On-Site Sewage Disposal Systems to Groundwater, Streams, and the Ocean Waters of Kāne‘ohe Bay, O‘ahu, Hawai‘i, USA

On the Hawaiian Island of O‘ahu, nearly 1,500 on-site sewage disposal systems (OSDS) exist within the Kāne‘ohe Bay drainage basin, releasing an estimated 3,800 cubic meters (one million gallons) of untreated wastewater into the groundwater each day, threatening stream and coastal water quality. The study area—Kahalu‘u, Hawai‘i—is characterized by the highest density (units per area) of OSDS in the Kāne‘ohe Bay drainage basin. This study evaluates hydrologic flow paths from wastewater point sources to groundwater and surface waters by utilizing a combination of unmanned aerial vehicle thermal infrared (UAV-TIR) imaging, stream gauging and seepage runs, and numerical groundwater models (MODFLOW and MT3DMS). Eight coastal groundwater seep locations were identified with UAV-TIR, with all seeps occurring through coastal valley fill sediments. Geochemical analysis of seeps revealed significantly elevated concentrations of all major nutrients compared to surrounding ocean waters. Groundwater nitrogen transport was modeled with MT3DMS and compared to measured concentrations. Most OSDS are located within the valley fill, and MODFLOW results suggest that the valley fill is more hydraulically conductive than the surrounding dike-intruded basalt, thus controlling groundwater and contaminant dispersion to streams and the ocean. Modeling also reveals that groundwater flow from leeward of the Ko‘olau ridgeline and from adjacent watersheds to the southeast may be significant hydrologic inputs to the study area.