Hydrology of Contaminant Flow Regimes to Groundwater, Streams, and the Ocean Waters of Kaneohe Bay, Oahu

Abstract

Study focus: Groundwater is an important source of nutrients for coastal waters, but can act as a pathway for anthropogenic contaminants to reach the ocean. On the Hawaiian Island of O‘ahu, nearly 1500 on-site sewage disposal systems (OSDS; e.g., cesspools and septic tanks) exist within the Kāne‘ohe Bay drainage basin, releasing one million gallons of untreated wastewater into the groundwater each day, threatening stream and coastal water quality. This study evaluated hydrologic flow paths from OSDS to surface water bodies by utilizing a combination of unmanned aerial vehicle thermal infrared imaging (UAV-TIR), stream gauging and seepage runs, and numerical groundwater modeling. New hydrological insights for the region: This study found that approximately 1.12 x 107 m3 of groundwater is directly entering the ocean each year, primarily in the form of diffuse submarine groundwater discharge with significantly elevated concentrations of all major nutrients. Eight groundwater seep locations were identified by UAV-TIR, with all seeps occurring through coastal valley fill or beach sediments. Model results confirmed that valley fill, which extends as deep as 425 meters in some locations, is the most hydraulically conductive substrate in the study area, with hydraulic conductance 16x greater than dike-intruded basalt. This is a marked contrast to many other coastal settings on O‘ahu, where valley fill deposits act as a semi-confining barrier to groundwater flow through the aquifer.