Assessment of Land Use Impacts on the Coastal Waters of Waialua Bay, North Shore of Oahu, Hawaii

Excessive nutrient loading from anthropogenic sources such as agricultural runoff and wastewater enter coastal ecosystems through surface runoff into streams and through the groundwater system as submarine groundwater discharge (SGD). The large island of Oahu in Hawaii contains approximately 14,000 onsite sewage disposal systems (OSDS), with the majority located in coastal areas. Coastal cesspools are of particular concern because they are susceptible to inundation and overflow as the water table rises from heavy rains and/or by seawater intrusion, including King Tide events. Lands adjacent to Waialua Bay, North Shore of Oahu, contain nearly 2,000 OSDS, of which ~75% are cesspools. The high density of OSDS, combined with the area's history of fertilizer use for agriculture make Waialua Bay an area at high risk of excess nutrient pollution. Our objective was therefore to differentiate the amount and location of contaminant sources, and their connection to the coastal waters of this region. Using a combination of thermal infrared imaging (TIR) by UAV (drone), surface and groundwater geochemistry and stable isotope analyses, we map and quantify the amount, location, and temporal variability of contamination sources and their connection to the coastal waters of Waialua Bay. UAV-TIR was used to identify locations of point-source and diffuse SGD. Geochemical sampling was conducted throughout streams, groundwater wells, coastal waters and beach face. Water samples were analyzed for total-N and total-P and specific nutrients (NH₃, NH₄, NO₃^-, NO₂^-, and PO₄^3-), and the δ¹⁵N and δ¹⁸O values of dissolved nitrate. In addition, total N and δ¹⁵N of both field-deployed and wild benthic algae in the coastal waters were used as another way to track contamination sources, as well as long-term nitrate contamination trends. When combined, these data permit accurate determination of the amount and source of excess nutrients in ground-, surface- and ocean water. The TIR imagery and coastal surveys identify previously unknown sources and conduits of SGD, and the surface and groundwater geochemistry combined with δ¹⁵N algae identifies the significant geochemical connection between terrestrial sources of contamination and the ground and surface waters of Waialua Bay due to OSDS, as well as the historic use of agricultural fertilizer.