Identifying wastewater pollution hotspots in coastal Hawaiian waters with high freshwater inputs

Identification of wastewater pollution hotspots and sources is a management priority because wastewater threatens human and ecosystem health in coastal waters. We assessed wastewater pollution from onsite sewage disposal systems (OSDS) and an offshore wastewater treatment plant (WWTP) outfall in the coastal waters of Hilo, Hawai'i, an area with high freshwater inputs from groundwater and rivers. Twenty shoreline stations were sampled monthly from July 2020 to October 2021 for fecal indicator bacteria, nutrients, and stable nitrogen isotopes of macroalgae and nitrate. Wastewater hotspots were identified using a wastewater pollution score and stable isotope mixing models. Wastewater was detected at all stations, although most wastewater indicator concentrations were low. High groundwater inputs likely diluted wastewater indicators making their detection challenging. Stable isotope mixing models support this conclusion as groundwater was the dominant nitrate source at all stations (44-87%), while wastewater (OSDS + WWTP) comprised 8-49%. Addition of OSDS density to the wastewater pollution score improved identification of hotspots against the high freshwater input background. This finding is supported from concurrent dye tracer tests that confirmed the presence of wastewater and rapid transport from OSDS to shoreline springs at hotspot locations. Our results illustrate the importance of using a variety of approaches to understand the wastewater pollution threat, especially in areas with high freshwater inputs.